# Bullet and block

1. Jan 6, 2006

### nrc_8706

a rifle fires a(n) 6g bullet with a muzzle velocity of 390m/s into a block of wood. the bullet comes to rest after it has made a(n) 18cm deep hole in the wood. what is the magnitude of the average frictional force slowed the bullet?

can i use this formula?
J=FT=pf-pi=mv

2. Jan 6, 2006

### Staff: Mentor

You could, but you'd first have to figure out the time. Instead, consider the work done by the frictional force on the bullet.

3. Jan 6, 2006

### Tide

What would you use for the time?

I suggest asking yourself how much work the frictional force does.

4. Jan 6, 2006

### nrc_8706

is it W=Kf-Ko ?

5. Jan 6, 2006

### andrewchang

yes, but make sure not to forget that the work done by friction will be negative.

6. Jan 6, 2006

### nrc_8706

so.....W=1/2mVf^2-1/2mVo^2 does Vf cancel since it comes to rest?

7. Jan 6, 2006

yes, it does

8. Jan 6, 2006

### nrc_8706

the answer is W=456.3 is that all?

9. Jan 6, 2006

### andrewchang

you've found the work, but the question wants to know the force. so what is the equation for work?

10. Jan 6, 2006

### nrc_8706

is it W=Fd

11. Jan 6, 2006

### andrewchang

yes, that's right

12. Jan 6, 2006

### nrc_8706

Thank You So Much!!!!!!