# Homework Help: Calculate the constant force

1. Jan 3, 2008

### mortho

1. The problem statement, all variables and given/known data
A 77 kg pole-vaulter falls from rest from a height of 4.8 m onto a foam-rubber pad. The pole-vaulter comes to rest 0.24 s after landing on the pad.
a)Calculate the athlete's velocity just before reaching the pad.
b)Calculate the constant force exerted on the pole-vaulter due to the collision.

2. Relevant equations

F=p/t

3. The attempt at a solution

for part a i did Vf=Ft/m and 755.4(0.24)/77 and got 2.35 m/s downward

for b i did F=p/t and (77)(2.35)/0.24 and got 754 N upward.

I think i calcuated the force for part a wrong and that 's why my answer was wrong.

2. Jan 3, 2008

### dynamicsolo

All right, one problem right off the bat is that the 0.24 seconds is the time it takes for the force from the foam pad to bring the pole-vaulter to a stop, not the amount of time it took them to fall 4.8 meters. To answer (a), you will need to calculate how long that takes.

Your equation and how you used it otherwise is fine, although a bit roundabout: you used the vaulter's weight and divided it by their mass, which gives you g. So you could have just used V_f = gt for their fall. Now you need to find t .

Last edited: Jan 3, 2008
3. Jan 3, 2008

### cryptoguy

For part A, keep in mind that you do not know the time it takes for him to fly until the pad, it is not .24 seconds (that is the time he is in contact with the pad itself). You can use kinematics or energy to do part A.

Part B looks right.

edit: yep ditto dynamicsolo, posted at the same time :D

4. Jan 3, 2008

### mortho

Oh thanks guys i figured this one right after i posted it. but thanks anyways