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Homework Help: Work and average force

  1. Dec 12, 2004 #1
    A meteor has a speed of 79.8 m/s when 787 km above the Earth. It is falling vertically (ignore air resistance) and strikes a bed of sand in which it is brought to rest in 3.19 m.
    ~How much work does the sand do to stop the meteor (mass = 556 kg)?
    ~What is the average force exerted by the sand on the meteor?
    ~How much thermal energy is produced?

    A meteor falls to earth and hits sand. The work done by the sand to stop it should just equal the change in kinetic energy. I found the final velocity before it hit the sand, and from that I found the decceleration of the meteor in the sand. That acceleration times the mass and multiplyed by the distance it took to stop should give the total work done by the sand as long as energy is conserved. But it doenst work!

    As for the average force, that should be the mass of the meteor*the acceleration. that doest work either
  2. jcsd
  3. Dec 12, 2004 #2
    Hmm, how are you sure that these are wrong?
    OK, KE = 1/2mv^2 and v at that low point = (79.8^2 + 19.6(787000))^0.5 = (15431568.04)^0.5 = 3928.30ms^-1
    So KE = 4289968367.42J = W, is that what you got?

    F = ma
    0^2 = (3928.30^2 + 2(-3.19)(a))
    a = 2418736.817ms^-2
    F = ma = 556(2418735.817) = 1344817670N
    Q = I suppose the work, which was mentioned above.

    Is this what you got? Just wanted to try it myself. :-\
  4. Dec 12, 2004 #3
    thats wat i got too, but i know that they're wrong becuase we have to plug our answers into this website. however, the website says the answer is incorrect...
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