*11.  A student (m = 63 kg) falls freely from rest and strikes the ground. During the collision with the ground, he comes to rest in a time of 0.040 s. The average force exerted on him by the ground is +18 000 N, where the upward direction is taken to be the positive direction. From what height did the student fall? Assume that the only force acting on him during the collision is that due to the ground.

 

Impulse-Momentum

Since the ground brings the student to rest.  Since the made up positive and we know  is positive, we know that  is down and must be negative, so - must be positive so we can drop the vector signs.  Solving for  we get

 

Now consider conservation of energy.  We have no non-conservative forces, so before the fall we have all gravitational potential energy and just before they hit the ground we have all kinetic energy

Solve for h

 

 

 

Please send any comments or questions about this page to ddonovan@nmu.edu

This page last updated on January 11, 2020