# Homework Help: Collisions - did I solve correctly

1. Dec 31, 2005

### haki

Hi,
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
p=v*m
v=sqrt(2*g*h)
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. Dec 31, 2005

### finchie_88

Yer, what you've done looks right

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