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Fellowroot
If a person is in a train traveling at speed v and they observe outside their window a ball set stationary on a nearby road. According to relativity you can say that the ball is moving at speed v while the train remains stationary. Now under this situation, does the ball have kinetic energy?
train has a speed of v with respect to ground. According to train, ball is stationary. so Ball speed with respect to ground is V Yes the ball does have kinetic energy
According to Relativity Principle if an object A is moving and another object B is at rest then both the objects are moving with respect to each other. So, if you are sitting in a train looking the ball in a stationary situation then you realize the train in a stationary situation and the ball moving in reverse direction to that of train. And if you keep yourself in place of ball then you will find that train is moving and subsequently you can say that both, ball and train, are moving wrt each other.