Newton's Third Law (AQA GCSE Physics): Revision Note
Exam code: 8463
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Newton's Third Law
Newton's third law of motion describes the effects of the forces involved when two different objects interact with one another
This is different to Newton's first and second laws, which describe the effects of forces acting on a single object
Newton's third law of motion states:
Whenever two bodies interact, the forces they exert on each other are equal and opposite
Newton's third law explains the following important principles about forces:
When two objects interact, the forces arise in pairs
if object A exerts a force on object B, then object B exerts an equal and opposite force on object A
Force pairs are of the same type of force
for example, if object A exerts a gravitational force on object B, then object B exerts an equal and opposite gravitational force on object A
Newton's third law explains the forces that enable someone to walk
The image below shows an example of a pair of equal and opposite forces acting on two objects (the ground and a foot):
The foot and the ground are the two objects interacting
The foot exerts a push force on the ground
The ground exerts a push force on the foot
The forces are equal in magnitude and opposite in direction
Recognising Newton's third law
Use the following three rules to help you identify a third law pair:
The two forces in a third law pair act on different objects
The two forces in a third law pair always are equal in size but act in opposite directions
The two forces are always the same type: weight, reaction force, etc.
If you are struggling to name the force, just describe it as a push or pull force
Worked Example
A physics textbook is at rest on a table. Student A draws a free body force diagram for the book and labels the forces acting on it as weight and reaction force.
Student A says the diagram is an example of Newton's third law of motion. Student B disagrees with Student A.
By referring to the vector diagram, state and explain who is correct.
Answer:
Step 1: Identify the forces and objects involved
The gravitational pull of the Earth acts downwards on the book (weight) and the push force of the table acts upwards on the book (normal contact force)
Step 2: State Newton's third law of motion
Whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction
Step 3: Check if the diagram satisfies the two conditions for identifying Newton's third law
Newton's third law identifies pairs of equal and opposite forces, of the same type, acting on two different objects
In this example:
both forces are acting on the book
the forces acting on the book are different forces: normal contact force and weight
the image below shows how to apply Newton's third law correctly in this case, considering the pairs of forces acting:
The third law pairs in this scenario would be:
The gravitational pull of the Earth on the book (weight) and the gravitational pull of the book on the Earth (weight)
Both forces are the same type (weight)
Both forces are equal and opposite
The arrows in the vector diagram of the book on the table are equal and opposite, which is where lots of students get confused
This is because the forces are balanced
But the forces are both acting on the book (one object)
Remember that Newton's third law describes two different objects interacting
Step 4: Conclude who is correct
In this case, Student B is correct
The vector diagram in the question is an example of Newton's first law
In the vector diagram of the book on the table, both forces are acting on one object, and the forces are not the same type
Examiner Tips and Tricks
Remember that pairs of equal and opposite forces in Newton's third law act on two different objects. It's a really common mistake to confuse Newton's third law with Newton's first law, so applying this check will help you distinguish between them. Newton's first law involves forces acting on a single object. These differences are shown in Scenario 1 (Newton's first law) vs. Scenario 2 (Newton's third law)
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