# Where does the energy required to speed the object in this question come from if the object actually slows down from another perspective?

I was thinking about mechanics and energy and stumped myself with this scenario I made up:

Suppose you are in a room and you are motionless. In the same room, a 1 Kg ball is moving to your right with a speed of 1 m/s. You arrest the ball's motion, so the ball loses its one joule of kinetic energy and also becomes motionless.

However, I lied. The room is actually in space, and is going to my left at a speed of 100 m/s, and the ball is also going to the left from my perspective, but slightly slower at 99 m/s. From my perspective, the ball gains kinetic energy, despite the opposite being true from your perspective.

How is this possible?

• You do work on the ball regardless of whether you slow it down or speed it up. I don't understand the question. – ACuriousMind Oct 31 '15 at 21:31
• As 'CuriousOne' said, the value of a kinetic energy depends on the observer or frame of reference. You can't just say that the kinetic energy of a ball or other object is some value. One way or another you have to state what frame of reference you are using. Sometimes the frame of reference is obvious, so it isn't explicitly stated but that doesn't mean that it's not important. Your example merely illustrated the fact that kinetic energies do depend on the frame of reference. – Samuel Weir Oct 31 '15 at 23:42