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Homework Help: Force exerted to a falling object

  1. Sep 9, 2012 #1
    1. The problem statement, all variables and given/known data

    A rock with a weight of 120 N free falls from height of 2.00m collides with with earth and digs 60 mm into it. What is the Earth's average force of resistance

    2. Relevant equations

    F=m*g => m=F/g

    m*g*h = 1/2*m*v^2


    3. The attempt at a solution

    I found the velocity of a rock just before collision using the law of conservation of energy. I now know the velocity which I got 6.3 m/s but now im stumped on how do i find the forces exerted by earth to stop the rock from digging deeper.
    Last edited: Sep 9, 2012
  2. jcsd
  3. Sep 9, 2012 #2
    Hmm i would do it another way

    Before hitting ground its velocity is 6.3m/s right??

    So that velocity becomes 0 due to some acceleration (due to resistance force)

    so using v2-v02=2(a)(s)

    u find acceleration (retardation in this case)

    now that multiplied to the mass will give u the force....

    Hope it helps
  4. Sep 9, 2012 #3


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    Gold Member

    Think of the 20mm as a 'stopping distance'.
  5. Sep 9, 2012 #4
    i assume you substitute s with 0.006 m right. I tried that in that case

    [itex]v^{2}_{f}[/itex]=[itex]v^{2}_{i}[/itex] + 2 * a * x

    -0.012a=39.69 / (-0.12a)
    a=330.75 m/s^2


    F= 12 * 330.75 = 3969 N

    however the answer should be 4.1 kN
    Last edited: Sep 9, 2012
  6. Sep 9, 2012 #5
    use mass = 120/9.8

    it gives an approximate answer
  7. Sep 9, 2012 #6
    getting closer :P thanks I guess it was just an error in rounding the number
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