# Homework Help: Find the magnitude of the force that stops the bullet

1. Dec 9, 2004

### Tabe

Ok, I am totally lost on how to solve this problem: A 5 g bullet moving at 600 m/s penetrates a tree trunk to a depth of 4 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 trunk and the moment the bullet stops moving.

I know the answers are 2.25*10^4 N, and 1.33*10^4 s, but I have no idea how to get them

2. Dec 9, 2004

### futb0l

a) Using the work energy theorem we get taht...
* remember to convert the units

$$\frac{1}{2}(0.005)(600)^2 = F(0.04)$$

Solving for F gives 22500N

3. Dec 9, 2004

### Tabe

Thanks, I really was lost because of the "depth" part.

4. Dec 9, 2004

### futb0l

b)

m = 0.005
F = 22500N

a = F/m
a = (v-u)/t

that gives
t = (v-u)/a

v = 0
u = 600

so.. i found that:
t = 0.000133333333

So you see :)