4.5.6.2.3-Newton's-Third-Law---AQA-GCSE-Science-Physics-by-Brainjar.pptx

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GCSE Science / Physics
Newton’s Third Law

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Key: Items in a green box are covered in this
lesson.

Teacher Notes
This lesson covers all of AQA GCSE Physics 4.5.6.2.3 / Combined Science
6.5.4.2.3.
A balloon can be useful for a very simple demo. If you blow it up and
release the end while holding the balloon, air will rush out and the force
can move little bits of paper etc. If you do this again and let go of the
balloon it will obviously fly around the room.
The colours chosen for the equilibrium situation (red and blue) have been
chosen as they are apparently the best colours for all types of colour
blindness. Thank you to the customer who emailed with feedback that
prompted me to make this change.

1. Describe the difference between contact and non-contact
forces.
2. Describe Newton’s First Law of Motion.
3. State Newton’s Second Law of Motion.
4. State the equation that describes Newtons Second Law of
Motion.
5. (HT) What do we call the tendency of objects to continue in
their state of rest or of uniform motion?
Atomic Structure Topic:
6. Why do the nuclei of some atoms undergo radioactive decay?
7. What are the four types of nuclear radiation?
16 September 2025
Starter

Answers
1. Describe the difference between contact and non-contact forces. Contact forces are where
the objects are physically touching (1) whereas non-contact forces are where the objects
are physically separated (or “not touching”) (1).
2. Describe Newton’s First Law of motion: An object’s motion will not change unless a
resultant force acts on it.
OR if the resultant force on an object is zero then if it is stationary, it will stay stationary
and if it is moving it will continue to move at the same speed in the same direction (i.e.
with the same velocity).
3. State Newton’s Second Law of motion: The acceleration of an object is proportional to the
resultant force acting on the object, and inversely proportional to the mass of the object.
4. State the equation that describes Newtons Second Law of motion.
Force (N) = mass (kg) x acceleration (m/s2
).
5. (HT) What do we call the tendency of objects to continue in their state of rest or of uniform
motion? Inertia.
Atomic Structure Topic:
6. Why do the nuclei of some atoms undergo radioactive decay? They are unstable and so
release nuclear radiation to become more stable.
7. What are the four types of nuclear radiation? Alpha, beta, gamma and neutron radiation.

Lesson Focus and Learning Objectives
Lesson Focus:
What is Newton’s Third Law of Motion?
Learning Objectives (To Be Able To…):
• Describe Newton’s Third Law of Motion.
• Apply Newton’s Third Law of Motion to explain the forces
acting in different situations.
• Describe what is meant by an object being in “equilibrium”
and apply Newton’s Third Law of Motion to equilibrium
situations.

Pushes and Pulls
Heavy
Thing
Push
Heavy
Thing
Pull
Heavy
Thing
• In the first lesson we learned that “a force is a push or pull that acts on an
object due to the interaction with another object.”
• It is important to remember this when we learn about Newton’s Third Law
today, because there are always two objects interacting.
Gravity (also
a “pull”)
Recap

Newton’s Third Law of Motion
We have already learned Newton’s first two Laws
of Motion.
Newton’s Third Law of Motion states that:
whenever two objects interact, the forces they
exert on each other are equal and opposite.”
The above is the exam board definition of
Newton’s Third Law, that you should state if you
are asked to do so in an exam.
However, in order for us to understand Newton’s
Third Law of Motion, it can be helpful to expand
that definition slightly:
whenever two objects interact, the forces they
exert on each other are equal in size
(magnitude) and opposite in direction.
Sir Isaac Newton

Force Pairs
• The best way of understanding Newton’s Third Law is to see examples of it
in action.
• When we look at these examples there will always be two equal and
opposite forces that occur when two objects interact. These are
sometimes referred to as “force pairs”.
• These “force pairs” always:
• act on two different objects.
• are of the same type - they must both be either contact forces or
non-contact forces.
• We can make a little checklist list to make sure that what we are seeing is
Newton’s Third Law in action.
Newton’s Third Law Checklist.
 Forces acting on two different objects.
 Forces equal in size.
 Forces opposite in direction.
 Forces both of the same type.

