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?