If you push something and it doesn't move does it have energy? So if I push something to apply a force, that requires energy right, but if the object like a building for example, doesn't move where does the energy go? Another way I was looking at it was if a ball collides with another stationary ball and the collision is completely elastic, then the first moving ball will hit the stationary ball and stop and the target ball will move away with the same velocity as the first one. But let's take the same moving ball and let's say it collides with a stationary building, but instead it bounces back because it obviously cannot give up all its momentum and kinetic energy to the target (the building) like it did in the example with the stationary ball. So why exactly does the ball bounce back then?