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Homework Help: Mechanical Energy

  1. Oct 24, 2008 #1


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    1. A 1100 kg car at rest at the top of a hill accelerates down the road, reaching a speed of 20 m/s after descending a height of 120 m. What was its total change in mechanical energy (KE plus PEG) as of that moment in Megajoules?

    2.KE = 1/2 mv^2 and PEg = mgh

    3. I used the above formulas to figure out KE = 2.20E5 and PEg = 1.3E6. When I add both together, I get 1.51E6 and convert it to MJ = 1.51MJ. When I put in the answer, this is incorrect. Am I missing something?
  2. jcsd
  3. Oct 24, 2008 #2


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    The KE at the bottom of the hill already includes the m*g*h from the descent. You probably want to just use the KE as the answer.
  4. Oct 24, 2008 #3
    It starts high and ends low, so it LOSES potential energy. It starts at rest and finishes in motion, so it gains kinetic energy. (I think)
  5. Oct 24, 2008 #4
    kraus is right: so how much mechanical energy does the car lose?
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