PSBS
PHYSICS
2012/2013
Forces and Motion
Exceed the speed limits.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 1
PSBS
PHYSICS
2012/2013
Forces and Motion
In this unit we will study the idea of speed and the way forces affect how things move. We
will also look at how streamlining is used to reduce the effect of water and air resistance.
We will use the idea of balanced and unbalanced forces to explain how falling objects move.
Describing how fast something moves
When we want to know how fast something moves we measure its speed. A unit tells you
what something is measured in. The speeds of the car and the lorry have been measured in
the unit 'Meters per second'. We use the symbol m/s. For example, the speed of the car is
15 m/s.
The car and the lorry go past the lamp-post
at the same time.
the next picture shows where they are one
second later
One second later.
The car has travelled 15 meters. So, it
has a speed of 15 meters per second.
In the same time the lorry has only
travelled 10 meters per second.
The car has a higher speed.
Questions
1) Is the car or the lorry moving at a lower speed? Give a reason for your answer.
2) What is the unit used to measure the speed of the car and the lorry?
3) What is the difference between the speed of the lorry and the speed of the car? Explain
how you worked out your answer.
Different units
Meters per second can be a very useful unit for speed, but sometimes it is
more useful to use a different unit. Road signs in Britain use the unit 'miles
per hour'. Similar signs in France use the unit 'kilometers per hour'. The
shorthand symbol for 'miles per hour'
is 'mph'. The Symbol for the unit 'kilometers per hour' is 'km/h'.
The speed of a tortoise walking across the floor is probably about 1
centimeter per second. We would write this as 1 cm/s.
Questions
1) What is the symbol for the unit 'kilometers per hour'?
2) Both photographs show speed limits of 40. The top photo is from
Thailand and the bottom photo is from Britain.
3) What is the important difference between the two?
4) Which speed is higher, 2 mm/s or 2 cm/s? Give a reason for your
answer.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 2
PSBS
PHYSICS
2012/2013
Working out the speed
Sometimes you can get a direct measurement of speed
from a measuring instrument that has been designed
especially for the job. Police use radar guns and speed
cameras to measure the speed of cars. People who
study the weather use an instrument called an
anemometer, which measures the wind speed.
1) Name two devices that give a direct measurement
of speed.
2) What does an anemometer measure?
Timing things
If you want to compare the speeds of things that travel the same distance, you only need to
measure the time they take. This happens in races since everyone runs the same distance.
Why can time alone be used to compare the
speeds in an athletics race?
The timing method in the picture is the one
used about 50 years ago. Today, the starting
signal sounds from a small speaker behind each
athlete. A clock is started electronically at the
same time as the signal sounds. When the
athletes cross the finish line, a camera linked to
the clock registers the times. If two athletes
pass the finish together, then the photograph
from the camera is used to work out the
winner.
It is sometimes called a 'photo finish'.
Questions
1) What is different about the starting signal for an athletics race
today and one about 50 years ago?
2) The person with the stopwatch used to start it when they saw the
smoke from the gun, and not when they heard the bang. Why do
you think this is more accurate than starting the watch when you
hear the gun go off?
3) Suggest two reasons why electronic timing is better than using a
stopwatch.
Calculating the speed
You calculate the speed of something from the distance it travels and
the time it takes to travel that distance. The swift has travelled 240
m in 6 s. assuming the swift travels at a constant speed; you can
work out its speed like this:
Speed = distance travelled time taken
Questions
1) What formula do you use to calculate speed?
2) Which two measurements are needed so you can calculate speed?
3) If the pupil calculated the correct speed for the giraffe, what answer did she get?
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 3
PSBS
PHYSICS
2012/2013
Speed
Speed is the distance travelled per unit time.
S.I. unit of speed is m/s
Speed (v) = distance (d) / time (t)
Calculating distance and time from the speed formula
You can change the formula round to get other versions of it.
Time taken = distance travelled /speed
Distance travelled = speed x time taken
Average speed
Speed of a car will never be a constant speed all over the trip, so we can calculate
the average speed
Average speed = total distance travelled / total time taken.
Example
A cyclist covers 20 m in 2 seconds then it covers another 20 m in 4 second.
Solution:Average speed
= total distance / total time.
= 40 m / 6 s = 6.66 m/s
Question:Calculate the average speed in meters per second of a runner who runs 1500 m in 5
minutes?
