Grade 8 Science
Forces and Their Effects and Motion
http://commons.wikimedia.org/wiki/File:Jal.747.newcolours.arp.750pix.jpg
Brainstorm about Forces
Brainstorm everything you know about forces,
in groups of 2 or 3. It should cover:
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Types of forces
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Effects of forces
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Theories or ideas related to forces?
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Famous people who developed or promoted
such ideas?
Can you bring in numerical relationships?
The Solar System
What keeps the planets moving?
http://commons.wikimedia.org/wiki/File:Solar_sys.jpg
Forces
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A force is a push or a pull.
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Forces are measured in Newtons, after Isaac Newton.
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Find something which:
a) has a force of 1 Newton.
b) a force of 5 Newtons which
isn't a weight force.
c) a force of 10 Newtons which
isn't a weight force or a friction
force.
What force is required to:
1. open a drawer.
2. untie a shoe lace.
3. turn on a tap (difficult)
4. drag a chair along the ground.
http://commons.wikimedia.org/wiki/File:Isaacnewton_1.jpg
Forces: The Basics
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A force is a push or a pull.
A force has a size (in N________) and a
direction.
A force can be drawn as an arrow. The length
of the arrow represents its size and the
direction of the arrow represents the direction of
the force.
A force can cause an object to change its
speed (a_________), change its direction or
change its shape.
Gravity
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Gravity is a very weak but important force.
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Gravity pulls everything together.
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We notice it as pulling us down towards the
Earth.
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Gravity also pulls the Earth towards the sun.
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What causes gravity?
Mass and Weight
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Mass measures
“quantity of matter”. It is
measured in kilograms
(kg).
Item
Mass
(kg)
Weight
(N)
Weight measures the
force of gravity. It is
measured in Newtons
(N).
Short task:
Measure the mass and
weight force of four
items in the laboratory.
Find something with a weight force of 1 Newton.
Mass and Weight Relationship
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The relationship between mass and weight is:
Weight (Newtons) = mass (kilograms) * 10
We MUST convert grams to kilograms first.
10 is the value for gravity on Earth.
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Calculate the weight forces of a 60kg person,
an 800 kg car and a 50 gram feather.
Extension exercise: Look up Newton's Law of Gravity, find
appropriate values and use them to show where the value of
10 (actually 9.8) comes from and determine its units, and
show why.
Gravity on Earth
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Weight = mass * 10, but only at the surface of the
Earth.
In space, weight force is nearly zero.
Every place in the universe has its own number for
gravity.
1. Calculate the weight force of a 60kg person on The
Moon (1.7), Mercury (4), or Jupiter (26).
2. Explain why things weigh more on Jupiter.
3. What would you feel on Jupiter, as a result of a greater
weight force?
4. What is wrong (scientifically speaking) with saying: “I
weigh seventy kilograms.”
OR extension exercises in previous slide.
Quick Review
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Explain the difference between mass and
weight, in your own words.
What is the weight of a 2kg object (on Earth)?
Calculate the weight of a 200g object (on
Earth). Don't forget to convert it to kilograms
first!
Reaction Force
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Hard surfaces provide a force which opposes anything pushing
against them.This often supports the weight of something on
them.
The reaction force is also sometimes called a support force.
The reaction force increases to match the weight
force until it reaches its limit.
Maximum
Reaction force
Reaction
Reaction
Reaction
Weight
Weight
Weight
Breaking Strength of Tissue Paper
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Write the prediction and your own procedure in your notebook. Remember
that it should be able to be followed by a Grade 6 student independently.
Prediction:
I think the tissues will break when the mass on them is _________ grams.
Therefore, I think the maximum reaction force will be ________ Newtons.
Mass (grams)
Weight (Newtons)
Trial 1
Trial 2
Trial 3
Average Weight
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Equipment: container, tissues, tape, masses
In your conclusion, answer the following:
A) Was the maximum reaction force of tissue greater or less than you
expected?
B) Name one use of a substance with a strong reaction force.
C) Is there a tension force occurring here?
D)How do you think your results would change if the box were larger?
