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L1 Forces

All will: Define important terms

Most will: Describe resultant forces
Some will: identify the effects of resultant
forces

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What are forces?

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What are forces?

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What are forces?

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What are forces?

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Push and pull

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Push and pull

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Push and pull

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Types of forces
All forces involve interactions
between objects. There are
several different types.

Gravity and magnetism

are forces that can act over
distances.

Friction and upthrust are

forces that involve direct
contact between objects.

All types of forces can occur whether objects are still or moving.

What forces are acting during this rocket launch?

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Forces affecting objects

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Forces affecting objects

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Forces affecting objects

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Forces affecting objects

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Forces affecting objects

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Measuring forces

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Measuring forces

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Measuring forces

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Measuring forces

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Forces

• In everyday language we use the word ‘force’ is

lots of ways, but in physics the word force has a
very particular meaning.

• A force is a push or a pull that acts on a system

(an object, a planet, an atom, or anything else)

• Different forces are calculated in different ways

depending on the situation, but the standard unit
for forces is Newtons (N).

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Effects of Forces

• Forces can three possible effects on a system. They can change

(1) the size
(2) the shape
(3) the motion

• Examples:
(1) stretching an elastic band (pull)
(2) squishing a large lump of plasticine into a smaller lump (push)
(3) throwing a ball (either)

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Balanced forces

There are two wind machines applying forces to the ice skater.
The forces acting on the skater are equal in magnitude and
opposite in direction.
The forces are balanced, so they cancel each other out.
The skater does not move.

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Unbalanced forces

What if only one wind machine is blowing on the skater?

The forces acting on her are no longer balanced so she
will start to move to the left. Her speed will change – this
is called acceleration.
Unbalanced forces lead to a change in speed or direction.

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Newtons first law of motion

• an object in motion will remain in motion

unless acted upon by another force

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Force pairs

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Force pairs

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Force pairs

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Balanced and unbalanced forces
Imagine a car travelling at a constant speed of 50km/h.

The engine provides sufficient force to balance all the

frictional forces that are acting to decrease the speed.

500N 500N

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Balanced and unbalanced forces
A crosswind acting on the car produces a sideways force.

500 N 500N

100 N
cross wind
The crosswind causes the direction of the car to change
– this happens because the sideways forces on the car
are not balanced. The car will veer sideways.
If the car turns right so that the wind is now behind the
car, what will happen to the speed?

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Resultant force
The sum effect of more than one force is called the
resultant force.
The resultant force is calculated by working out the
difference between opposing forces in each direction.
What is the resultant force on this truck?
A resultant force of 100 N is accelerating the truck.

400 N 500 N

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Balanced and unbalanced forces – summary

If the forces on an object are balanced:

⚫ and the object is stationary, it will remain stationary
⚫ and the object is moving, it will continue to move at
the same speed and in a straight line.
In other words, the object will continue to do what it is
already doing without any change.

If the forces are unbalanced, two things can happen:

⚫ The speed can change. This is called acceleration.
⚫ The direction of motion can change.

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Resultant forces – question 1
1. What is the resultant force on the satellite?
5N 5 N
Resultant force = 20 N – 10N
= 10 N down

The satellite will accelerate

downwards.

20N

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Resultant forces – question 2
2. What is the resultant force on the bird?
The forces acting in each
direction horizontally are equal
in size, so there is no resultant
force in this direction.
Resultant force = 5 N – 5 N = 0 N

5N 5 N The vertical forces are not

balanced, the bird will accelerate
in a downwards direction.

5N Resultant force = 5 N – 0N
= 5 N down

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Resultant forces – question 3
3. What is the resultant force on the yacht?
10 N

10N
13 N 20 N

10N
The vertical forces are equal in size and opposite in direction
so there is no resultant force in the vertical direction.
The horizontal forces are not balanced, so the yacht will
accelerate to the right.
Resultant force = (20 N +10 N) – 13N
= 17 N right
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Resultant Force

• You may have noticed that Forces not only have a size
but also a direction. Forces are called vectors because
they have these two pieces of information.

• Vectors are extremely important in physics, but for now

we only need to understand them in 1 dimension (or a
straight line).

• The resultant force is the sum of all forces acting on a

system.

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Free Body Diagrams
• The best way to analyse the forces acting on a
system is with a free body diagram.