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 4
PHYSICS
PSBS
2012/2013
MOTION GRAPHS
(Plotting graphs)
1. Distance-time graphs
Slope = Y-axis/X-axis = gradient = distance (m)/time (s) = speed (m/s)
Distance
0
10
20
30
40
50
60
Time
9:45
10:00
10:15
10:30
10:45
11:00
11:15
Slope = velocity = distance/ time
Slope = velocity = 0
Body at uniform velocity
Body at Rest
Question
The figure on the right shows the distance/time graph for two
trucks, A and B, on an expedition across the Mongolian desert.
a) i) How far did truck A move in the first hour of its journey?
ii) What was its speed during this time?
b) How did the speed of truck A change in the second hour of its
journey?
c) Was truck B moving faster or slower than truck A in the first
hour of its journey?
d) What do you think might have happened to truck A in the third
hour of the journey?
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 5
PSBS
PHYSICS
2012/2013
Showing a bike ride on a graph
If you plot a graph of the distance travelled against the time you take you get something
called a “distance/time” graph.
It gives you a picture of what is happening on the journey.
A horizontal part of the line means you have stopped.
A steep part of the line represents a high speed.
Questions
1) On which part of the journey is the speed of the cyclist the highest?
2) On which part of the journey is the speed of the cyclist zero?
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 6
PHYSICS
PSBS
2012/2013
2- Speed-time graphs
Slope = gradient = Y-axis/X-axis= velocity (m/s)/time(s) = [a] acceleration (m/s2)
Slope = a
= velocity/ time
(+ve a)
Slope = a
= velocity/ time
(-ve a)
Slope = a = 0
(no increase in
velocity)
Slope = a = 0
(no increase in
velocity)
Body at
uniform
acceleration
Body at
uniform deceleration
Body at
uniform velocity
Body at
Rest
Questions
1) The table gives some data for a Ferrari racing car at the start of a Grand Prix
race;
Plot a speed-time
graph for the car
2) Describe in as much details
as possible, the motion of a
car which has this graph:
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 7
PSBS
PHYSICS
2012/2013
Forces
Pushing and pulling, stretching and turning these are some of the things a force can do.
• You use a force to push a broken-down car. “Motion forces”
• You use a force to pull a drawer open. “Motion forces”
• You use a force to stretch a rubber band. ”Stretching Force”
• You use a force to turn a door handle. “Moment”
Push, pull, stretch and turn - these are some of the ways in
which a force can act. (We say that a force 'acts' on an object.)
Forces cannot be seen
Our bodies allow us to feel forces. There are nerve endings in our skin which can detect
pressure.
• Press gently with your finger on the tip of your nose; you will feel the force of your
finger pushing on your nose.
• Sit on a chair; you can feel the upward push of the chair.
• Put your hand on the chair and sit on it. Your hand is squashed by two forces: the
force of your body pushing downwards and the force of the chair pushing upwards.
We can't see these forces but we can feel their effects.
In the drawings, the forces will be represented by arrows.
A force arrow is a good way to represent a force because it
shows the direction in which the force is acting.
Labeling force arrows
A force arrow shows us the direction of a force. We label the
arrow to show two things: the object that the force is acting
on and the object that is producing the force. The picture
shows an example. The woman is pushing the shopping
trolley. The force arrow is labeled to show which object is
doing the pushing, and which object is being pushed.
This helps us to understand where forces come from. Forces
appear when two objects interact with each other.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 8
PSBS
PHYSICS
2012/2013
Labeling forces
A magnet can attract an iron nail. The magnet and the nail interact. The magnet is pulling.
The nail is being pulled.
The picture shows the force of the magnet on the nail.
Questions
While they are playing together, Sam picks up his little brother Joe.
Think about the force that acts on Joe.
a) In which direction does this force act?
b) What are the two objects that are interacting?
c) Draw a diagram to show the force that acts on Joe. Take care
to label the force arrow correctly.
d) Draw a line below Sam represent the ground below Sam, and
the forces acting at his foot, his weight “W” and Push of the
ground “UP”
Comparing Forces
“big and small”
Forces can make things move. You have to push a
shopping trolley to start it moving around the shop.
You have to pull on a handle to open a drawer.
Question
The pictures show some forces making things move.
