# Homework Help: Conservation of energy and work

1. Mar 17, 2009

### Knfoster

1. The problem statement, all variables and given/known data

A 70.4 kg man steps out of a window and falls (from rest) 1.33 m to a sidewalk. at is his speed just before his feet stke the pavement?

If the man falls with his knees and ankles locked, the only cushin for his fall is a 0.508 cm give in the pads of his feet. Calculate the erage force exerted on him bye ground in this situation. The avera force is sufficient to cause cartilage dage in the joints or to break bones.

2. Relevant equations

PE+KE=PE+KE (before and after)
W=F(xf-xi)cos theta
W=delta KE
PE=mgh
F=ma
KE=.5mv^2

3. The attempt at a solution

I already solved the first question and got 5.11 m/s.
I don't know how to set up the second problem to find the force.
Thank you.

2. Mar 17, 2009

### LowlyPion

I think they want you to calculate the force (acceleration) that will be required if the deceleration is over the space of just 1/2 cm (.508 cm).

3. Mar 17, 2009

### Knfoster

ok. So F=ma, and I need to find acceleration to finish the problem. So Do I use Vf^2=Vi^2+2a(xf-xi) ? If so do I use 5.11 m/s for Vf and 0 m/s for Vo? Is xf=0 and xi=.508 cm?

4. Mar 17, 2009

### LowlyPion

Looks to me like what they are asking.