1. Represent your system as a point.

2. Represent each force as an arrow coming

+
out from the point. 𝑭𝟐 𝑭𝟏
(Larger forces = Bigger arrows)

3. Choose a positive direction and calculate the

resultant. 𝑅 = 𝐹1 − 𝐹2

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Balanced and Unbalanced

• If the resultant force is not 0, we say the forces

are unbalanced. This means our system will
accelerate. (Because the system is being
pushed or pulled)

• If the resultant force is equal to 0, then our

system is not accelerating and we say that our
forces are balanced. Then there are two
possibilities:
(1) It is not moving at all or
(2) It is moving with a constant speed

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Classwork/ homework
• Complete questions 1 – 11 on the N: Forces page in your
Onenote class notebook
• Complete the page A: forces simulation using the link
provided
• Learn the key terms using Quizlet or do the challenge
activities

Textbook pages: chapter 4 pp 113 – 122

Challenge Activities
• Complete research to answer the questions in green
• Try the motion simulations linked on the learning resources
page
• S
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L2 Gravity, Mass and Weight

All will: Define important terms

Most will: calculate weight
Some will: distinguish between mass and
weight

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What is gravity?
When the netball is thrown, why does it fall back down?
There is a gravitational force
pulling it towards the Earth.
Gravity is a force that occurs between all objects.

Gravity always acts to pull objects

towards each other.
The Earth and the ball are
pulling each other together.
However, the ball moves much
more than the Earth because
it has a much smaller mass.

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Sir Isaac Newton
Sir Isaac Newton is a very
famous physicist who lived in
England 1643–1727.

Newton wrote down his ideas

in the Philosophiae Naturalis
Principia Mathematica; a very
important book about forces
and gravity.

Some accounts suggest that one

of Newton’s greatest discoveries
occurred when an apple fell on
his head and it made him think
about the reason it fell…

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Gravity and Newton
Newton realized that the motion
of falling objects and objects
orbiting in space must be caused
by the same force – Gravity!

He wrote in the Philosophiae

Naturalis, “It is an attractive
force that makes apples fall
from trees and the planets orbit
the Sun.”

Other scientists had already

noted the effects of gravity but
Newton was the first to calculate
the force of gravity on objects.

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What is gravity?
Gravity is an attractive
force that acts between Satellite’s
gravity
all objects that have
mass. The size of the
force depends on the
masses of the objects. Earth’s gravity

All objects produce a

gravitational force. This
is very large for huge
masses such as planets.
When you jump, the gravitational force of the Earth pulls you
down. Your gravitational force also pulls the Earth towards
you, but you don’t notice it because the Earth is too heavy to
be visibly affected by your gravity.

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Gravity and distance
The force of gravity between two
objects depends on their masses
and the distance between them.

Spacecraft produce a very

large force, called thrust, to
overcome the force of gravity.

As a spacecraft gets further

away from Earth, the force
of gravity gets smaller.

Why do spacecraft lose their large fuel tanks and booster

rockets once they have left the Earth’s surface?

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Gravity during a rocket launch

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Gravity during a rocket launch

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Gravity during a rocket launch

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Gravity during a rocket launch

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Gravity during a rocket launch

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Gravity during a rocket launch

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What are mass and weight?
Mass is the amount of
matter in an object and is
measured in kilograms.
Mass is not a force.
An object, such as this
satellite, has the same
mass at any point in the
Universe.
Weight is a force and is caused by the pull of gravity acting
on a mass.
Weight is measured in newtons and has both magnitude
and direction. An object’s weight changes depending on
where it is in the Universe.

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Investigating mass and weight

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Investigating mass and weight

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Investigating mass and weight

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Investigating mass and weight

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Investigating mass and weight

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Investigating mass and weight

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Investigating mass and weight

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Mass and weight on the Moon
The force of gravity on the Moon
is only one-sixth of that on Earth
because the Moon has a much
smaller mass.
Any object on the Moon weighs
one-sixth of the amount it would
weigh on Earth.
Astronauts can jump up 20 feet
on the Moon due to there being
such a low gravitational force.
However, the astronaut still has
the same mass – they just weigh
less because gravity is weaker.

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Mass and weight on different planets

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Weight and mass activity

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Weight and mass activity

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Weight and mass activity

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Missing words about gravity

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Missing words about gravity

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Missing words about gravity

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Mass and Weight

• “How much does it weigh?” – we often answer this

question with “10 kg” or “70 pounds” or something
similar. But in physics we need to make an important
distinction: Mass and weight are different.

