A book of weight W is at rest on a table. A student attempts to state Newton’s third law of motion by saying that ‘action equals reaction’.

Question 24

A book of weight W is at rest on a table. A student attempts to state Newton’s third law of motion by saying that ‘action equals reaction’.

If the weight of the book is the ‘action’ force, what is the ‘reaction’ force?

A the force W acting downwards on the Earth from the table

B the force W acting upwards on the book from the table

C the force W acting upwards on the Earth from the book

D the force W acting upwards on the table from the floor

Reference: Past Exam Paper – June 2017 Paper 12 Q9

Solution:

Answer: C.

For Newton’s third law, the two forces in each pair must be the same type of force and act on different bodies. (The forces should also be equal in magnitude and opposite in direction.) 

Same type of force: For example, the weight of the book is the gravitational force exerted by the Earth on the book while friction between the book and the table is a type of electric force between the atoms of the book and the table. So, weight and friction are 2 different kinds of forces; they are not of the same type and so cannot constitute the pair of forces referred to in Newton’s third law. 

When an object is resting on a table, the weight of the object is equal and opposite to the contact force from the table but this is also not an action and reaction pair as the forces are of different types. The weight of the object is a gravitational force and the support force is an electrical contact force. [B incorrect]

The equal and opposite reaction force to the weight of the object is the force the object pulls upwards on the Earth. [C is correct]

The other pair of equal and opposite forces is the force the table exerts upwards on the object and the force the object exerts downwards on the table. In each pair, the forces must act on different bodies.