Which of these things needs the biggest force?
Look at the pictures. Put the forces in order, from
smallest to biggest. (Write 1, 2 and 3)
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 9
PSBS
PHYSICS
2012/2013
Forces act in pairs
When things touch there are two forces acting in opposite
directions, one force on each object. When the legs of a stool
touch the floor there are push forces on the legs and push
forces on the floor.
When you carry a bag, the handles and your hand touch each other.
Two forces are produced where your hand and the bag touch. There is
a pull force downwards from the bag on your hand and a pull force
upwards from your hand on the bag. You can show these forces on a
diagram with arrows. The length of the arrow shows how big the force
is.
Look at the pictures of the person and the book.
Questions
1) Does the up or down arrow show the force of the person on the
floor?
2) What force is pushing on the bench?
3) What is its direction?
4) What can you say about the size of the forces that happen where the book touches the
bench?
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 10
PSBS
PHYSICS
2012/2013
Measuring forces
In science, if we want to know if one force is bigger
than another, we don't simply guess. We make
measurements.
How can we measure forces?
We use an instrument called a forcemeter to measure
a force. (Another name for this is a newtonmeter.)
The picture shows one type of forcemeter.
This is how you use it to measure the force needed to
pull a block of wood along the bench.
Notes:• Check that the forcemeter reads zero before
you start.
• Attach the hook of the forcemeter to the block.
• Hold the ring at the other end of the forcemeter and pull the block.
• Read the value of the force from the scale.
How a forcemeter works
There is a spring inside a forcemeter.
The pulling force stretches the spring and this moves the
indicator along the scale.
The bigger the force, the further the indicator moves.
The unit of force
We measure forces in newtons (N).
This unit is named after Isaac Newton, an English scientist who
explained how forces affect the way things move. To make it
easy, we can write N instead of 'newton'.
How much is a newton?
If you hold an apple on the palm of your hand, it presses down
with a force of about 1 N. If you hold 5 apples, that's about
5N. Palm of your hand, it presses down with a force of about I
N. If you hold 5 apples, that's about 5N.
Questions
Look at the picture of the laboratory stool hanging from the
forcemeter.
1) What is the biggest force this forcemeter can measure?
2) How big is the force lifting the stool?
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 11
PHYSICS
PSBS
2012/2013
Measuring pushing forces
If you stand on weighing scales, you press down on the
scales and the reading on the dial increases. You can use
scales like this to measure pushing forces.
You need a set of scales which measures in newtons. If it
gives readings in kilograms, you need to know that 1kg
means 1O N, 2 kg means 20 N, and so on.
The pictures show three ways of using weighing scales to
measure forces.
• You can stand on the scales. This measures the
downward force of your weight.
• You can use your hands to press the scales against the
wall; this measures the pushing force of your arms.
 You can use your feet instead, this measures the pushing
force of your legs.
Weight “the pull of gravity”
We live on the Earth. It is difficult to get away from the Earth. If you jump upwards, you fall
back down again. The Earth's gravity pulls you downwards.
The Earth's gravity causes a force that pulls any object
downwards.
This force is called weight. Like any other force, weight is
measured in newtons (N).
Gravity always pulls you towards the center of the Earth. It
doesn't matter where you are on the surface of the Earth.
When we draw a force arrow to represent an object's weight,
the arrow points towards the center of the Earth.
Questions
1) Draw a diagram to show yourself, standing on the
ground. Add a force arrow to show your weight.
2) Draw a diagram to show the Earth. Mark the center of
the Earth. Show yourself, standing on the Earth. Add a
force Arrow to show your weight.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 12
PHYSICS
PSBS
2012/2013
Falling through the floor
The Earth's gravity is pulling on us all the time. It pulls us
downwards, but we don't fall through the floor. Why not?
The floor pushes upwards on us with a force. This force is
called the contact force.
Any object that you push on pushes back with a contact
force. Usually the force is big enough to balance the pull of
gravity. But if you stand on something that isn't very
strong, its upward push may not be enough to support
you.
Question
3) Go back to the diagram you drew for Question 1,
Add a contact force arrow, to show the force of
the ground acting on you.
Mass and weight
Again; already you studied mass and weight in past
chapter; so when you weigh yourself at home, the scales
show the value in kg.