Example 1: Punching A Wall
• If you punch a wall (please don’t) you will exert a force onto the wall.
• This then causes an equal (in size) and opposite (in direction) force from
the wall to act on your fist.
• The forces in this case are both the normal contact force.
We can see that this is an example of Newton’s Third Law as the forces are:
• Acting on two different objects. 
• Equal in size. 
• Opposite in direction. 
• Both of the same type (contact forces in this case). 
Normal
Contact Force
Normal
Contact Force

Understanding Newton’s Third Law
• Punching something hard like a wall is a bad
idea because it can easily hurt you.
• This is because, as Newton’s Third Law tells us,
whatever force you exert onto the wall, the
exact same size force is exerted by the wall in
the opposite direction - onto your hand.
• Because the forces are acting on different
objects, even though they are the same size,
the effect they have on the objects may be
different.
• In this case the size of the force may not be big
enough to damage the wall but it is big enough
to damage your hand.
• Remember that the same size forces may have
different effects on different objects when we
look at the next example.

Example 2: A Car Driving
• When a car is driving along the wheels are turning. As they do so they
push the road backwards. The force here is friction.
• This then causes an equal (in size) and opposite (in direction) force on
the wheels that push the car forward.
• Although the forces are equal in size, the size of the force is not enough
to move the road, but it is enough to move the car.
We can see that this is an example of
Newton’s Third Law as the forces are:
• Acting on two different objects. 
• Both of the same type (contact
forces in this case). 
Friction force
from wheel
on road.
Friction force from
road on wheel.
(Driving Force)

Example 3: A Balloon
• When we let the air come out of this balloon there is a force to the left
caused by the air being pushed backwards.
• This then causes a force that is equal in size and opposite in direction
which acts to the right and pushes the balloon itself forward.
• This is pretty much how a jet engine works – hot gases are forced
backwards out of the jet engine and this pushes the aeroplane forwards.
We can see that this is an
example of Newton’s Third Law as
the forces are:
• Acting on two different
objects. 
• Both of the same type
(contact forces in this case). 
Push force
on air
Push force
on balloon

Example 4: Non-Contact Forces
• Our examples so far have all been for contact forces.
• However, Newton’s Third Law also applies to non-contact forces such as
gravity.
• The Earth pulls on the Moon, but the Moon also pulls on the Earth.
Earth and Moon size to
scale but distance
definitely not to scale!
We can see that this is an
example of Newton’s
Third Law as the forces
are:
objects. 
• Opposite direction. 
• Both of the same type
(non-contact forces in
this case). 
Gravity
Gravity

Falling Through Air
Weight
• Remember when we threw a teddy bear out of an aeroplane?
• While it is in free fall, the force of gravity acting on the teddy bear (its weight)
accelerates it towards the Earth.
• But Newton’s Third Law tells us that there must be an equal (in size) and opposite
(in direction) force on the Earth.
• This in turn accelerates the Earth towards the teddy bear – we could even use
Newton’s Second Law (F = ma) to calculate it.
• However because the mass of the Earth is very large and the size of the force on
the Earth is very small, the acceleration of the Earth is incredibly tiny – it is too
small to measure so we say it is “negligible” and we essentially ignore it.
Optional

Newton’s Third Law applies
whenever ??? objects ???
The force pairs are always ??? and ???
Newton’s Third Law applies whenever
two objects interact.
The force pairs are always equal (in
size) and opposite (in direction).
The force pairs always act on…
…two different objects.
The force pairs are always of the same
??? – either both ??? or both ???
forces.
The force pairs are always of the same
type – either both contact forces or
both non-contact forces.
If you hit a wall and damaged your
hand, which force would be larger –
the force acting on the wall or the
force acting on your hand?
Neither – they would be the same
size. Even though the forces on both
objects are the same size, the effect
on each object may be different.
The arrow below shows the force
acting on a shopping trolley when you
push it. How would you draw the
force acting on you?
Learning Check: Quick Questions
1 minute to discuss with the person next to you, before I choose people at random.
A force arrow the same length
pointing in the opposite direction.
E.g.

