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Homework Help: Mechanics question on dropping soldiers

  1. Oct 4, 2008 #1
    1. The problem statement, all variables and given/known data

    this question is from the book introduction to mechanics.. question 4.12

    During the Second World War the Russian, lacking sufficient parachutes for airborne operations, occasionally dropped soldiers inside bales of hay onto snow. The human body can survive an average pressure on impact of 30 lb/in2. Suppose that the lead plane drops a dummy bale equal in weight to a loaded one from an altitude of 150 ft, and that the pilot observes that it sinks about 2 ft into the snow. If the weight of an average soldier is 144 lb and his effective area is 5 ft2, is it safe to drop the men?

    2. Relevant equations

    So, I use work-energy theorem to solve it.. but i could not find the later part after this equation..

    work-energy equation i got is : Kb-Ka = integration(a,b) (F •dR)

    3. The attempt at a solution

    I am not sure whether my work energy equation is correct.. So anyone here would like to help me? sorry for trouble.. thx..
    maybe anyone of you guys want to give me some hint on it.. thx alot..
    Last edited: Oct 4, 2008
  2. jcsd
  3. Oct 4, 2008 #2


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    There are a number of ways to solve this problem, but if you use the work energy theorem, you've got to include the work done by gravity also. Also, the work done by the snow on the man can be obtained by its average force times the distance through which it acts..no need to integrate if you don't have to. Rather than get confused by the work-energy approach, you might want to try the conservation of total energy principle (Ka + Ua - W_snow = Kb + Ub), where Ka and Kb =0. Or otherwise, find the man's speed as he hits the ground, then his deceleration over the 2 foot snow pit, then the force acting on him per newton 2, then the pressure.
  4. Oct 5, 2008 #3
    hey jay thankx.. but i am not really understand why is ka n kb = 0 ?
  5. Oct 5, 2008 #4
    anyone would like to give me some hint on my question ?

    y is kb n ka = 0 from the previous.. thx..
  6. Oct 5, 2008 #5


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    In the absence of other data, this problem is looking in the vertical Y direction only, and ignoring the horizontal motion of the soldier (he would have an initial speed in the X direction due to the plane's horizontal speed). So, I'm assuming this is like a soldier dropped from a cliff rather than an airplane. Point A is the start point; his vertical initial speed is 0, thus Ka = 0. Point B is the end point at 2 feet below the snow; he comes to a stop at that point, thus his speed is also 0 at that point, and thus Kb=0.
  7. Oct 5, 2008 #6
    wow... i should have thought so..
    really thankx for your patience in answering my question..
    really thankx alot.. and i found the answer.. thankx......
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