Forces Kit
Key Stage 2
BOOK I: Lesson Plans,
Extensions, Links
and Teachers’ Quick
Reference Notes
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Forces Kit
Key Stage Two
We would like to acknowledge
our debt to our science and education advisors,
Tom Syson and Anne Goldsworthy.
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Forces Kit
Teachers’ Notes: Book I
TABLE OF CONTENTS
How to use this kit ................................................................. 7
Lesson Plan
Optional pre-activity session................................ 9
Activity one
Grip Test......................................................... 13
Activity two
The Force of Magnetism.................................... 15
Activity three
See Stress........................................................ 17
Activity four
The Octopus.................................................... 19
Activity five
Wind Turbines................................................. 21
Activity six
Floating on Air................................................. 23
Activity seven
Spring Force.................................................... 25
Activity eight
Earthquake Table.............................................. 27
Activity nine
Magnet Game................................................. 29
Activity ten
Air Power........................................................ 31
Activity eleven
Arch Bridge..................................................... 33
“What I learned” sheet..................................... 35
Lesson Plans
Optional follow-up sessions:
Measuring forces.............................................. 37
Forces can make things change direction............ 41
Magnetism...................................................... 47
Forces on structures.......................................... 51
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How to use the Key Stage 2 Forces Kit
Setting up the Kit
Place the Forces Kit trays on work tables, plug in those that require electricity and the Forces Kit is ready
to use.
What to do
• Optional Pre-Activity Session: a lesson plan is provided for an hour-long session to introduce or
revise Key Stage 2 Forces National Curriculum points. This session could take place the week before
the Forces Kit arrives.
• Using the Kit: The Key Stage 2 Forces Kit consists of 11 activities, with built-in easy-to-follow
instructions. The kit is designed to be used in a session of about an hour. Children work in pairs,
spending five minutes on each activity.
• Optional Follow Up Session(s):
1) Measuring forces (includes springs).
3) Magnetism.
2) Forces can make things change direction.
4) How forces affect structures.
The follow-up sessions are themed teacher-led sessions of about an hour on aspects of the Forces
Curriculum. Full lessons plans are provided. These sessions employ elements of the kit, so it is
intended that classes have the follow-up session on the same day as they use the kit.
Through question-and-answer sequences, directed activities and discussion in small groups, children
develop their scientific enquiry skills: prediction, observation, repetition to check results, measurement
and recording of data.
Support Material
BOOK I: Lesson Plans and Teachers’ Quick Reference Notes
• Optional Pre-Activity Session Lesson Plan: about an hour.
Lesson Plan pages have a green strip down the side.
• Session with the Forces Kit: about an hour.
For each activity, the book gives:
– Forces Concepts: Key Stage 2 National Curriculum points.
– The Activity: a description and photograph.
– Background: a brief explanation of the relevant science.
– Extensions: suggestions for additional activities.
– Web Links: websites for additional investigation.
• What I Learned Sheet: a fill-in sheet to help children to assess their session with the Forces Kit.
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• Four Optional Follow-Up Session Lesson Plans: about an hour each, with additional “Further work”
activities to develop and deepen children’s understanding of the forces concepts.
BOOK II: National Curriculum (DCELLS) and Background Material
• National Curriculum (DCELLS): assessment criteria are defined for each activity and
a check list provided.
• More Background: additional science notes are provided for each activity.
• Practical Applications: material is provided on relevant technology.
BOOK III: Risk Assessments
8
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Pre-Activity Lesson Plan
YOU WILL NEED the Newton Meter, a weight, small compression and extension springs, a straw and
two magnets from the PRE-ACTIVITY LESSON PROPS WALLET. Please provide a few paper clips,
your own beaker of water and a plastic receptacle to pour it into.
Ask children to consult with “talk partners”, before asking pairs to respond to questions.
The lesson is divided into sections. You only need to go through the sections you feel your class can
benefit from.
Teacher’s prompts appear in this typeface: Sample.
Suggested “answers” appear in this typeface: Sample.
Requests for volunteers to come up to try something – or for children to try something at their places –
appear in this typeface: Sample.
Introduction to the Concept of Force
A force is basically a push or a pull. Place your two hands together, palm-to-palm in front of you. Push
your right palm against your left palm. Now, push your left palm against your right palm. That was
force. Now, clasp your hands together in front of you. Pull with your left hand. Pull with your right
hand. That was force, as well.
When you kick a ball, it’s force from your foot that makes it go forward. When you spin a top, it’s
force from your hand that makes it go ‘round.
Measuring Force
Pushes and pulls come in different sizes.
Take a partner. Clasp hands. Try a big pull, but not too big. Try a little pull.
Now, clasp little fingers, hardly touching and try the smallest pull you can.
However big the pull is, that is the size of the force. So, we can measure force, just as we measure
length or distance.
What do we use to measure length? (for instance, a ruler or tape measure)
What are some units of length? (for instance, millimetres, centimeters, metres, kilometers)
We can measure force, as well. The unit of force is a newton. Here is a Newton Meter that we can
use to measure force.
We’re going to use the Newton Meter, which is also called a force meter, to measure the force of a
pull from your hand.
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How large a force do you think your pull would have? Give your guess in newtons.
I want a few volunteers to come up and try pulling down on the Newton Meter to
measure the force of their pull.
Measuring the force of gravity on an object
Now, we’ll hang a weight on the Newton Meter. That will give us a result, as well. Remember, a
Newton Meter measures force. How can there be a force in the weight? Can anyone guess why?
(The force pulling the Newton Meter down, when you hang a weight on it, is the force of gravity.)
It’s because the Earth’s gravity is a force, just like the force in your hand. Some people think of
gravity as something like an invisible elastic band between objects and the Earth that pulls objects
downwards. Gravity isn’t quite like that, but you can think of it like that for now, if it helps you to
remember what gravity does.
Your hand can apply force upwards or sideways or in any other direction, because the force comes
from you, not from the Earth.
Let’s try it. Let’s have a new set of volunteers come up and try pulling the Newton
Meter in other directions to measure the force of their pull.
Force and springs
Let’s have a look at the Newton Metre. What can you see inside it? (a spring)
There are three main kinds of spring. We’ll look at two of them. One stretches. The other compresses.
What kind of spring is in the Newton Metre? (the kind that stretches)
Here’s the other kind, the type of spring that compresses. Here’s how it works. I’m going to pass
around some compression springs and you can try it.
