Say there's the following situation: A bullet with some velocity strikes a block connected to a spring. The bullet passes right through the block and the spring is compressed by x cm to the right of the block after the impact. Some internal energy is lost due to deformation of the block while bullet passes through it. Because the question states "compressed AFTER the impact", i was able to assume that the only energy being converted into spring energy is the kinetic energy of the block (When it begins to compress when the bullet has already exited the block), so I solved the problem. HOWEVER, WHAT IF the question had stated "the block compresses the spring DURING the impact", in other words, it compresses WHILE the bullet is passing through the block? Then would the spring energy come from BOTH the kinetic energy of the block and some of the kinetic energy of the bullet while it is moving through the block?