# Homework Help: Frictional Energy?

1. Dec 13, 2006

### gloryrentgirl

Frictional Energy??

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

A 145 g baseball is dropped from a tree 8.0 m above the ground.
(b) If it actually hits the ground with a speed of 7.80 m/s, what is the magnitude of the average force of air resistance exerted on it?

2. Relevant equations

I've tried this problem several times, but I do not know how to find air resistance using the energy of the problem. Any help would be appreciated!

(air resistance) h= (delta)KE +(delta)PE

3. The attempt at a solution

(air resistance)=(m(.5(v^2)-gh))/h
Using this I got 5.56 N as an answer, but apparently that's wrong.

Any help whatsoever is appreciated!

2. Dec 13, 2006

### Kurdt

Staff Emeritus
You can use the kinematic equations to find the actual acceleration of the ball. You know that minus any air resistance it should be the acceleration due to gravity g. You can then take g away from the acceleration you find to find the net acceleration due to air resistance. Multiply this by the mass of the ball to find the force.

3. Dec 13, 2006