• Mass is the amount of ‘stuff’ a system has – it cannot be

described in terms of anything else. If you are 70 kg on
earth, you will be 70 kg anywhere else in the universe.

• Mass and motion are related – the more mass

something has, the harder it is to move. This idea is
often called inertia.

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Weight

• Weight is a force. It is measured in Newtons (like all forces) and it

depends on where in the universe you are. Usually, weight refers to
the force of Earth’s gravity.

• On Earth, we can calculate the weight of any system:

𝑊 = 𝑚𝑔

• W is the weight (in Newtons), m is the mass (in kilograms), and g is

the strength of the Earth’s gravitational field (equal to 9.8 N/kg)

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Gravitational Fields

• Everything with mass produces a

gravitational field and so there is a
‘weight’ associated with the pull of all
matter in the universe.

• The person next to you is producing a

gravitational field and pulling towards
them, and so is the furthest galaxy in
the universe.

• ‘Weight’ usually refers to the biggest

and closest thing – in most cases, the
Earth. So ‘weight’ is the effect of the
largest gravitational field nearby.

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Examples

Calculate the weight of a 14

kg dumbbell on Earth.

Solution
𝑊 = 𝑚𝑔 Always start with the
relevant formula

Include units in every

step

𝑊 = 137.2 𝑁

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Gravity on the Moon

• Since the moon is much less massive

than the earth, it also has less
gravitational pull.

• Your weight on the moon is different

than on the earth (even though your
mass is the same!). That’s why
astronauts can jump much higher
and further.

• (not assessable) for interest, we

could calculate your weight on the
moon using the same formula but
𝑁
𝑔𝑚𝑜𝑜𝑛 = 1.6 𝑘𝑔

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Newtons second law of motion

• The acceleration of an object depends on

the mass of the object and the amount of
force applied

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Classwork/ homework
• Complete questions 13 – 20 on the N: Forces page in your
Onenote class notebook
• Complete the page A: Changing gravity
• Learn the key terms using Quizlet or do the challenge
activities

Textbook pages: chapter 4 pp 113 – 122

Challenge Activities
• Complete research to answer the questions in green
• Try the gravity simulations linked on the learning resources
page

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L3 Friction

All will: Define important terms

Most will: explore how forces change
Some will: explore forces and acceleration

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Other Forces

• Some other important forces include

1)Friction (including air resistance)
2)Tension
3) Normal forces

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Normal Forces

• Are “push” forces – they occur when

matter gets compressed together.
Examples:
(1) a person pushing a crate along the floor
is exerting a normal force on the crate.
(2) A bottle of water does not fall through the
table because the table exerts a normal
force on the bottle.

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Tension Forces

• are “pull” forces – they occur when matter

gets stretched apart.

• Examples:
(1) A picture hanging on the wall with a
small wire. The wire exerts a tension force
on the picture.
(2) A person pulling a crate along the floor
exerts a tension force through their arms

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Friction

• is a force that opposes motion. There are two main

kinds of friction forces we need to know about.

1) Sliding friction – when two solid surfaces are in

contact and sliding against each other, sliding
friction tries to oppose (or stop) this motion.
• Sliding friction also transforms mechanical energy
into heat energy (try rubbing your hands together
very fast)

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Friction

2) Air Resistance – is the friction that results from

a body moving through the air. Air molecules
oppose the motion and slow the body down. This
also occurs in any other fluid like water.

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Newton third law of motion

• For every action there is an equal and

opposite reaction

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What is friction?
If you rub your hands together, they get warm because there
is resistance to the rubbing motion.
What is the name of this resistive force?
It is called friction.
What causes this force?
Your hands might look smooth,
but on a microscopic level they
have rough surfaces.
When you rub your hands
together you feel the resistive
force of friction.

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Friction and motion
Friction always tries to slow moving objects down – it
opposes motion. Frictional forces occur when two touching
surfaces move past each other, in this case the box moving
across the ground.

pull of rope
friction on box

Friction also occurs when things move through air.

This is called air resistance.