Springs are very interesting objects. Where have you seen springs? (maybe inside a ball point pen, a
mattress, toys, supporting a trampoline, on a motorcycle)
What do springs do? (they make things “spring” back)
Because springs “spring back”, they are used to absorb shock – force – on a motorcycle, for instance.
The force of that shock can come from a bump in the road, for instance.
Every force has a direction
In pairs, put your palms together, again. Push with one of your palms against the
other. Don’t make your other hand resist. Which way did you push? Which way did your
hand move?
Now, still in pairs, grip your hands together and pull with one hand. Don’t make
your other hand resist. Which way did you pull? Which way did your hand move?
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When you push or pull, you are always pushing or pulling in one direction or another.
Sometimes, though, there are two forces involved. Put your own palms together. This time,
push with both hands and try to make the push from both sides the same. What
happens? (Your hands don’t move.)
This time, the forces applied by your two hands balanced each other.
Let’s try it with a pull, this time. Grip your two hands together and pull both ways.
What happens? (Your hands don’t move.)
Once again, the two forces have balanced each other.
Now, let’s try using two forces with the Newton Meter. We’re going to hang a weight on the Newton
Meter, but this time, we’re going to apply another force to oppose – or balance – the force of gravity.
Let’s have some new volunteers to lift the weight on the end of the Newton Meter –
the force meter – so it can’t hang down.
Can you feel the weight pressing down on your hand? That’s the force of gravity – the force that pulls
all objects downwards towards the centre of the Earth – acting on the weight. You should be able to
measure that force with the Newton Metre, but what does the reading on the Newton Metre actually
say? (It should say “0”.)
Why isn’t the Newton Metre measuring the force of gravity on the weight? (Because the children are
putting a force on the weight – in the other direction – to lift it. The two forces acting in opposite directions are
balancing each other and cancelling each other out.)
What do you think would happen if you used the force you can apply with your hand to pull
downward on the weight – in the same direction as gravity?
Let’s try it. Let’s have some new volunteers to pull downward on the weight hanging
from the Newton Metre, the force meter.
Now, there is much more force to measure. We’d need a bigger Newton Meter to measure the size of
the combined forces – the force you are putting on the weight and the force of gravity.
Force can come from other sources, as well
Have you seen machines push and pull, applying force? What are some examples? (A toaster makes
toast pop up; a crane on a building site lifts heavy loads; a car engine makes a car go forward.) When you
understand more about forces, you will be able to understand how many machines work.
Just as you can apply force with your hand or your foot, air and water can apply force, as well. Let’s
experiment with this. Everyone pick up a piece of paper and hold it lengthwise in
front of your mouth. Now, blow hard.
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What happened to the piece of paper? (It was blown outwards.)
Have you ever felt a really strong wind? Could you feel the wind pushing you or pulling you? That was
the force of the wind.
There are special force meters that can measure the force of the wind in newtons, just as we can
measure the force of a pull on the Newton Metre we have here.
Have you seen the giant wind turbines or wind mills that are used to generate electricity? They are
using the force of the wind.
Water can apply force, as well.
Let’s see how. I need a volunteer to come up and hold the straw horizontally like
this. Now, I’ll pour water over it into this bowl. Let’s try that again, so you can be
sure to see what happens.
What did the water do to the straw? (It pushed it downwards.)
So, was the water applying force to the straw? (yes)
Have you heard of dams? A dam is a massive barrier that holds back the water in a river. The water
is then allowed to fall in a controlled way, so that the force of the water can be used to generate
electricity.
Magnetic force
Magnets have force, as well. What do you know about magnets? What do magnets do? Let’s try an
experiment. I’m going to put some paperclips down. What do you think will happen if I put this magnet
a little distance away?
Alright, let’s see what actually does happen, using the weaker magnet. Let’s have some
volunteers up. (The paper clips jump to the magnet, as long as you get the distance right.)
What do you think would happen if we used a stronger magnet? Let’s have some new
volunteers and we’ll see. (Place the magnet farther away and the paperclips will still jump to it.)
So, it turns out that magnets have “magnetic force”. They pull on objects containing iron and
certain other metals. Magnets can also push. Let’s have volunteers to make to make the
magnets push each other.
For more work
In groups, choose a sport where there are pushes and pulls. I want you to work out a mime where each
child takes part. Each group will come up and act out its mime. The audience will have to say what
sport you are miming, what the forces are and what the forces are acting on. They will have to count
how many pushes and pulls they see.
If there is time, each group can make a poster about the sport one of the other groups mimed, showing
the pushes and pulls.
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Grip Test
Forces Concepts
Supports an understanding of how to measure forces between objects in newtons; objects that are
stretched or compressed exert a force on whatever is changing their shape.
Activity
Children squeeze a handgrip.
Software built into the exhibit
automatically charts the force of
their grip in newtons. They find
out whether there is a difference
between their right and left hands.
They find out whether the force of
their grip decreases over the period
of the activity.
Background
When you squeeze the handgrip, you are exerting a force. This force can be measured in newtons.
Children will find that there is a definite resistance in the handgrip. An object that is compressed exerts
a force on whatever is changing its shape.
Extensions:
The balloon test (to show that objects that are pushed, push back). We are told that if you apply a
force downwards on a table and the table doesn’t move, your force must be balanced by a force
in the table pushing back. The upward force from the table top is due to it being bent downwards just
enough to balance whatever is resting on it. Unfortunately, you can’t see this – but towards the end of
the following experiment, you are going to feel a similar force in your own neck!
Blow up a balloon and press it gently between your two hands. To make it change shape, you’re pushing
on it from two sides. If you now put the balloon on the table and press downwards on it, it changes
shape in just the same way. You can see that there is a force pressing upwards on the balloon from the
table that it is equal to the force you are pressing down with, because the balloon is squashed in the
same way from both sides. You can try the same thing by pressing the balloon against a wall.
Now, stand in front of a mirror and put the balloon on your head. Reach up and press down on the
balloon. See it change shape, once again. This time, though, you can feel your neck working hard.
You can feel that you are applying an upward force to balance the downward force from your hand.
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The Force of Magnetism
Forces Concepts
Supports an understanding that forces can change in size; forces act in a particular direction; there
are forces of attraction and repulsion between magnets and forces of attraction between magnets
and some materials.
Activity
Children use wand-mounted
magnets to see the amazing effects
of magnetic force on ferromagnetic fluid, a fluid in which
nano-particles of black iron oxide
are suspended in water (or oil).
