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Homework Help: Collisions - did I solve correctly

  1. Dec 31, 2005 #1
    I would like somebody to verify if I have correctly solved a problem.
    The problem is this: Rock of mass m falls from height 20 m to the ground. What is the average force of the ground acting on the rock if it takes 0.025 s to bring the rock to full stop. Express the force in relation to the mass of the rock.
    Keywords that came to my mind are: Collisions, momentum, impusle, mom.-impul. theorem.
    The trick should be in expressing the 2nd newton law with momentum.
    F = m*a is equivalent to F = p/t.
    To get the momentum p. I would use energy conservation. all the potential energy goes to the kinetic energy just before the collision:
    m*g*h=1/2*m*v^2 -> v = sqrt(2*g*h) soo
    F = p/t
    Solution:F= sqrt(2*g*h)*m/t Putting the numbers in it would get us:
    F = sqrt(2*9.8*20)*m/0.025 = 729 * m
    Was my resoning correct?
  2. jcsd
  3. Dec 31, 2005 #2
    Yer, what you've done looks right
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