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What are the sources of friction?
Label all sources of friction acting on this bike.

pedal
bearing air
resistance

links in brake
chain pad

tyre and wheel

road bearing

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Sources of friction

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Sources of friction

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Sources of friction

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Friction experiment

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Friction experiment

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Friction experiment

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Friction experiment

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Friction experiment

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Cars stopping
Friction is a very important force for the movement of cars.
It acts in the opposite direction to the movement of the car.

thrust from
engine friction

The time it takes for a car to brake is affected by the frictional

forces between the car’s tyres and the road surface.

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Factors affecting stopping distance
One of the most important
sources of friction in cars
is that between the tyres
and the road.

When the car brakes, it’s important to have as much

friction as possible so that the car does not skid.

The friction between the tyres and road is affected by the:

⚫ inflation pressure of the tyres
⚫ road surface
⚫ surface condition caused by the weather (rain, ice, etc).

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Explaining air resistance
Air resistance is a type of
friction that occurs
whenever an object moves
through the air, and is
caused by the frictional 400 N
forces acting between the
air and the object.

If the area of contact

between the air and object
is reduced, the object is
said to be streamlined. 300N

Which of these cars is

more streamlined?

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Air resistance and drag
The photo shows a streamlined car which has been shaped so
that the flow of air around the body is made as smooth as
possible. This minimizes the air resistance.
If you looked at the car
from the front, how
much surface area
would you see?
The air resistance is
determined by:
⚫ the size of the car
⚫ the shape of the car
⚫ the speed of the car.

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Falling objects on the moon

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Effects of frictional forces

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Effects of frictional forces

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Effects of frictional forces

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How does a plane take-off?

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How does a plane take-off?

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How does a plane take-off?

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How does a plane take-off?

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How does a plane take-off?

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Classwork/ homework
• Complete questions 21 – 34 on the N: Forces page in your
Onenote class notebook
• Complete the page A: Friction Simulation
• Learn the key terms using Quizlet or do the challenge
activities

Textbook pages: chapter 4 pp 113 – 122

Challenge Activities
• Complete research to answer the questions in green
• Try the acceleration simulation linked on the Friction
simulation resources page

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L4 Moments

All will: Define the moment of a force

Most will: calculate moments
Some will: apply the principal of moments

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Force and rotation
A force acting on an object can cause it to turn about a pivot.

100 N

pivot

What happens to the see-saw when a force is applied on the left-hand side?

Does the see-saw turn? If so, clockwise or anticlockwise?

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Force and rotation – a moment
The left-hand side of the see-saw moves downwards when a force is applied to it
– this is an anticlockwise turn.

100 N

The turning effect of a force is called a moment.

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Using moments
A spanner is a lever that can be used to unscrew a nut.

The spanner exerts a moment or turning force on the nut.

pivot

distance from
force to pivot

force
force

If the moment is big enough, it will unscrew the nut.

If not, there are two ways of increasing the moment.

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Using moments – increasing the moment
1. Increase the distance from the force to the pivot –
apply the force at the end or use a longer spanner.

pivot

distance from
force to pivot

force

If the same force is applied over a greater distance, a larger moment is

produced.

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Using moments – increasing the moment
2. Increase the force applied – push/pull harder or get
someone stronger to do it!

pivot

distance from
force to pivot

force

If a greater force is applied over the same distance, a larger moment is

produced.

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Moment equation
The moment of a force is given by the formula:

moment = force (N) × distance from

pivot (cm or m)

This can also be represented in a formula triangle:

moment

f d
Moments are measured in newton centimetres (Ncm) or newton metres
(Nm).

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Moment calculation
The counterweight on the trebuchet weighs 300 N and is attached to the short arm.
It is 1 m from the pivot. It exerts a clockwise moment. What is the size of this
moment?

moment = 300 × 1 = 300 Nm

3m 1m

100 N 300 N
pivot

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Principle of moments
The lead shot on the trebuchet weighs 100 N and is attached to the long arm. It is
3 m from the pivot. It exerts a clockwise moment. What is the size of this moment?

moment = 100 x 3 = 300 Nm

3m 1m

100 N 300 N
pivot

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Principle of moments
If the anticlockwise moment and clockwise moment are equal, then the trebuchet
is balanced. This is known as the principle of moments.

When something is balanced about a pivot:

total clockwise moment = total anticlockwise moment

3m 1m

100 N 300 N
pivot

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Principle of moments
What happens if the counterweight is increased to 1000 N?
The moments will no longer be balanced, so the trebuchet will be able to fire.