Background
Ferro-magnetic fluid was originally
developed by NASA in the 1960s.
It is a suspension of tiny particles
of black iron oxide (magnetite) in
water or oil.
When children bring a magnet against the base of the dome, they will see smooth spikes and other
shapes appear in the ferro-magnetic fluid. These spikes are created by the strong magnetic field around
the magnet.
For information on magnets, see the background material on the “Magnet Game”, p.29.
Extensions:
“Dip” a magnet into a box of pins (sewing pins, not safety pins). Pull the magnet out. See how the pins
point outwards in a thistle shape, just as the spikes in the ferro-magnetic fluid do.
Links:
For ferro-magnetic fluids:
http://www.mie.utoronto.ca/labs/MUSSL/Ferrofluids.htm
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See Stress
Forces Concepts
Supports an understanding that forces act in particular directions; that forces can make things
change direction; that objects that are stretched or compressed exert a force on whatever is changing
their shape.
Activity
Children put on goggles with
special polarising filters. They apply
pressure to acrylic bridge models
above a cross-polarising light
screen. They see the effects of stress
in rainbow colours. Colours appear
wherever there are problem areas in
the structure.
Background
Unless a solid object actually breaks, it can be difficult for children to understand that forces can have
an effect on it. That is why it is important to give them a chance to “see stress”.
When you put pressure on an object, you are “stressing” it. A stressed object shows “strain”, a change
in its shape, but this change in shape may usually be hard to see. The See Stress activity allows
children to see the effects of stress with the aid of cross-polarised filters.
Children may be interested to realize that their own bones are subject to stress, just as the plastic
bridges are.
Extensions:
Stressed materials. Stick some sellotape on a piece of clear rigid plastic in layers: sticky side
down, sticky side up, sticky side down. Make sure that in some places the sellotape is one layer thick,
in some places the sellotape is two layers thick, and in some places it is three layers thick.
Now, look at the piece of plastic between “crossed” polarising filters. Sellotape has been stretched
in one direction during manufacture, so it is permanently “stressed”. The colour that you see with the
polarising filters is the result of that pre-stressing.
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With coloured pencils or markers, draw a diagram showing the coloured patterns in the layers
of sellotape.
In which direction do you think the stress was applied? Label your drawing to show this.
Next, try looking at a plastic ruler under the between “crossed” polarising filters, as well. Try to draw
the patterns you see here.
Links:
For more on machines that use polarised light to show stress, see:
http://www.sharplesstress.com/
For bridge design:
http://pghbridges.com/basics.htm
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The Octopus
Forces Concepts
Supports an understanding that forces can change in size; forces act in particular directions; forces
can make things speed up, slow down or change direction; the weight of an object is the force of the
Earth on the object; friction is a force that slows moving objects and may prevent them from starting
to move.
Activity
Air blows a ball up each arm of
The Octopus. By using their hands
as flaps to block and unblock the
ends of the tubes, children make
the balls go up, go down, change
direction and hover. Children are
balancing the forces acting on the
balls, gravity and the force of the
airflow.
Background
There are two forces acting upon the ball in an arm of The Octopus, gravity and the force of the
airflow. When a child blocks the end of a tube, he or she is blocking the airflow. This changes the
size of the force of the airflow on the ball. Changing the size of this force can make the ball change
direction.
All forces have direction. Gravity is the force on a body exerted by the mass of the Earth. This pulls
all objects down towards the centre of the Earth. If you hold an Octopus arm up, gravity and the force
of the airflow are acting in opposite directions. If you hold the arm down below the table, gravity and
the force of the airflow are acting in the same direction.
The airflow around the ball produces friction. Friction is a force that resists the motion of an object,
whatever direction it is travelling in.
Extensions:
Streamlining one. “Stream lines” are the lines of flow of air past, for instance, a car, an
aeroplane or a ship. If something is not streamlined, the flow of air past it is chaotic. That kind of
airflow wastes energy.
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What shape of car do you think would produce the best flow of air past it, producing the least “drag”?
Draw your “streamlined” car.
Why do you think ordinary passenger cars are not designed in this way?
Have you seen any cars that are more like your sketch?
Which is more streamlined, a tractor or a formula one car? Why do you think this is so?
Cut out pictures of cars from a newspaper or magazine and make a poster, with labels comparing how
“streamlined” you think they are and why.
Links:
To see a streamlined car shape:
http://www.discoverychannelasia.com/car/race_design/index.shtml
To see how manufacturers try to streamline lorry design:
http://www.scania.com.au/trucks/cabs/aerodynamics/
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Wind Turbines
Forces Concepts
Supports an understanding that forces change in size; forces act in particular directions; forces can
make things speed up, slow down or change direction; friction and air resistance are forces.
Activity
Children set up a variety of wind
turbines to catch the “wind” from
a hidden fan. Children quickly
observe that blades with a curve
and a twist work best and they
are invited to improve the flat
bladed propellers. Children
experiment to find out whether
placing the turbines nearer to the
airflow makes them turn faster,
and whether they can see any
interference if they place turbines in front of one another.
Background
Children may be surprised to find that wind power is actually a by-product of solar energy. Warm air
rises and creates low pressure zones; cooler air blows in. Wind is caused by air flowing from high
pressure to low pressure areas. The rotation of the Earth also plays a part in how the wind blows.
The blades of a wind turbine are designed to be similar to aeroplane wings. Propellers are designed in
the same way. Adapting propeller design for a wind turbine, you use long thin blades that spin freely
to generate the greatest amount of electricity.
Extensions:
There are a number of different types of wind turbine for generating electricity from the force of the
wind. Follow up the links below to find out about them. You can also collect data on the size of various
wind turbines, how much steel goes into them, the length of their blades, etc. Print out pictures of the
different types of wind turbine. Make a poster and include data you have collected.
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Links:
For information on standard wind turbines:
http://www1.eere.energy.gov/windandhydro/wind_how.html
http://en.wikipedia.org/wiki/Wind_turbine
For information on different types of wind turbines:
http://www.windpower.org/en/tour/design/horver.htm
http://en.wikipedia.org/wiki/Darrieus_wind_turbine
For information on propellers:
www.mh-aerotools.de/airfoils/prophist.htm
http://en.wikipedia.org/wiki/Propeller
For the history of windmills:
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Floating on Air
Forces Concepts
Supports an understanding that force can change in size; forces act in a particular direction; forces
can make things speed up, slow down or change direction; friction, including air resistance, is a force.
