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Which has higher Kinetic Energy?

  1. Jul 25, 2012 #1

    utkarshakash

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    Gold Member

    1. The problem statement, all variables and given/known data
    A man in an open car moving with high speed. throws a ball with his full capacity along the direction of motion of the car. Now, the same man throws the same ball when the car is not moving. In which case the ball possesses more kinetic energy:
    a)in ground frame
    b)in car frame

    2. Relevant equations



    3. The attempt at a solution
    As seen from the ground frame and when the car is moving the ball's inital velocity is (vel of car+vel of ball). But when the car is at rest the ball's init velocity is only the ball's velocity. So in the first case KE must be higher but I think my solution is wrong as this is not the correct answer.
    As seen from the car frame and when the car is moving the ball's relative velocity = Ball's Velocity-Car's Velocity.But when the car is at rest, the ball's relative velocity is equal to its own velocity. So in the second case KE must be higher but this too is wrong. I can't understand where I'm going wrong. Please help me to point out my mistakes and also tell the correct answer.
     
  2. jcsd
  3. Jul 25, 2012 #2

    jedishrfu

    Staff: Mentor

    So the man throws the ball while riding in the car so the balls velocity relative to the car is the balls velocity. No need to subtract the cars velocity from it.
     
  4. Jul 25, 2012 #3

    utkarshakash

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    Gold Member

    But what about the first case when seen from ground?
     
  5. Jul 25, 2012 #4

    utkarshakash

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    Gold Member

    Thanks. I knew where I did wrong. Actually the ball's velocity as seen from the ground frame when the car is moving = ball's velocity+car's velocity whereas when the car is at rest, the ball's velocity is equal to its own velocity. So in the second case KE must be higher and this is the correct answer. thanks again for the help.:)
     
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