Finding the average force

1. Sep 9, 2008

physics19

A 68 kg man drops to a concrete patio from a window only 0.60 m above the patio. He neglects to bend his knees on landing, taking 2.0 cm to stop. What is the magnitude of the average stopping force?

-From this problem I have already found that the average acceleration is 293.95 m/s^2. I tried to find the average force by using the formula F=ma. F=68kg(293.95m/s^2)=199988 N. However that is not right. I need is a formula to compute the average stopping force. Thank you.

2. Sep 9, 2008

Redbelly98

Staff Emeritus
Welcome to Physics Forums, physics19!

Your method looks good, but you have an extra factor of 10 for some reason.

Try multiplying 68 x 293.95 again.

3. Sep 9, 2008

physics19

Thank you.. I actually copied down the wrong number when I inputed the answer. I got 19988N as my answer but that is not correct. My friend came up with the same result too and both of us are scratching our heads because we are not sure why it is not the correct answer. I was thinking that there may actually be another force that I have to account for but am not sure.

4. Sep 9, 2008

PhanthomJay

Yes, there is another force. The wording of the problem is a tad bit confusing, but it does ask for the stopping force, which I take as the normal force of the ground on the person, and not the net force, which you have calculated. The normal force acts up; what other force on the person acts down??

5. Sep 9, 2008

Redbelly98

Staff Emeritus
Okay, I might know what's going on now.

Acceleration x mass gives the net force acting on the man.

"Stopping force" refers to the force of the concrete/ground on the man. What other force also acts on him?

6. Sep 9, 2008

physics19

hmmm I am not sure... this is a very confusing problem that the teacher has asked us!

7. Sep 11, 2008

Redbelly98

Staff Emeritus
What force caused the man to drop in the first place?

8. Feb 12, 2009

the gravity.