Activity
Children “float” flat plastic objects
of various sizes on an airtable.
They discover that they can use
objects with a large surface area
to “carry” objects that would not
“float” on their own. There are
half-cut holes in the surface of the
objects, so that children can count
the number of airholes each object
covers. This helps them to see the
connection between the surface area of an object and the force of the air on that object.
Background
In this activity, you are producing high pressure in the air underneath the flat plastic objects, which
balances the force of gravity. Objects with a large surface area can carry items that would not “float”
on their own, because the weight is spread over a greater surface area. If an object covers more
airholes, there is more force to lift it than there is to lift an object that covers fewer airholes. So, we
see that the force of the air pressure on various objects on the table differs in size.
Extensions:
Balloon “rocket” car. Let’s try using the force of air in another way. If you blow up a balloon and
let it go, without tying the end, it will fly around the room. Rockets use fuel, but they work in a similar
way. Let’s try making a balloon “rocket” car.
Find a toy car. Use strong tape to attach a plastic drinking straw lengthwise on top of the car. Make
sure the straw is sticking out both at the front and the back of the car.
Take a balloon and blow it up several times to make sure it is very well stretched. Let the air out of the
balloon. Now, tape the end of the balloon around the end of the straw at the front of the car. Tape it
well, to make the connection airtight. Next, bend the straw upwards, so that when you blow up the
balloon, it won’t push the back wheels into the air.
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Now, blow up the balloon. Make sure you pinch the ends of the straw as soon as you stop blowing, so
that the air won’t escape. Put the toy car on the floor. Release the end of the straw. Watch the car go!
Balloon “rocket” boat. Try a similar experiment, but this time use a burger tray in a large shallow
container of water. In water, you can vary the angle of the straw to see which way works best. You can
also try the balloon in and out of the water.
Links:
To learn how a rocket works:
http://curious.astro.cornell.edu/question.php?number=681
To learn how a hovercraft works:
http://www.quicktechhobby.com/Hovercrafts/what_are_hovercrafts.htm
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Spring Force
Forces Concepts
Supports an understanding of how to measure forces between objects in newtons; that forces can
change in size; forces act in particular directions; objects that are stretched or compressed exert a
force on whatever is changing their shape; the change in shape of a spring is used in force meters for
measuring forces.
Activity
Children are asked to test and
measure the force they exert
on the two “arms”. Each arm is
controlled by a clearly visible
spring – one a compression
spring and the other a stretching
(extension) spring. Children will
measure the forces involved by reading a scale in newtons positioned below each “arm”.
They will be asked to consider why they have to use force to push or pull the “arm”. This will help
children to realise that the springs exert force back on them, as each spring tries to “spring back” into
its original shape.
Background
A spring is any mechanical device that is capable of storing energy. Extension springs get longer
when you put a load on them. They are designed to resist a stretching force. Compression springs get
shorter when you put a load on them. They are designed to resist compressing force.
When you remove a load, a spring returns to its original shape. A spring will always stretch or
compress to the same degree, when the same amount of force is applied to it. That makes springs
ideal for measuring force.
Extensions:
Make a spring scales: You can try making a very simple spring scales to learn more about how
springs can measure force. Find a medium sized cardboard box. Cut off the flaps and stand the box
on end, so that the bottom of the box stands vertically. Now use gaffer tape to secure the box to a
table top, so that it won’t fall over.
Find a medium-sized extension spring, the kind of spring that stretches. Use strong gaffer tape to fix the
spring to the inside of the roof of your box frame, so that the spring hangs down as straight as possible.
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Now, find a few weights with their size marked on them. There should be some available in the school.
Make sure the weights aren’t too heavy for your spring.
Hang a weight on the hook of the spring. See how far the spring stretches. Mark this point on the back
inside wall of your box frame. Do the same with the other weights.
Now, you have a very rough scales that you can use to measure other objects in your classroom.
Links:
For more information on springs:
http://www.technologystudent.com/cams/sprng1.htm
http://en.wikipedia.org/wiki/Springs
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Earthquake Table
Forces Concepts
Supports an understanding that forces change in size; forces act in a particular direction; forces
can make things speed up, slow down or change direction; that the weight of an object is the force of
the Earth on the object.
Activity
Children build a structure. They
then press the buttons on the
Earthquake Table to make the
Earth “quake” with different
amounts of force. Children
can experiment to see which
configurations of building blocks
are most stable.
Background
Earthquakes start deep below the surface of the Earth, when stressed rocks of the Earth’s crust suddenly
“give”. The ground above a point of movement jolts and moves like the deck of a ship in a rough sea.
The activity shows what happens to buildings when the earth shakes. To begin with, the upper part
of the building stays still as the foundations lurch sideways. This dislocates the materials, so that the
subsequent swaying may demolish the building.
Many earthquakes occur around the edges of the large tectonic plates which form the rocky crust of the
Earth. These move slowly, relative to one another, driven by the convection currents in the semi-molten
rock (magma) underneath. Others are associated with cracks in a plate (called faults), like that at the
Rift Valley in Eastern Africa and the San Andreas fault in California, USA.
Extensions:
For this activity, you will need to obtain some breadsticks. Lay a couple of breadsticks on the top of a
mug. Now, press down on the breadsticks at each end. What happens? The Earth’s crust breaks in the
same way, under pressure from below.
Links:
http://en.wikipedia.org/wiki/Earthquake
http://tsunami.geo.ed.ac.uk/local-bin/quakes/mapscript/home.pl
http://www.earthquakes.bgs.ac.uk/
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Magnet Game
Forces Concepts
Supports an understanding that forces change in size; forces act in particular directions; there are
forces of attraction and repulsion between magnets.
Activity
Children find that when they
flip a magnet over, the force of
magnetic repulsion flips a nearby
magnet, which in its turn flips
another magnet. Children may
already be aware of magnetic
attraction. Now, they become
more aware of magnetic repulsion
– the key force operating in
the magnet game. Children can
experiment at will, or they can use
the activity as a puzzle, attempting to turn all the magnets the same way up.
Background
Magnetism is a force that affects matter at the atomic level and is mainly due to spinning electrons in
the atoms. In a magnetic material, the electrons spin much more in one direction than any other and so
produce a net effect. Naturally occurring magnets are pieces of black iron ore, magnetite.
Magnetic Poles
Magnetic force is strongest at the two ends of a bar magnet. Although we speak of the poles of a
magnet individually, these poles cannot exist in isolation. They are an effect of the atomic organisation
of the entire magnet. In other words, if you cut a magnet in half, each half will still have two poles.