Friction
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Friction is a force which opposes all objects moving.
The friction force between two surfaces depends on:
-the type of surface
-the perpendicular force between the surfaces
-the area of the surfaces
Reaction
Friction
Weight
More Pulling Force
Reaction
Friction
Weight
More Pulling Force
We can determine the maximum
friction force by pulling until the block
just starts moving.
Reaction
Friction
Weight
Increasing the Weight
Reaction
Friction
Weight
Friction and Perpendicular Force
Measure the perpendicular force (weight) and the friction
force for at least three different masses.
1. Describe the pattern between the size of the friction force and
the perpendicular force.
2. Would it make a difference if the perpendicular force wasn't
weight, for example if it were pressed against a wall by hand.
3. Can you determine an approximate rule for the relationship
between the two forces? If so, how would it
change if the surface in contact was
changed? How about if the surface
area was changed?
Reaction
Friction
Weight
Book Busters
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Video:
It's said that two books with the pages
interleaved require the force of _____ to
separate them.
Determine the relationship between the
number of pages interleaved and the force
required to pull the books apart.
Extension exercise:
Can you determine a pattern between the two
quantities, and if so can you describe it
algebraically?
The Bed of Nails
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I predict that it will take ______ N (_____
grams) to pop the balloon with ONE nail
because ______________________________.
I predict that it will take ______ N (_____
grams) to pop the balloon with the 'bed' of nails
because ______________________________.
Pressure
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Why does a sharp knife cut and not a blunt
knife?
Why does a pin prick but not a finger?
Why does a chair leave marks
in the carpet?
http://commons.wikimedia.org/wiki/File:Dwie_pinezki_(ubt).JPG
http://commons.wikimedia.org/wiki/File:Knife_01.JPG
Pressure
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Pressure occurs when a force is applied to an
area.
The higher the pressure, the more damage a
force will do to the surface.
pressure=
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force
area
Pressure is measured in Newtons per square
metre(N/m2), or Newtons per square
centimetre(N/cm2).
One N/m2 is called one Pascal, after the
French genius Blaise Pascal.
Pillows
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Why is a soft pillow more comfortable than a
brick floor?
http://commons.wikimedia.org/wiki/File:Body_pillow.jpg
http://commons.wikimedia.org/wiki/File:Worn_floor_-_geograph.org.uk_-_771522.jpg
New Unit: Motion
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Key Concept:Change
Related concepts:
Energy, Models, Interaction, Consequences, Balance
Concept statement:
The consequences of unbalanced forces on an object are
changes in its motion.
Global context statement:
The impact of scientific and technological advances on
society and on the environment.
Inquiry Questions:
-How can we use forces to predict changes in something’s
motion?
-How can we apply our understanding of forces and motion
to help us get around?
A Long Walk (and Run)
After the earthquake,
many people needed
to walk home from
Tokyo to Yokohama.
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How long would it
take to walk to
Shibuya? How about
to run to Shibuya?
What information
would we need to
know?
A Long Walk (and Run)
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Distance: 30km
Walking speed: 2
metres per second.
Running speed: 5
metres per second.
Speed, Distance and Time
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If it takes 4 hours to travel 200 kilometres, what
was the average speed, and how did you
calculate it?
How long
would it take to
travel 1000km
at this speed?
How did you
calculate it?
Speed
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Speed is a measure of how fast something moves.
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Units are m/s (or cm/s, km/s etc).
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Eg. Katie is running during PE. After 20 seconds she
has run 120m. What is her speed?
1. Calculate how far Katie would run in one minute
(using the speed from the previous example).
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2.A. Yuki is swimming at 2m/s. How long will it take
him to swim a length of a swimming pool (25m)?
B. How far can Yuki swim during a 45 minute PE class
if he swims for the whole class?
Hint: you will need to convert units here.
Measuring Walking / Running Speed
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We will use the roof of the central building.
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Equipment: meter rulers and stopwatches.
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Work in groups of 4 or 5.
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Measure the speed of a person walking and
running. (It does not have to be the same
person walking as running).
