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Homework Help: Potential Energy/ The Isolated system

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

    A ball of mass m = 8 kg is dropped from rest at a height h = 13.9 m above the ground, as in Figure 7.4. Ignore air resistance.

    If the ball is being released with a downward speed 4.9 m/s initially, what will be its final speed when it hits the table 0.7 m below the release point?

    2. Relevant equations

    Kf+Ugf=Ki+Ugi which is 1/2mvf2+mgy=1/2mvi2 +mgh

    3. The attempt at a solution
    I am confused in the wording of the question. What do they mean by "final speed when it hits the table 0.7m below the release point?"
  2. jcsd
  3. Apr 1, 2009 #2


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    Gold Member

    They mean the full speed it was going right when it hits the table. Of course the table stopping it will quickly accelerate it down to a velocity of 0, but we want to know the velocity before the table actually stops it.

    The important information is .7m. How much kinetic energy will it have when it's fallen .7 meters from where it was dropped?
  4. Apr 1, 2009 #3
    So it's just asking for the speed of the ball after it has fallen 0.7m?
  5. Apr 1, 2009 #4


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    Homework Helper

    Apparently so.

    I will presume that the first question was the speed or kinetic energy when it hit 13.9m below?
  6. Apr 1, 2009 #5
    Nope. The first question asked for the speed at an arbitrary height before the ball hit the ground. In that case it was 5.2m
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