Extensions:
Make a floating compass. A compass shows us which way is north. An early form of the compass
was invented in China in the 11th century. To prepare for making your own compass, ask your teacher
to determine which way is north. Next, take a pan of water. Float a bar magnet on a piece of cork mat
and you will see it rotate until it is aligned on a north – south axis. The end that points north is known
as the north pole of the magnet, because it “seeks” the north. This is the principle behind the magnetic
compass used for navigation.
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Now, use a pencil to mark “N” on the correct end of the bar magnet. You can take your floating
compass into the corridor or outside. The north pole of the magnet will always point north.
Links:
Magnets are used very widely in industry. To find out how electromagnets work:
http://www.essortment.com/hobbies/electronicsques_sbbr.htm
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Air Power
Forces Concepts
Supports an understanding that force can change in size; forces act in particular directions; forces
can make things change direction; the weight of an object is the force of the Earth on the object
(gravity).
Activity
Children place the ball in the
airstream and observe its amazing,
gravity-defying behaviour. What
better way to start thinking about
gravity than to see it apparently
missing? Children turn the blower
and watch the ball move further
away, still hanging in the air. They
take the ball in and out of the
airstream – and still it defies gravity.
Background
When gravity doesn’t seem to be operating, some other force must be opposing or – in other words
– balancing it. In this activity, the ball stays in the air for the same reason that an airplane is able to
stay airborne. When the fast moving air meets the ball, it is just like the air rushing past the wing of an
aircraft in flight.
A wing is shaped and angled, so that the air moves faster over the top surface than under the bottom
surface. When air flows quickly, the pressure in the airstream gets lower. This is the Bernoulli effect.
Since the pressure of the fast moving air above the ball is low, the higher pressure of the atmosphere
beneath it pushes the ball up. The Bernoulli Force or “lift” is the force that holds the ball in the air, just
as it provides lift for an aeroplane.
The second force involved in this activity is the blow-away force of the air from the Blower. This force
is termed “drag”. The third force is, of course, the gravitational pull of the Earth, which acts straight
downwards. The balance of these three forces acting upon the ball keeps it hovering.
Extensions:
See how an aeroplane wing works. Take a piece of A4 paper. Tape the top edge nearly to the
bottom, but leave a couple of centimetres sticking out on the bottom. Don’t crease the paper.
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Now you have a model aeroplane wing. It should be quite flat at the bottom and very curved at the
top. Use sellotape to secure the bottom edge of the paper to a table, with the curved part facing
towards you. Now, blow hard straight at the middle of the bulge. The “wing” rises, just as an
aeroplane wing does.
Funnel Test. Test the Bernoulli effect in another way. Place a table tennis ball under a clear plastic
funnel. Extend the funnel with a bendy drinking straw, taped securely into the funnel, so that no air
escapes. Now, blow into the straw. Watch the ball rise.
Links:
For more on why aeroplanes stay in the air:
http://www.nasa.gov/audience/forkids/home/F_How_Do_Planes_Fly_Slideshow.html
For more on how an aeroplane wing works:
http://www.boeing.com/companyoffices/aboutus/wonder_of_flight/airfoil.html
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Arch Bridge
Forces Concepts
Supports an understanding that force can change in size; forces act in particular directions;
forces can make things change direction; the weight of an object is the force of the Earth on the
object (gravity).
Activity
Children fit the blocks together to
make a bridge. They press down
on it carefully to test its strength.
They try to discover how the
bridge “works”.
Background
An arch bridge is a bridge with
abutments – or supports – at
each end. It takes the form of a
curved arch. An arch bridge works by transferring the weight of the bridge and its load across to the
abutments on either side.
The blocks in the model are all wedge shaped. When you stand on the bridge, the wedges are
squeezed into the gap. This transfers the force down into the abutments at the two ends. The side
pieces are fixed, so that the bridge is supported.
Extension Points:
The steel used for reinforcement in modern concrete construction is positioned at the bottom of a beam,
because it is the bottom that tends to stretch and steel is strong in tension. Concrete, on the other hand,
is strong in compression and the top of the beam is compressed when the beam carries a load.
The steel rods used in a concrete beam are “pre-stressed”, that is, they are stretched while the concrete
is setting. Pre-stressing the steel means that the concrete at the bottom of the beam is compressed.
When the beam is under load, the concrete at the bottom only becomes less compressed, never
stretched.
You can experiment with this yourself:
1) Make a bridge by balancing a plastic ruler between two piles of books. Have two partners press
down on the ends of the ruler to hold it in place.
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2)
Press down on the middle of the ruler. How does the shape of the “bridge” change?
3)
Now stretch some adhesive tape tightly along the bottom of the ruler.
4)
Press down on the “bridge” again. Is the bridge stronger or weaker, now?
5)
Next, turn the ruler over, so that the adhesive tape is on top.
6)
Press down on the middle of the “bridge”. What happens to the tape that shows you that the top
of the bridge is in compression?
7)
Is the bridge stronger with the adhesive tape on top or on the bottom?
The adhesive tape in this experiment plays the part of steel in a bridge.
Links:
Useful websites:
http://en.wikipedia.org/wiki/Bridge
http://www.monolithic.com/thedome/pantheon/
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What I
Learned...
Forces Kit
Which activity was most interesting?
______________________________________
______________________________________
Which activity surprised you most?
______________________________________
______________________________________
What is one thing you learned?
______________________________________
______________________________________
______________________________________
______________________________________
Which activity would you most like to do again?
______________________________________
______________________________________
36
Optional Follow-Up Session
Lesson Plan 1: Measuring Forces
(Using the “Grip Test” & “Spring Force” activities
from the Forces Kit)
Have the children consult with “talk partners”, before asking various pairs to respond to questions.
Teacher’s prompts appear in this typeface: Sample.
Suggested “answers” appear in this typeface: Sample.
Requests for children to try something appear in this typeface: Sample.
Suggested approaches continue to appear in this typeface: Sample.
Introduction
Do you remember when we learned about measuring forces with the Newton Meter?
What are some of the ways we measured our own force? (For instance, we measured the force of a pull
from a hand.)
When you used the Forces Kit, do you remember any activities that were about measuring force? (the
“Grip Test” and the “Spring Force”)
In this lesson we’re going to use the “Grip Test” and the “Spring Force” activities to look more at
measuring force.
