Consider the two carts in the figure below

 

There is a compressed spring between them, but a rope is holding them together.  Since the carts are in equilibrium, the total force on each of them is .  if the rope is cut, however, the compressed spring exerts an force on each of the objects.

Newton's third law (the action-reaction law) tells us about the forces on the carts that they are equal and

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Reference

Basic Physics: A Self-Teaching Guide