You might say, 'I weigh 50 kg.' However, in science, we would say that your: mass is 50 kg.
The mass of an object is measured in kilograms (kg).It tells you the amount of matter the
object is made of
The Earth's gravity pulls on each kg with a force of about 1O N. So, if your mass is 50 kg,
your weight on Earth is about 500 N.
Weight-the pull of gravity
Imagine going to the Moon. The Moon's gravity is
weaker than the Earth's. You weigh a lot less up
there. You can jump much higher on the Moon - but
you still fall back downwards.
If you go far out into space, far from the Earth,
Moon or any other object, your weight is zero.
Your mass stays the same, however - you are still
made of 50 kg of matter.
Questions
1) Look at this table.
Quantity
Description
a force caused by gravity
an amount of matter
Units
In the first column, write the words 'mass' and 'weight' in the correct spaces.
Add the correct units in the last column.
2) A set of weighing scales gives values in kilograms. Are the scales measuring mass or
weight?
3) When astronauts went to the Moon, they found it much easier to lift heavy objects than
on Earth. Explain why.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 13
PHYSICS
PSBS
2012/2013
Friction Forces
Press your hands together. Rub them together. You should be able
to feel the force of friction that each hand exerts on the other.
Rub your hands together hard and they will start to get warm. You
have observed the heating effect of friction. That's useful on a cold
day.
Friction is a force than can appear when two objects are in contact
with each other. ('In contact' means 'touching'.)
The picture shows a heavy box lying on the floor. Imagine that you
try to push it. If you try to push it to the right, the force of friction
pushes back in the opposite direction - to the left.
Eventually, if you push hard enough, the box will move. Your pushing
force is greater than the force of friction.
Questions
1) When will the box move?
2) If you try to push the box to the left, in which direction will
friction act? Draw a diagram to show the two forces.
The direction of friction
We say that friction acts to oppose motion. To draw a force arrow
to represent friction, you must ask yourself:
Which way is an object moving or trying to move?
For example, the heavy weight in the picture is trying to slide
down the slope. This tells us that friction acts up the slope
Question
3) Omar is sliding along the school corridor.
Here is a picture of Omar sliding along the floor.
Draw a force arrow to show the force of friction
acting on Omar.
4) Which of these names represents the friction
with air “air resistance” for this airplane?
Air resistance is “……………..”
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 14
PHYSICS
PSBS
2012/2013
Investigating friction
1) Where does the friction force occur?
2) What is the direction of the friction force
compared to the pushing force?
3) What happens to the box when the pushing
force is bigger than the friction force?
Investigating friction again
You can u e a forcemeter to measure the force of
friction. The diagram show how. Place a wooden
block on the bench and pull it with a
Forcemeter. When the block just starts to move,
the forcemeter will show you the value of the force.
You can investigate the different factors that affect
the size of the force of friction. Here's how.
a) Add weight on top of the block to make it
heavier.
b) Turn the block so that a different face is in
contact with the bench, this changes the
area of contact.
c) Use a material such as paper or cling film to
cover the surface, making it rougher or
smoother.
Factors affecting friction
1) You are going to investigate how friction depends on two of the factor mentioned
above. Start by changing the weight of the block.
2) First, make a prediction. If you increase the weight of the block, will friction increase,
decrease or stay the same? Give a reason for your prediction. Carry out an
experiment to test your prediction.
3) Now investigate how friction depends on the area of contact between the block and
the bench.
Question
Press your hands very gently together and rub them. Now press much Harder and rub
again. Describe what you observe. What does this tell you about the force of friction?


Friction is a force that acts when two surfaces in contact with each other.
Friction acts to oppose motion.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 15
PSBS
PHYSICS
2012/2013
Air resistance
If you drop something, it falls to the ground. Its weight the pull of the Earth's gravity – makes it fall.
The photograph shows some parachutists falling.
Eventually, they will reach the Earth's surface.
The parachutists will not be travelling very fast when they
hit the ground. This is because they are falling through the
air. This means that there is force acting on them.
This extra force is the force of air resistance.
This slows them down to a safe speed.
Balanced forces
As the parachutist falls, air pushes upwards on the inside
of the parachute. We can represent this force using a force arrow,
pointing upwards, There are two forces acting on the parachutist.
They are equal in size but point in opposite directions, so they
cancel each other out. The parachutist falls at a safe speed.
