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Homework Help: Conservation of mechanical energy of ball of mass

  1. Apr 1, 2008 #1
    1. The problem statement, all variables and given/known data
    A ball of mass 240 g is moving through the air at 20.0m/s with a gravitational potential energy of 70J. With what speed will the ball hit the ground?

    2. Relevant equations
    Eg = mgh
    Ek = 1/2mv^2
    W = mgd

    3. The attempt at a solution
    this is what i did:
    at 0m, potential energy is 0 so kinetic energy must have 70J now.

    Ek = 1/2mv^2
    70 J = 1/2(0.24kg)v^2
    v = 24.15 m/s

    I dont know what to do with the 20 m/s that was given and yeah....please help!
  2. jcsd
  3. Apr 1, 2008 #2
    The potential and kinetic energy that it had initially is now all kinetic by the time it reaches the ground.
  4. Apr 1, 2008 #3
    you have

    potential energy + kinetic energy = total energy = constant

    What is the total energy whe the ball is moving through the air? it's not 70 J.

    At a height of 0 m, potential energy is indeed 0, so all the energy must be kinetic.
  5. Apr 3, 2008 #4
    since he total mechanical energy is conserved equate the initial total mechanical energy
    (pe+ke)to the final toal mechanical enrgy.
    remeber pe=mgh ,so what is pe at ground level?
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