1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    v=√(2*g*h)

    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

    CAF123

    User Avatar
    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=6.3^{2}+2*a*0.06
    0=39.69+0.12a
    -0.012a=39.69 / (-0.12a)
    a=330.75 m/s^2

    F=m*a

    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
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Force exerted to a falling object
Loading...