A train moves along the tracks at a constant speed u. A woman on the train throws a ball of mass m straight ahead with a speed v with repsect to herself. What is the kinetic energey gain of the ball as measured by a person on the train? By a person standing by the railroad track? How much work is done by the woman throwing the ball and the train? I'm not completely sure I have these concepts down. With respect to the woman, wouldn't kinetic energy just be (1/2)mv^2? I think this because the woman, ball, and reference frame are all moving at the same speed, initially. But for someone on the side of the tracks, I know the same logic can't apply. The woman and ball are moving at the speed of the train, while the reference frame is stationary...I'm not quite sure where to go with this. Work is just the change in kinetic energy. But I don't know how to deduce the work done by the train. The ball is moving with the horizontal speed of the train. Do I just use that speed in the equation of KE? I'm not really sure how to figure this out. Thank you for your help!