Tasks:
1. State Newton’s Third Law. [3 marks]
2. “Force pairs are always the same type.” What does this mean? [1 mk]
3. Draw the forces acting in the following two scenarios:
a) A person pushing against a wall.
b) The wheels of a car driving along the road. [2 marks for each]
4. I flick a toy car on the desk. Why does it accelerate but I don’t? [2 mks]
5. A car is driving down the road. A fly hits the windscreen of the car and
is obliterated. Which of the two forces is greater: the force on the fly or
the force on the car? [1 mark]
Extension: Write down the “Newton’s Third Law”
checklist.
Challenge: Look at the image of two ice
skaters. Blue skater pushes on red skater
and they move apart. Describe
a) the forces on each skater [1 mark]
b) the acceleration of each skater. [3 mks]
Learning Check: In-Depth Questions
75 kg 50 kg

Answers
1. State Newton’s Third Law. Whenever two objects interact (1), the forces they
exert on each other are equal (in size / magnitude) (1) and opposite (in
direction) (1).
2. “Force pairs are always the same type.” What does this mean? They must both
be either contact forces or non-contact forces. (1)
3. Draw the forces acting in the following two scenarios:
a) A person pushing against a wall. See below.
b) Wheels of a car driving along the road. For both diagrams they should show
forces acting in opposite directions (1) that are the same size. (1)
4. I flick a toy car on the desk. Why does it move but I don’t? The force acting on
your finger is the same size as the force acting on the car (1) but the force is
large enough to accelerate /move the car but not large enough to accelerate /
move you (1).

Answers
5. A car is driving down the road. A fly hits the windscreen of the car and is
obliterated. Which of the two forces is greater: the force on the fly or the
force on the car? Neither – both forces are the same size (1), however the
force is large enough to destroy the fly but not to damage the windscreen.
Extension: Write down the “Newton’s Third Law” checklist.
• Forces acting on two different objects.
• Forces equal in size.
• Forces opposite in direction.
• Forces both of the same type.
Challenge: Look at the image of two ice skaters. Blue skater pushes on red skater
and they move apart. Describe
a) the forces on each skater. Equal in size and opposite in direction.
b) the acceleration of each skater. Red skater will experience a larger
acceleration. (1) This is because the force on each skater is the same size,
but red skater has a smaller mass (1). Newton’s Second Law tells us that
acceleration is inversely proportional to the mass of the object.(3)

Equilibrium Situations
• We need to look at what is meant when we say an object is “in equilibrium”.
• In this diagram we are looking at only the forces acting on the car. This is not an
example Newton’s Third Law because we are looking at the forces acting on one
object and not two!
• Recap: When two opposing forces are the same size we say they are balanced.
• All the opposing forces on this car are balanced, so we say it is in equilibrium.
• The resultant force on the car is therefore zero.

Newton’s First Law and
Equilibrium Situations
• We need to go back to Newton’s First Law again for a moment.
• Newton’s Third Law always involves two objects interacting, however
Newton’s First Law is about the forces acting on one object.
• When all the opposing forces on an object are balanced we say it is “in
equilibrium.” The resultant force on the object will be zero.
• Newton’s First Law tells us that if the resultant force on an object is zero
then if it is stationary, it will stay stationary, and if it is moving it will
continue to move at the same speed in the same direction (i.e. with the
same velocity).
• When an object is in equilibrium, understanding what is happening in
terms of Newton’s Third Law can sometimes be confusing, but the
following examples should help you to understand.

Equilibrium
Think, Pair, Share: This tin of beans on a table is in equilibrium. Is this an example of
Newton’s Third Law?
Answer: No. Let’s go through our checklist:
• Acting on two different objects. X
• Both of the same type. X
Normal Contact Force
on Tin of Beans
Weight of Tin of Beans

applies to each pair of
forces.
• Acting on two
different objects. 
• Both of the same
type. 
Equilibrium
• If we look more closely, we can see that there are actually two pairs of forces
acting here. In this image each pair is colour coded.
• Newton’s Third Law applies to each pair of forces. For example, the force the tin
exerts on the table is causing an equal and opposite force to act on the tin.
• The reason the situation may have been confusing is that the object was in
equilibrium. In this case it was stationary and was remaining stationary. Therefore
– as Newton’s First Law tells us – the forces acting on it were balanced (as the
resultant force was zero). The two forces being the same size can cause confusion.
• The key was spotting that they were acting on one object not two.
on Tin of Beans
Weight (Force of Gravity
on) Tin of Beans
on Table.
Force of Gravity on Earth
Worked Example

