A train is moving along the tracks at a constant speed u. A girl on the train throws a ball of mass m straight ahead along the direction of motion of the train with speed v with respect to herself. Then
1
Kinetic energy of the ball as measured by the girl on the train is mv2/2.
2
Work done by the girl in throwing the ball is mv2/2.
3
Work done by the train is mvu.
4
The gain in kinetic energy of the ball as measured by a person standing by the rail track is mv2/2.