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Homework Help: Impact Speed

  1. Feb 23, 2009 #1
    1. The problem statement, all variables and given/known data
    Using energy conservation and assuming negligible air resistance, calculate the impact speed of a 4.00 kg cannon ball after being dropped from a height of 55.0 m.

    2. Relevant equations
    KE sub total=KE sub 1+KE sub 2

    3. The attempt at a solution
    I'm completely lost as to how to get this with energy conservation. Can someone please help?
  2. jcsd
  3. Feb 23, 2009 #2


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    Homework Helper

    Welcome to the physics forums! Hope you ask lots of questions here.

    As the ball drops, its gravitational potential energy is converted to kinetic energy. Start with:
    Potential energy at the top = kinetic energy at the bottom

    Fill in the detailed formulas, solve for the quantity you want to find, then put in the numbers you know.
  4. Feb 23, 2009 #3
    Thank you!

    TE=KE+PE sub g
    PE sub g=mgh
    PE sub g=4*9.8*55=2200 J
    v=33 m/s
  5. Feb 23, 2009 #4


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    Homework Helper

    That's it!
    Maybe simpler to write mgh = 1/2m*v^2, cancel the m's, then solve for v:
    v = sqrt(2gh) = sqrt(2*g*55) = 32.8 m/s
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