When we measure length, can you think of some units we use? (millimeters, centimeters, meters,
kilometers)
When we measure force, what unit do we use? (the newton)
What are some of the tools we use to measure length? (a ruler, a tape measure, a trundle wheel)
What do we measure force with? (a newton meter, also known as a force meter)
In the activities, we found out that many different things could apply force. What are some of them?
(for instance, your hand, air, water, magnets, gravity)
There are force meters that can measure the force of all those things – even force meters that can
measure the wind and force meters that can measure the strength of a magnet. Now, we’re going to
look at force meters from the activities.
Grip Test
Let’s start with the “Grip Test”. You all had a go of this in the activity session. What is it for? (for
measuring the force of a grip).
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What did we say every force can be described as? (a push or a pull)
Everyone take a pen or pencil and grip it. Does your grip apply a push or a pull?
(a push)
When you used the “Grip Test” activity, did you notice the “scale” that showed you how big your
force was in newtons? We’re going to have a look at that now.
In a few minutes, each of you is going to test your grip and write down the results, but first let’s look at
the other activity that measures force.
Spring Force
Springs are good for measuring a push or a pull. That is why some Newton Meters use springs. There
are two main kinds of springs: extension springs and compression springs.
What happens when you pull an extension spring? (It gets longer.)
What happens when you push a compression spring? (It gets shorter.)
When you pull an extension spring, the amount of stretch is always the same for the same amount of
pull. That means you can use it to measure the force of the pull.
When you push a compression spring, the amount of compression is always the same for the same
amount of push. That means you can use it to measure the force of the push.
Did you notice the “scale” that showed you how big your push and pull were, when you used the
“Spring Force” activity? We’ll have a look at that now.
Measuring the size of your force
At this point, you may want to hand out the activity sheets, which prompt children to ask certain
questions connected with the activities. “Are people with longer arms stronger?” “Does your hand get
tired, if you hold the grip bar for half a minute?” There are blank spaces for children’s own questions.
Ask children to come up with their own questions, while they take it in turn to do the “Grip Test” and
the “Spring Force” activities. Fill in the data from each child’s “Grip Test” and “Spring Force” work on
a class spread sheet. Look at the children’s suggestions for questions to investigate and help the class to
select two or three.
If the children need to collect more data to answer the chosen questions, take it in. In a subsequent
lesson, you could present the data to answer the children’s questions.
For more work
Let the children work in groups to produce concept maps, showing what they have learnt about
measuring forces. The concept maps should include as many as possible of the following:
• We learned about measuring force.
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• Force is measured in newtons.
• We can use a Newton Meter, the Hand Grip and the Spring Force activity to measure force.
• The two main kinds of spring are extension and compression springs.
• When you pull on an extension spring, it gets longer. When you push on a compression spring, it gets shorter.
• Springs are useful for measuring force, because they always stretch or compress the same amount for the
same size push or pull.
• Other types of force meter measure the force of the wind or measure the force of water.
Extensions
Those who are interested can look for other types of force meter on the internet.
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Let’s
Investigate...
Forces Kit
Are people with longer arms stronger?
What happens if you keep squeezing the grip tester?
Does your force get smaller, as your hand gets tired?
Do left handed people have stronger left hands?
Do right handed people have stronger right hands?
Add some questions of your own
in the blank spaces...
______________________________________
______________________________________
______________________________________
______________________________________
______________________________________
Optional Follow-Up Session
Lesson Plan 2: Forces can make
things change direction
(Using the “Octopus” from the Forces Kit and
True and False cards you can photocopy from
pages 56-57)
Have the children consult with talk partners, before asking various pairs to respond to questions.
Teacher’s prompts appear in this typeface: Sample.
Suggested “answers” appear in this typeface: Sample.
Requests for volunteers to come up or for children to try something appear in this typeface: Sample.
Suggested approaches continue to appear in this typeface: Sample.
The teachers’ key to the True and False cards appears in this typeface: Sample.
Introduction
What is a force? (a push or a pull)
Do you remember when we talked about how every force has a direction?
What are some of the things we did to find out about that? (For instance, we put our hands together and
pushed one hand against the other.)
What are some of the other things we did? (For instance, we gripped our hands together and pulled with
one hand.)
Now, we are going to look at how forces can make things change direction. We’re going to use
some activities from the Forces Kit.
The Octopus
I’m going to ask four volunteers at a time to come up and try various things. Get in groups and sort
the cards I’m going to pass out into True and False piles, as you find out whether they are true or
false during the lesson.
Let’s have four volunteers come up. Each of you take one arm of the Octopus and stand so that
the rest of the class can see you.
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I’d like the volunteers to use the palm of their hand as a flap on the end of the Octopus arm – opening
and closing the end – to find out what they can make the ball inside do. Keep doing it, so that
everyone can observe what is happening inside the tubes.
Now let’s have four new volunteers come up. Each of you take one arm of the Octopus and
stand so that the rest of the class can see you.
I’d like the volunteers to experiment and see if they can get the ball to hover part way down the tube.
Any ideas on what our volunteers should do to make the balls hover? (Children may suggest partly
covering the end of the tube or placing a hand near to the end of the tube, but not on it. Both will work.)
Now let’s have four new volunteers come up. Each of you take one arm of the Octopus and
stand so that the rest of the class can see you.
I’d like the volunteers to lift the octopus arms right up in the air and experiment with the airflow. Can
you make the ball hover now? Can you make it change direction? First send it up the tube. Now make
it go down the tube.
Now let’s have four new volunteers come up. Each of you take one arm of the Octopus and
stand so that the rest of the class can see you.
I’d like the volunteers to use the end of the tube to blow air at themselves. Now, I’d like them to blow
air at each other. So, does the force of the air always have to blow upwards?
Now, I want the volunteers to lower the end of their tubes below the table. What happens? Can you
make the balls hover now? Why?
Alright, volunteers, now lift the tubes back up. Can you make the balls hover now? You last volunteers
can sit down now.
Think about how this shows that forces can make things change direction. Make sure all the
groups get their True and False cards sorted into True and False piles. If anyone needs to
come up and check anything, please do.
You may want to have the groups stick the true and false sentences onto separate sheets to show
their work.