3m 1m
1m

100 N 1000 N

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Principle of moments
The principle of moments can be investigated using 10 g masses with this
balance. 10 g exerts a force of 0.1 N.

Anticlockwise Clockwise
moment = 0.1 × 7 moment = (0.1 × 3) + (0.1 × 4) = 0.7
= 0.7 Ncm Ncm

Both moments are equal, so the see-saw is balanced.

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Principle of moments – calculation
Two girls are sitting on opposite sides of a see-saw. One girl weighs 200 N and is
1.5 m from the pivot. How far from the pivot must her 250 N friend sit if the see-
saw is to balance?

200 N 250 N
1.5 m ?m

total clockwise moment = total anticlockwise moment

200 N × 1.5 m = 250 N × distance

200 × 1.5 = distance = 1.2 m

250

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How do tower cranes work?
Tower cranes are essential at any major construction site.

trolley
load arm

counterweight

loading platform

tower

Concrete counterweights are fitted to the crane’s short arm. Why are these
needed for lifting heavy loads?

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Moment calculation – crane
If the crane below is balanced, how heavy is the load?

3m

6m

10,000 N
?

moment of = moment of
load counterweight

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Moment calculation – crane
Moment of load = load × distance from tower

= ? × 6

Counterweight = counterweight × distance

moment
= 10,000 × 3

= 30,000 Nm
6m 3m

Moment of load = moment of

counterweight

?×6 = 30,000 10,000 N

? = 30,000 5,000
?N

6
? = 5,000 N

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Crane operator activity

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Classwork/ homework
• Complete questions 1 – 10 on the N: Moments and levers
page in your Onenote class notebook
• Learn the key terms using Quizlet or do the challenge
activities

Textbook pages: chapter 4 pp 113 – 122

Challenge Activities
• Complete research to answer the questions in green
• Try the Moments game linked with the moments simulation

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Levers

• A lever is a simple machine made of a rigid beam and

a pivot or fulcrum. The effort (input force)
and load (output force) are applied to either end of the
beam. The fulcrum is the point on which the
beam pivots.
• When an effort is applied to one end of the lever, a load
is applied at the other end of the lever. This will move a
mass upward. Levers rely on torque for their operation.
• Torque is the amount of force required to cause an
object to rotate around its axis (or pivot point).

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Mechanical advantage

• A lever provides mechanical advantage.

• Mechanical advantage refers to how much a
simple machine multiplies an applied force. The
location of the effort, load, and pivot or fulcrum will
determine the type of lever and the amount of
mechanical advantage the machine has. The farther
the effort is away from the pivot or fulcrum, the
easier it is to move the load.
• Mechanical advantage can be calculated using this
formula:

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Levers

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First class levers

• The pivot or fulcrum is located between

the load and the effort.

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Second class levers

• The load is located between the pivot or

fulcrum and the effort.

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Third class levers

• The effort is located between the load and

the pivot or fulcrum.

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Force multipliers

• These allow a small effort to be used to

move a large load. This works because
the effort is at a larger distance from the
pivot.

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Distance multipliers

• These allow a small movement of the

effort to produce a large movement of the
load. This works because the load is at a
larger distance from the pivot.

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Pulleys
• A pulley is a simple machine consisting of a string (or rope)
wrapped around a wheel (sometimes with a groove) with
one end of the string attached to an object and the other
end attached to a person or a motor.
• Pulleys may seem simple, but they can provide a powerful
mechanical advantage so lifting tasks may be done easily.

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Classwork/ homework
• Complete questions 11 – 16 on the N: Moments and levers
page in your Onenote class notebook
• Learn the key terms using Quizlet or do the challenge
activities
• Complete the Edpuzzel assignment linked on teams
• Ensure all work from the topic is complete

Textbook pages: chapter 4 pp 113 – 122

Challenge Activities
• Complete research to answer the questions in green

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L6 Consolidation

All will: Complete outstanding work in

black
Most will: Try the Seneca activity
Some will: Conduct individual research

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Classwork/ homework

• Ensure all work at your level from the topic is complete

• Complete the seneca activity (The link is in assigments and
this will be graded)

Textbook pages: chapter 4 pp 113 – 122

Challenge Activities
• Complete independent research on an aspect of the topic
that you find interesting
• Or
• Begin your revision for the exams

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