This is a suggested answer to the GCE O Level Pure Physics examination, year 2019, Paper 2, Part B Question 11.

In this question, a diagram drawn by a student showing an action-reaction pair of forces described by Newton’s 3rd Law of Motion was acting on a book. The teacher states that the diagram does not show an action-reaction pair of forces. She adds that forces in an action-reaction pair have similarities and differences.

Students answering this question has to (i) state 2 ways in which the 2 forces in action-reaction pair are identical and (ii) 2 ways in which they are different.

Part (iii) of this question also state “in the diagram, the Earth exerts a force of 10N on the book resting on the table. This is one force in an action-reaction pair.”  Students are required to describe the other force in the pair.

Suggested Solution

(i) The Action and Reaction forces are identical in:

      1. They are equal in magnitude
      2. They are vectors, i.e. they have magnitude and direction.

(ii) The Action and Reaction forces are different in that:

      1. They are opposite in direction
      2. They are on different bodies.

(iii) The other force is the contact force from the surface of the table acting upwards. The contact force is equal and opposite to 10N force exerted by the Earth.