Newton's Third Law (Edexcel IGCSE Physics (Modular))
Revision Note
Written by: Katie M
Reviewed by: Caroline Carroll
Newton's third law
What is Newton's third law?
Newton's third law of motion can be defined as follows:
Whenever two objects interact, the forces they exert on each other are equal in magnitude and opposite in direction
Newton's third law explains the forces that enable someone to walk
The foot exerts a push force on the ground
The ground exerts a push force force on the foot
The forces are equal in magnitude and opposite in direction
Newton's third law of motion applied to walking
The foot pushes the ground backwards, and the ground pushes the foot forwards. Newton's third law explains the forces that enable people to walk
Recognising Newton's third law
Force diagrams can be used to represent 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.
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 rection 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
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
Newton's third law in collisions
According to Newton's Third Law:
When two objects collide, both objects will react, generally causing one object to speed up (gain momentum) and the other object to slow down (lose momentum)
Newton's third law applied to a collision
The force of Trolley A on Trolley B is equal and opposite to the force of Trolley B on Trolley A
Consider the collision between two trolleys, A and B:
When trolley A exerts a force on trolley B, trolley B will exert an equal force on trolley A in the opposite direction
In this case:
FB–A = –FA–B
While the forces are equal in magnitude and opposite in direction, the accelerations of the objects are not necessarily equal in magnitude
From F = ma, acceleration depends upon both force and mass, this means:
For objects of equal mass, they will have equal accelerations
For objects of unequal mass, they will have unequal accelerations
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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