Use a distance of 5-10 metres.
Repeat each set at least twice (three total), and
take the average of all trials.
Present your results in a summary.
The Investigation
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Aim: To determine how fast _____ walks and ______ sprints.
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Apparatus: stopwatches, meter rulers
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Prediction: ________ will walk at _____ m/s and _________ will
sprint at _____ m/s.
Procedure:
1. Measure a suitable distance for someone to
walk, and sprint.
2. Arrange a system for ensuring that the timing is
accurate.
3. Record at least three trials, and calculate the
average speed for walking and running.
Results and Conclusion
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__________ Walking
Trial number
Distance (m)
Time (s)
Speed (m/s)
Time (s)
Speed (m/s)
1
2
3
Average speed
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__________Running
Trial number
Distance (m)
1
2
3
Average speed
1. Compare your results to others in the class. Who is the
fastest a) runner? b) walker?
2. The result obtained is an average speed. Explain what is
meant by an average speed. You may need to do some
research for this question.
3. Two years ago in Berlin, Usain Bolt from Jamaica set the
world record for the 100m sprint, coming in at 9.58s.
Calculate his average speed, and compare it to the fastest
student in the class.
4. A. Convert your group's walking and sprinting speed to
km/h. This may require some thought.
B. Use this, and google maps (directions) to calculate how
long it would take to walk AND run to
i) your home
ii) Tokyo tower
Forces and Acceleration
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If an unbalanced force acts on an object, it will cause it to
accelerate.
Acceleration is a change in speed or a change in direction.
http://commons.wikimedia.org/wiki/File:Hatchback_5_doors.png
Streamlining
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Streamlining is the shaping of a (usually fastmoving) object to minimise air resistance.
http://en.wikipedia.org/wiki/File:JR_East_Shinkansen_lineup_at_Niigata_Depot_200910.jpg
Motion Graphs
Motion = movement
A motion graph is used to show how something moves, and
how its movement changes over time. It can show:
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whether an object is stationary or moving
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an object's speed.
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if and how its speed is changing (acceleration or
deceleration).
A motion graph can show all of the above on the same
graph.
Types of Motion Graphs
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Time always goes on the x axis.
A distance-time graph shows how far
something has travelled on the Y axis.
Distance can not go backwards.
A speed-time graph shows how fast something
is moving on the Y axis.
Daniel Walking at 1m/s
Distance (m)
Speed (m/s)
8
8
6
6
4
4
2
2
0
0
0
2
4
6
8
Time (seconds)
10
0
2
4
6
8
Time (seconds)
10
Marii walking at 2 m/s
Speed (m/s)
Distance (m)
8
8
6
6
4
4
2
2
0
0
0
2
4
6
8
Time (seconds)
10
0
2
4
6
8
Time (seconds)
10
1. Where is each of the following on the graph above:
Stationary, constant (slow) speed, constant (fast) speed, acceleration,
deceleration
2. If this shows a car's motion
a) At which point is thrust greater than friction?
b) At which point is friction greater than thrust?
c) Are there any forces acting on the car at point a? If so, what?
d) Are there any forces acting on the car at point b? How can you tell?
s
p
e
e
d
(m/s)
c
b
d
e
a
0
Time (seconds)
Acceleration
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Acceleration is a constantly changing speed.
Deceleration is a form of acceleration, in which the speed
is decreasing at a steady rate.
Exercise:
On the roof, mark off 1m, 2m, 3m and 4m. Everyone must
run 1m in the first second, 2m in the second second, 3m
in the third second and 4m in the fourth second.
Extension: what units could be used for acceleration?
What would the acceleration of the class from the class
exercise be? Could we write a formula for acceleration?
Yongwon Accelerating
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In the first second, he travels one metre per second.
In the second second, he travels at two metres per second.
In the third second, he travels at three metres per second.
How would we describe Yongwon's motion?
Distance (m)
Speed (m/s)
8
8
6
6
4
4
2
2
0
0
0
1
2
3
4
Time (seconds)
5
0
1
2
3
4
Time (seconds)
5