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Homework Help: A football being kicked through air

  1. Nov 14, 2009 #1
    1. The problem statement, all variables and given/known data

    The sentences below might be used to describe the motion of the football as it went through the air during the field goal kicked in the last seconds of the RU vs Louisville game. If you neglect air resistance, which of the following is FALSE?

    The work done by the kicker on the ball was converted entirely to gravitational potential energy of the ball at the top of its path.

    The ball had its lowest speed at the top of its path.

    The total mechanical energy of the ball-Earth system didn’t change after the ball left the kicker’s foot.

    The kicker did work on the ball before it left his foot.

    I said D was the false statement.

    Since the ball was kicked, its a parabolic motion so at the top of it's path, the velocity is zero so B is true. The energy you started with initially is the amount you always ends up with, so C is true.

    I was stuck between A and D. I think D is false since Work is FD, so it's possible he can apply a force on the ball before it leaves his foot, but it needs to travel a distance to be considered work?

    Letter A on the other hand, I'm not sure if the energy is completely converted to potential at the highest point. The ball still has a velocity in it's horizontal component so would've it have KE as well at the highest point?
  2. jcsd
  3. Nov 14, 2009 #2


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    Homework Helper

    It's true that no work is done on something that isn't moving, but that doesn't apply to the football. The kicker's foot stays with the football for a short time while the football begins to move, and that's how the work is done on it. (If you think about it, the ball definitely gains some kinetic energy, and there's no way that could happen without some work being done)

    Sounds reasonable to me...
  4. Nov 14, 2009 #3
    Thanks for the clarification, i guess sometimes a little common sense is needed :tongue:
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