# Homework Help: Energy and Work of a bullet

1. Jan 10, 2010

### lilwigz

1. The problem statement, all variables and given/known data
A 5.00g bullet moving at 600.0 m/s penetrates a tree trunk to a depth of 4.00 cm.

A) Use work and energy considerations to find the magnitude of the force that stops the bullet.

B) Assuming that the frictional force is constant, determine how much time elapses between the moment the bullet enters the tree and the moment the bullet stops moving.

2. Relevant equations
Ek= 1/2mv^2
W=fdcos(0)

3. The attempt at a solution

I tried to do Ek=1/2mv^2
Ek= 1/2 x ( .005kg) x (600m/s)^2 but i think thats wrong

2. Jan 10, 2010

### jegues

This is how much energy the bullet intially has.

All this energy is transfered as the bullet travels through the tree a distance of 4cm.

So how can we solve for the magnitude of the force which stops the bullet?

HINT: Use kinetic energy-work theorem.

Last edited: Jan 10, 2010
3. Jan 10, 2010

### lilwigz

with the formula Fd(cos0)= (1/2 mvf^2)-(1/2mvi^2) i got -900J/.04m and i got the answer 22500N horizontally

4. Jan 10, 2010

### Matterwave

looks good to me

5. Jan 10, 2010

### jegues

They are asking for the magnitude of the force so you don't have to indicate a direction ("horizontally" in your case).

22500N is good enough! Now try part B!

6. Jan 10, 2010

### lilwigz

since the frictional force is constant, the acceleration would be constant, so

v²=u²+2as
0=600²+2a(0.04)
a=-360000/0.08 = -4.5x10^6

a=(v-u)/t
t=(v-u)/a
=0-600/(-4.5x10^6)
=0.133 ms

7. Jan 10, 2010

### jegues

Looks good to me!

8. Jan 10, 2010

### Matterwave

You can find the acceleration a by Newton's law: F=ma

Iono how you're trying to find it there...answer looks right though.

9. Jan 10, 2010

### lilwigz

thank you!

10. Jan 10, 2010

### jegues

How would you do that? You have no way of computing the frictional force.

11. Jan 10, 2010

### Matterwave

You just did in part A
...

12. Jan 10, 2010

### jegues

Doh! Brainfart :)