Question

Two blocks of unequal masses are tied by a spring. The blocks are pulled stretching the spring slightly and the system is released on a frictionless horizontal platform. Are the forces due to the spring on the two blocks equal and opposite? If yes, is it an example of Newton's third law?

Answer 1

Yes, the forces due to the spring on the two blocks are equal and opposite.

No, the given is not an example of Newton’s 3rd Law of Motion.

Explanation:

When the two blocks are pulled in the opposite direction attached to a common string, a tension is developed in the string.  

Force developed in the String  (F) = -k.x

Where k is spring constant and x is displacement produced.

This force is equal to the Tension developed in the spring. This will be for Body A and spring attached at body A, and Body B and spring attached at Body B.  

Newton’s 3rd Law Statement  is as follows

Every action has equal and opposite reaction.  

Here action-reaction is 2 body systems, equal in magnitude, opposite in direction and acting on two different bodies.

Here in this example, there are three body systems. 3rd Law is applicable to the body and spring system and not the Body A and Body B system.