Equilibrium
• Here we can see both diagrams together.
• Remember, both the First and Third Law are always applicable, the difference in
the diagrams is whether we are looking at only the forces acting on the tin of
beans in order to see what is happening from the point of view of the First Law…
• OR whether we are looking at all the forces acting on both the tin and the table in
order to see what is happening from the point of view of the Third Law.
on Tin of Beans
Weight (Force of
Gravity on) of Tin of
Beans
on Tin of Beans
Weight (Force of Gravity
on) Tin of Beans
on Table.
Force of Gravity on Earth

Forces on a Car: First Law
• When we learned about Newton’s First Law we looked at the example of a car
moving at a steady speed. This is an equilibrium situation – the forces acting on
the car are balanced so the resultant force is zero and the car is not accelerating.
• There is a driving force from its engine pushing it forward.
• There are also resistive forces (e.g. friction with the ground and air resistance /
drag) acting on it. For simplicity we are only looking at the horizontal forces.
• But of course these two forces are both acting on one object – the car. So this
helps us understand what is going on from the point of view of Newton’s First Law,
but not Newton’s Third Law.
• We’ll look at the forces in more detail on the next slide.
Steady Speed
Driving Force
Resistive Forces
(e.g. air resistance)
Newton’s Third Law does not apply:
• Acting on two different objects. X
• Both of the same type. 

Forces on a Car: Third Law
• Here we have again colour coded the two pairs of forces acting on the car. We
have also moved them around to make the image easier to understand.
• As we know, the wheels turn and exert a force onto the road. This causes an equal
(in size) and opposite (in direction) force to act on the car which pushes it forward.
• As the car moves through the air it exerts a force onto the air. The air exerts an
equal and opposite force onto the car, which is air resistance.
• (Of course there are other resistive forces acting on the car, but to keep this simple
we will pretend that air resistance is the only resistive force).
Worked Example
Force from
wheel on road
Force from road
on wheel.
(Driving Force)
Air resistance
on car.
Force from
car onto air.
Newton’s Third Law applies to
each pair of forces.
objects. 

Forces on a Car
• This diagram shows us what is happening to the car and the objects it is
interacting with from the point of view of Newton’s Third Law.
• If we want to view the car from the point of view of Newton’s First Law we
simply take away the forces that are not acting on the car.
• Now we are only seeing the forces acting on one object – the car.
Worked Example
Force from
wheel on road
Force from road
on wheel.
(Driving Force)
Air resistance
on car.
Force from
car onto air.
Newton’s Third Law applies to
each pair of forces.
objects. 

Not in Equilibrium
• In this diagram the car is not in equilibrium. There is a resultant force acting on it,
so it is accelerating.
• Again, this diagram shows us what is happening to the car and the objects it is
interacting with from the point of view of Newton’s Third Law.
• As with the last example, if we want to view the car from the point of view of
Newton’s First Law we simply take away the forces that are not acting on the car.
• However, because the car was not in equilibrium, the forces acting on the car are
not the same size, so it is easier to spot that this is not an example of the Third
Law.
Worked Example
Force from
wheel on road
Force from road
on wheel.
(Driving Force)
Air resistance
on car.
Force from
car onto air.

