Grade 8 Forces and Motion 2012
Transcript of Grade 8 Forces and Motion 2012
Grade 8 Science
Forces and Their Effects and Motion
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Brainstorm about Forces
Brainstorm everything you know about forces, in groups of 2 or 3. It should cover:Types of forces
Effects of forces
Theories or ideas related to forces?
Famous people who developed or promoted such ideas?
Can you bring in numerical relationships?
The Solar System
What keeps the planets moving?
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Forces
A force is a push or a pull.
Forces are measured in Newtons, after Isaac Newton.
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.
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Forces: The Basics
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
Gravity is a very weak but important force.
Gravity pulls everything together.
We notice it as pulling us down towards the Earth.
Gravity also pulls the Earth towards the sun.
What causes gravity?
Mass and Weight
Mass measures quantity of matter. It is measured in kilograms (kg).
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.
ItemMass (kg)Weight (N)
Find something with a weight force of 1 Newton.
Mass and Weight Relationship
The relationship between mass and weight is:
Weight (Newtons) = mass (kilograms) * 10We MUST convert grams to
kilograms first.
10 is the value for gravity on Earth.
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.
8A got to here 23 August
Gravity on Earth
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
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
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.
Weight
Reaction
Weight
Reaction
Weight
Reaction
Maximum Reaction force
Breaking Strength of Tissue Paper
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.
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?
Mass (grams)Weight (Newtons)
Trial 1Trial 2Trial 3Average Weight
Friction
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
Weight
Reaction
Friction
More Pulling Force
Weight
Reaction
Friction
More Pulling Force
Weight
Reaction
Friction
We can determine the maximum friction force by pulling until the block just starts moving.
Increasing the Weight
Weight
Reaction
Friction
Friction and Perpendicular Force
Weight
Reaction
Friction
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?
Book Busters
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
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
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?
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Pressure
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 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
Why is a soft pillow more comfortable than a brick floor?
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New Unit: Motion
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 somethings
motion?
-How can we apply our understanding of forces and motion to help us
get around?
After the earthquake, many people needed to walk home from Tokyo to Yokohama.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)
Distance: 30km
Walking speed: 2 metres per second.
Running speed: 5 metres per second.
A Long Walk (and Run)
Speed, Distance and Time
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
Speed is a measure of how fast something moves.
Units are m/s (or cm/s, km/s etc).
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).
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
We will use the roof of the central building.
Equipment: meter rulers and stopwatches.
Work in groups of 4 or 5.
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
Aim: To determine how fast _____ walks and ______ sprints.
Apparatus: stopwatches, meter rulers
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
__________ Walking
__________Running
Trial numberDistance (m)Time (s)Speed (m/s)
123Average speed
Trial numberDistance (m)Time (s)Speed (m/s)
123Average 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
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.
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Streamlining
Streamlining is the shaping of a (usually fast-moving) object to minimise air resistance.
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Streamlining
Streamlining is the shaping of a (usually fast-moving) object to minimise air resistance.
Motion Graphs
Motion = movementA motion graph is used to show how something moves, and how its movement changes over time. It can show: whether an object is stationary or moving
an object's speed.
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
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
0 2 4 6 8 10Time (seconds)
Distance (m)
8
6
4
2
0
0 2 4 6 8 10Time (seconds)
Speed (m/s)
8
6
4
2
0
Marii walking at 2 m/s
0 2 4 6 8 10Time (seconds)
Distance (m)
8
6
4
2
0
0 2 4 6 8 10Time (seconds)
Speed (m/s)
8
6
4
2
0
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?
Time (seconds)
speed
(m/s)
0
a
b
c
d
c
e
Acceleration
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
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?
0 1 2 3 4 5Time (seconds)
8
6
4
2
0
Speed (m/s)
8
6
4
2
0
Distance (m)
0 1 2 3 4 5Time (seconds)