This is an interesting topic. Energy in different Frames of reference. Assume There is a body moving with a velocity of 5 m/s on a flat surface,and I'm running with the same speed, I see that the initial kinetic energy of the body is 0. I stop moving now so my velocity is 0, relative to me the body has a speed now of 5m/s means he as kinetic energy! How is that possible? energy came from anywhere?how is that possible? The only thing i can think off is D'lambert's principle doing work on the body cause i had to stop.but with another example it can easily show that it has nothing to do with it. anybody suggestions ? I guess there has to be work done on the body in the frame reference of me moving(and stopping) but I can't spot a force in the right direction ( normal force is perpendicular to the displacement thus no work) surface is frictionless and I stop with a pole hitting my head. Maybe air friction(Drag)?? Thank You in advanced, Dweirdo BTW This is not an HW question , just something I was thinking about.