When forces cancel each other out like this, we say that the forces
are balanced.
Moving through air
It is easy to wave your hand through the air.
Air is a very 'thin' substance, so we can move easily through it.
That's why a parachute must have such a big area - a small
parachute would be useless.
Question
1) Name the two forces that act on a parachutist who is falling
towards the ground. Give the direction of each force.
2) Explain why a parachute would be useless if you went to the Moon.
The air resistance produced by a parachute is also used
to bring sky divers safely to the ground. The resistance
of the gases in the atmospheres of other planets in the
Solar
System is used to slow down space probes so they can
land safely and the devices on board are able to carry
out their investigations.
Question
How would the size of parachute required on a space
probe to allow it to land safely differ on:
a) A planet such as Venus which has a thick
atmosphere
b) A planet such as Mars which has a thin
atmosphere?
Explain your answers.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 16
PSBS
PHYSICS
2012/2013
Water resistance
When an object moves through water it pushes the water out
of the way, and the water moves over the object's sides and
pushes back on the object. This push on the object is called
water resistance or drag. Objects that can move through
the water quickly have a streamlined shape.
A fish such as a barracuda, which moves quickly through the
water, has a much more streamlined shape than a slowmoving sunfish.
Flying fish and squirrel have shapes help them to fly
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 17
PSBS
PHYSICS
2012/2013
Reducing friction
Friction is a problem when surfaces need to move over
each other. Anything with moving parts has a problem
with friction.
Friction will wear away a surface where that surface
meets another one. Friction also uses up energy. You can
show this by rubbing your hands together. The friction
soon warms them up! Movement energy is converted to
heat energy through friction.
There are three main ways to reduce friction:
• You can make the surfaces smooth;
• You can put a lubricant like oil on them;
• You can design the moving parts so they roll on each
other rather than slide.
You can judge how much friction there is between an object and a surface by tilting the
surface. If the friction force is big, the object will not slide until the surface is quite steep.
With a tin of beans standing on its flat end, the slope can get to 400 before the tin slides
down. If you put the tin on its side, it rolls as soon as there is any slope at all.
It takes a steep slope for the tin to slide.
The tin rolls much more easily.
Rollers can be used to reduce friction when you
are trying to move large blocks of stone. The
same idea is used on a small scale in the
wheels of a bike.
The wheels of a bike turn on sets of ball
bearings: These are designed to reduce friction
because they roll rather than slide. You can
also reduce the friction by using oil on the
moving parts. We say that the oil lubricates
the moving parts.
1) What does friction do to surfaces?
2) Name a substance that can be used as a
lubricant.
3) What three things can you do to reduce the
friction between surfaces?
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 18
PSBS
PHYSICS
2012/2013
Friction can be useful
If there were no friction you could not walk and your
shoelaces would be impossible to tie. When you walk you
push your foot backwards. The effect of the friction forces
between your foot and the floor is to move you forward.
This happens because the floor does not move. If you try the
same thing off a skateboard, which can move, then you won't
get very far because the skateboard moves back instead of
you going forward. This happens because the wheels of the
skateboard act like rollers and reduce the friction to a very
low level.
Questions
1) What is the effect of the friction force between your foot
and the floor when you walk?
2) Why is it difficult to step forward off a skateboard?
3) Why is it difficult to walk on ice?
Another friction force
The friction force when something slides through air or a
liquid is called drag. It slows things down and makes
speeding up harder.
People who design racing cars spend a lot of time working
out what shape the car has to be to make the drag as low
as possible.
The same idea is also used to design the shape of cars and
vans so they do not use as much petrol. A car with a shape
that moves through the air easily does .not use as much
petrol as a car of the same size going at the same speed
with a shape that gives more drag. We say that the shape
that goes through the air with less drag is more
streamlined.
Drag can also be very useful. If you want to slow down the
fall of an object you can increase the drag with a
parachute. Some plants use the drag of the air to allow
their seeds to be dispersed by the wind. The seed can travel
a long way on the wind before it hits the ground. This means
the new plant growing from the seed will not be competing
with the original plant for food and space.
Questions
1) What is drag?
2) How can you reduce the drag on something moving
through the air?
3) Describe some situations where drag is useful.
Hany El‐Gezawy
“Forces and Motion”
Chapter 2
Page 19