Teachers’ Key to the True and False Cards:
• There are two forces acting upon the balls in the arms of the Octopus: gravity and the
force of the blown air. (TRUE)
• When you change the size of one of these forces, you can make the balls move up or
down the arms of the Octopus. (TRUE)
• When the end of the tube is open, there are no forces on the ball. (FALSE)
• Usually, a ball falls, because gravity pulls it downwards. (TRUE)
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• The ball is at the top of the tube, when the tube is open, because something is
sucking it upwards. (FALSE)
• There are two forces acting on the ball: the force of the airflow is pushing it
upwards and gravity is pulling it downwards. (TRUE)
• Changing the amount of air getting out of the tube changes the size of the force of
the air flow pushing on the ball. (TRUE)
• The ball falls when the tube is closed, because the air in the tube can’t escape.
There is no room for more air to blow in to lift the ball. (TRUE)
• Changing how much air gets out of the tube doesn’t affect the force of the airflow.
(FALSE)
• When the ball hovers, there is more force from the airflow pushing the ball
upwards than there is from gravity pulling it downwards. (FALSE: the two Forces are
balanced)
• The ball hovers when the force from the air pushing upwards is the same size as
the force from gravity pulling it downwards. (TRUE)
• The balls hover when the force of the air flow stops pushing upwards. (FALSE)
• Gravity sometimes stops pulling downwards. (FALSE)
• The force of air flow always pushes upwards. (FALSE)
• The force of air flow can push in any direction. (TRUE)
• Forces can make objects change direction. (TRUE)
Further work
Let children pair up and interview each other “for local radio or for television” on how the Octopus
works. Afterwards, ask some children to come up and act out their interview for the rest of the class.
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16 True/False Cards
True or False?
True or False?
There are two forces acting
upon the balls in the arms of the
Octopus: gravity and the force
of the blown air.
When you change the size of
one of these forces, you can
make the balls move up or down
the arms of the Octopus.
True or False?
When the end of the tube is
open, there are no forces on
the ball.
True or False?
The ball is at the top of the tube,
when the tube is open, because
something is sucking it upwards.
True or False?
Changing the amount of air
getting out of the tube changes
the size of the force of the air
flow pushing on the ball.
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True or False?
Usually, a ball falls because
gravity pulls it downwards.
True or False?
There are two forces acting on
the ball: the force of the airflow
is pushing it upwards and
gravity is pulling it downwards.
True or False?
The ball falls when the tube is
closed, because the air in the
tube can’t escape. There is no
room for more air to blow in to
lift the ball.
for Follow-Up Lesson 2
True or False?
Changing how much air gets
out of the tube doesn’t affect the
force of the airflow.
True or False?
The ball hovers when the force
from the air pushing upwards is
the same size as the force from
gravity pulling it downwards.
True or False?
When the ball hovers, there is
more force from the airflow
pushing the ball upwards than
there is from gravity pulling it
downwards.
True or False?
The balls hover when the force
of the air flow stops pushing
upwards.
True or False?
True or False?
Gravity sometimes stops pulling
downwards.
The force of air flow always
pushes upwards.
True or False?
True or False?
The force of air flow can push
in any direction.
Forces can make objects
change direction.
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Optional Follow-Up Session
Lesson Plan 3: Magnetism
You will need a few small magnets, a small bar magnet, some paper clips, iron
filings and some cellotape.
Have the children consult with talk partners, before asking various pairs to respond to questions.
Teacher’s prompts appear in this typeface: Sample.
Suggested “answers” appear in this typeface: Sample.
Requests for volunteers to come up and try something or for children to try something with their talk
partner or individually appear in this typeface: Sample.
Suggested approaches continue to appear in this typeface: Sample.
Introduction
Have the children form groups. Distribute two magnets and a few paper clips to each group. Each
child should have a chance to handle them.
What are the little objects I’ve given you? (magnets)
How do you know? (The paper clips stick to them.)
What do you know about magnets? (For instance, some metals are attracted by magnets.)
Can anyone tell me what a force is? (a push or a pull)
We’ve been working with forces recently. What are some of the ways we’ve found to produce a
force? (for instance, pushing or pulling with our hands, kicking a ball, the force of air blowing a ball)
Magnetism is a force. A magnet can push or pull, just as your hand pushes or pulls. Let’s try them.
Place one of the magnets on the table. See how close you can bring the other
magnet, before the one on the table jumps to it. Everyone have a turn.
So, we’ve seen magnetic force pulling. That’s magnetism acting as a force of attraction.
Magnets can also push. That’s magnetism acting as a force of repulsion. Let’s try putting one magnet
on the table, again. One end of each magnet will push an end of the other magnet. Experiment
to find out which one it is. Once you’ve found out, try chasing one magnet with the
other, by magnetic force. Everyone take a turn.
Let’s have two volunteers. I’m going to hold up this piece of paper. I want one of my
volunteers to hold a magnet to one side of the paper and the other volunteer to hold another
magnet to the other side.
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Now take turns moving your magnet in all directions and see if the other magnet follows on the other
side of the paper. So, the magnets can attract each other even through the piece of paper. Everyone
have a try of this in their groups.
If you had a strong enough magnet, you could try the same thing with a magnet on your palm and
another one underneath your hand. This works, because every magnet has a magnetic field. The
magnetic field has magnetic force. You can’t see a magnetic field, but it’s possible to see exactly
where it is.
We can use tiny pieces of iron, called iron filings to show the force of a magnetic field. This is
because iron is attracted by a magnet.
Let’s have a couple of volunteers to come up and try a magnet on the iron filings. The
pattern that the iron filings are creating is the “outline” of the magnetic field.
Can everyone see the lines of force that the magnetic field is producing? They seem to
come out of the two ends, don’t they? The two ends are called the poles of the magnet. That’s because
a magnet will always line up to point to the North Pole, if it can move freely. We’re going to try this.
Place the bar magnet on a cork mat in a pan of water – on the floor if possible, for maximum visibility.
This way is north, so we know that this must be the north pole of the magnet. We’ll stick a green dot
on that end of the magnet and we’ll stand this big “north” signpost here. Let’s have a couple of
volunteers up to give the mat a gentle spin. Does it work?
Let’s have another couple of volunteers up for a second try.
It’s always a good idea to repeat a scientific experiment several times to be sure your results aren’t the
result of chance. So, let’s have a third set of volunteers up to see if the “north pole” of
the magnet will always point north.
What’s the answer? (yes)
This is a compass. It works in exactly the same way as our little experiment. The needle of the compass
is a magnet. The north pole of the magnet always points north. I’ll pass it around for everyone to try.
Look to see if the needle is pointing towards our “north” sign, (but be aware that a large
metal object can sometimes interfere with a compass). Why do you think this is? (Magnets attract
certain metals.)