Newton’s Third Law of motion tells us
about the forces acting on ???
object(s).
Newton’s First Law of motion tells us
about the forces acting on ???
object(s).
Newton’s Third Law of motion tells us
about the forces acting on two
objects.
Newton’s First Law of motion tells us
about the forces acting on one object.
When all the opposing forces acting
on an object are balanced (i.e. the
resultant force is zero) we say that it
is…
…in equilibrium.
Look at the diagram of a tin of beans
on a table below. Why is this not an
example of Newton’s Third Law?
The forces are not acting on two
different objects.
The forces are also not of the same
type.
Learning Check: Quick Questions
1 minute to discuss with the person next to you, before I choose people at random.
on Tin of Beans
Weight (Force of
Gravity on) of Tin of
Beans

Third Law in Equilibrium Situations
Tasks:
The image shows a bicycle that is travelling at a steady speed.
1. Is this object in equilibrium? Explain your answer. [1 mark]
2. Is this an example of Newton’s Third Law? Explain your answer. [2 marks]
Challenge: Explain what is happening in terms of Newton’s Third Law. Draw a
diagram with labelled forces to help.
Learning Check: In-Depth Questions
Force from
pedalling
Air
Resistance

Answers
The image shows a bicycle that is travelling at a steady speed.
1. Is this object in equilibrium? Explain your answer. Yes. An object is in
equilibrium when all the forces on it object are balanced / the resultant force
on the object is zero.
2. Is this an example of Newton’s Third Law? Explain your answer.
• This is not an example of Newton’s Third Law (1)
• Because the two forces are acting on one object (the bicycle) (1).
• Note: The two forces acting on the bicycle are the same size because the
object is in equilibrium (it is moving at a steady speed).
Challenge: Explain what is happening in terms of Newton’s Third Law. Draw a diagram
with labelled forces to help.
• There are two pairs of forces.
• Each pair of forces acts on two objects.
• Each force pair is equal (in size) and…
• Opposite (in direction).
Force from
wheel on road
Force from
road on wheel.
Air resistance
on bike.
Force from
bike onto air.

Exam Style Questions
Please mark this in your book as
“exam questions”

a) When two objects interact they exert forces on each other. Tick the correct
statement. [1 mark]
 The forces are equal in size and act in opposite directions.
 The forces are equal in size and act in the same direction.
 The forces are unequal in size and act in opposite directions.
 The forces are unequal in size and act in the same direction.
b) Which law is this describing?
1
• Newton’s Third Law (of Motion) (1)

The image below shows a cricket bat hitting a cricket ball.
a) Draw an arrow on the diagram to show the force of the ball on the bat.
[2
marks]
2
Force of bat on ball
• Arrow same length, drawn from the bat. (1)
• Arrow in opposite direction. (1)
• Example shown.

The aeroplane pictured below uses a propeller. When the aeroplane’s engine
is turned on the propeller forces air backwards and the aeroplane accelerates
forwards.
a) Explain why there is a forwards force on the aeroplane. [2 marks]
b) When in flight the aeroplane reaches a constant speed as the force from
the engine and propeller is balanced by the force from air resistance. Is
this an example of Newton’s Third Law? Explain your answer. [2 marks]
• Force (backwards / on the air) causes a force of equal size / magnitude (1)
• To act in the opposite direction (accept forwards / to the right) (1)
3
• No. (1)
• The two forces are acting on one object OR Newton’s Third Law describes
the forces acting on two different objects and the aeroplane is only one
object. (1)

Self-Evaluation
• Describe Newton’s Third Law of Motion.
• Apply Newton’s Third Law of Motion to
explain the forces acting in different situations.
• Describe what is meant by an object being in
“equilibrium” and apply Newton’s Third Law of
Motion to equilibrium situations.
For each of the learning objectives, rate your progress
towards completing them using Red, Amber or Green.
• If you are green in every area then what has helped
you / what have you done to make you successful?
• If you are Amber or Red what do you need to know,
do, or be helped with, in order to make you green?
Lesson Focus: What is Newton’s Third Law of Motion?

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Note on Copyright
Photographs and images below are public domain or otherwise copyright free.
Specific images require attribution:
• Light bulb graphic: CheChe, CC BY-SA 4.0 via Wikimedia Commons
• Isaac Newton: James Thronill after Sir Godfrey Kneller, Public domain, via
Wikimedia Commons
• Clenched fist: Ralpharama, CC BY-SA 3.0 via Wikimedia Commons
• Earth Moon: NASA, public domain, via Wikimedia Commons
• Aeroplane: Julian Herzog, CC BY 4.0 via Wikimedia Commons
All other aspects of this resource, including text, diagrams, animations and
photographs are © John Dovey.

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