Qstnid -I-15-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? - 05: Newton's Laws Of Motion

with Solutions - page 2

#15
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?
Ans : Yes, the forces due to the spring on the two blocks are equal and opposite.
But it's not an example of Newton's third law because there are three objects (2 blocks + 1 spring). Spring force on one block and force by the same block on the spring is an action-reaction pair.