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)