Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Action-Reaction forces in Newton's Third Law


To study action and reaction forces.


When hook of a spring balance is pulled by a force, the spring inside it gets stretched. A pointer attached to the spring reads the force on the scale of the spring balance.


two spring balances.


Attach the ring of a spring balance (A) to a fixed support on a table. Pass the hook of the second spring balance (B) through the hook of A. Now, pull B by its ring. Keep applying the same amount of pull while you take readings. Note the readings of A and B.



The readings of A and B are equal. What do these readings show? The reading on A gives the magnitude of the force exerted on its hook, i.e., the force exerted by A. The fact that the readings are the same shows that the force exerted by A on B and that exerted by B on A have the same magnitude.

Can you say that these forces have opposite directions? Both the springs are stretched. To stretch the spring, the hook must be pulled away from the balance. So B is exerting a force on A towards the right, and A is exerting a force on B towards the left. So, the forces have opposite direction.