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Homework Help: Kinetic energy due to heatloss

  1. Dec 31, 2011 #1
    1. The problem statement, all variables and given/known data

    A 1600 kg object is initially at rest 400 km above the earth’s surface. The object falls straight
    down and generates 8.3 x10^8J of heat energy while descending to an altitude of 100 km.

    2. Relevant equations

    3. The attempt at a solution
    i made PE at 100km above earth and got 1600x9.81x300000=800V^2+8.3x10^2J
    i got V=2200 but the answer should be 2100 about last 2 digits can be off by a bit im not sure where i go wrong
  2. jcsd
  3. Jan 1, 2012 #2
    What do we need to find here ?
    What is V? velocity?
  4. Jan 1, 2012 #3
    Hint: Your altitude is lofty.
  5. Jan 1, 2012 #4
    sorry for the late reply iv been working 8 hours everyday.

    even after i made PE at 0km and subtracted mgh100km from 400km i got 2200 im not sure what you mean any more hits? please lol?

    honestly i guess i dont really know what lofty means haha
    Last edited: Jan 1, 2012
  6. Jan 1, 2012 #5
    velocity yes sorry for late response iv have been working quite a bit
  7. Jan 1, 2012 #6


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    Hint: PE = mgh assumes gravity is constant. When you're talking about altitudes in the hundreds of kilometers, you can't necessarily make that assumption any more. Do you know any formulas for potential energy that don't assume constant gravity?
  8. Jan 2, 2012 #7
    yeah i got it i was kinda suprised they would make us us m1m2G/r and i found out what lofty ment lol thanks for the help
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