The north pole of a magnet points north, because the Earth has an iron core and it behaves like a giant
magnet.
The poles of a magnet can’t be taken apart. If you break a magnet in two, you will have two smaller
magnets, each with a north and south pole. A magnet is a magnet all the way through.
Let’s try that. Let’s have two volunteers up to cut this magnet in half.
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Now, let’s have another two volunteers up to see if the two pieces still work as magnets. Do we
still have a north pole and a south pole on each magnet? (yes)
Let’s check that with another two volunteers. Does it still work? (yes)
Let’s have two more volunteers up to try cutting both small magnets in half again.
Now, let’s have another pair of volunteers to check if they still work as magnets.
Let’s have one more pair of volunteers to check again.
We can see that a magnet is really and truly a magnet all the way through. In fact, one spinning
electron on its own is a tiny magnet. Iron makes a good magnet, because if you place it in a magnetic
field, the electrons in its atoms spin mostly in one direction, instead of in random directions.
Everything – even water – is affected by a magnetic field to some extent, but some materials are much
more affected by magnetic force than others.
What is very affected by magnetism? (for instance, iron, steel) Nickel and cobalt are two other metals
that are noticeably affected by magnetism.
Making a magnet
I’m going to give each group some paper clips and some cellotape. You still have two magnets in each
group. Try the paper clips on each other. Are they magnets? (no)
I want you to unwind a paperclip and wind each end with cellotape to make it safe.
Then, rub the magnet along the unwound paper clip - in one direction only, lifting the magnet off at the
end each time. Do that about twenty times. After that, test it on other paper clips to see if it has become
a magnet.
Now, try rubbing it some more and see if you can make the magnet stronger. Try it
with other paper clips, as well.
Paperclips are made of steel. Magnets have a strong effect on steel. By rubbing the magnet on the
steel, you are magnetising it. The magnetism won’t last, but there are treatments you could give the
steel that would make it a permanent magnet.
In your groups, make a poster to show what you have learned about magnets and
magnetism.
For more work
You can also have children make an electromagnet very simply.
http://www.energyquest.ca.gov/projects/electromagnet.html
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Optional Follow-Up Session
Lesson Plan 4: Forces on Structures
You will need a simple spring-operated newton meter, a plastic ruler and some
cellotape, as well as access to the See Stress activity.
Have the children consult with talk partners, before asking various pairs to respond to questions.
Teacher’s prompts appear in this typeface: Sample.
Suggested “answers” appear in this typeface: Sample.
Requests for volunteers to come up and try something or for children to try something with their talk
partner or individually appear in this typeface: Sample.
Suggested approaches continue to appear in this typeface: Sample.
Introduction
We’re going to look at how force can strain an object, so that we can see the effects of stress on
that object.
Let’s have three volunteers up. I’m going to stretch this plastic ruler between two piles of books. I
want one volunteer to hold down each end of my “bridge”. Volunteer number three, please press down
gently on the centre of the “bridge”. What is happening to our “bridge”? (It’s bending in the middle.)
Let’s have another three volunteers up. I need two to hold the bridge. The third volunteer is
going to measure a force applied to the bridge in newtons. I’m going to tie this elastic band around
the bridge and attach the Newton Meter to it. Instead of pushing on the bridge, you’ll pull on it from
below and measure the force of your pull. What is the reading in newtons?
Now, let’s have another three volunteers up. Would you expect the reading on the Newton
Meter to be exactly the same this time? (no) Why not? (Some people pull harder than others.)
Pull down with the Newton Meter and tell us your results. What is the reading in newtons? Is it bigger
or smaller than the first result? What are we measuring? (the force of your pull)
So forces can be of different sizes, even in a similar situation.
Sometimes, when you apply force to an object, it’s easy to see the effect. We can see the plastic ruler
bend when we push or pull it. A real bridge has to be much stronger, but you can still see the effects of
a force applied to it, if you have the right tools to look. If we look through a polarising filter, we can
see the effects of force on these plastic bridges.
Predicting the effects of force on an object
I have six bridges here. I’m going to pass out an activity sheet with a drawing of each of the
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bridges. Working in groups, I’d like you to try to predict which bridges will be
stronger and which will be weaker. Number the bridges from one to six, with six being the
bridge you think will be the strongest. Discuss among yourselves, why you think the various bridges
will be stronger or weaker.
Observation and recording
While everyone works on the activity sheet, I’ll call each group up in turn to try the “See Stress”
activity. As well as using the goggles, we’ll look together through a sheet of polarising material. Bring
your coloured pencils and your activity sheets. While you’re up here, use your coloured pencils to
draw the colours you can see onto the bridge drawings. Wherever there are colours, you are “seeing
stress” on the bridge.
As well as pressing down on the bridges, we’ll be using a Newton Meter to pull downwards on them.
Drawing conclusions from observations
Have the children also draw arrows to show what direction the forces are acting in. This reinforces an
understanding that forces act in particular directions. They can also circle the area where the greatest
amount of pressure is being applied.
Another type of bridge
Now, let’s look at the arch bridge. I need a couple of volunteers to come up and put it
together.
Now, I need another couple of volunteers to come up and take turns carefully pressing down
on it. This is a very old type of bridge. Can you see how it works?
The blocks are wedge shaped, so the force is transferred down into the two sides of the bridge. I
want each group to draw the bridge, while I call one group at a time up to examine the
bridge and to try pressing down on it.
Diagram on your drawings how the force from testing the strength of the bridge is being
transferred down into the two sides.
Further work
Have children look at other clear plastic objects through the goggles. Both cellotape and a plastic ruler
will work well. (Not all types of plastic “work” with polarised light.)
Sellotape has been stretched in one direction during manufacture, so it is permanently “stressed”. The
colour you can see through the polarising filter is the result of that pre-stressing. The plastic ruler will
show signs of stress from its manufacture, as well.
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Have the children layer the cellotape, so that in some places it is one layer thick, but in other places, it is
two or three layers thick. Have the children draw diagrams, with coloured pencils or markers, showing
the stress patterns in the sellotape.
Have them show in which direction they think the stress was applied.
Have the children bend the plastic ruler gently while looking through the goggles or through the
polarising sheet. Have them draw diagrams, with coloured pencils or markers, showing the stress in the
plastic ruler.
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Draw the stress colours onto the bridge drawings.
Which bridges do you think are strongest? Why?
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