While studying energy on Sears & Zemansky's University Physics, I came up with a doubt on the meaning of kinetic energy. The book gives two possible physical interpretations of this quantity. I'm okay with the first meaning of KE but I don't understand completely the second one. I don't understand how the particle can do work just because it owns KE. Consider a ball with velocity ##v## that meets a spring, the spring is compressed and the ball is stopped. Following the previous interpretation of the kinetic energy, the ball should do work on the spring because of its KE. But does this really happen? In the collision with the spring exerts a force ##f## on the ball and the ball exerts a force ##-f## on the spring. Are the two works done by the two forces equal and opposite?