# Homework Help: Bullet speed-

1. Feb 3, 2004

### Crashdowngurl

I am not to great at physics and I was hoping someone could point me where I am going wrong in this problem! :) My answer I came up with is 2.5 m/s but that isn't one of my options so I know something has to be off.

5. To find the speed with which a gun fires bullets, a bullet of mass 5 gm is fired horizontally into a 10 Kg block of wood resting on a frictionless table. The bullet gets embedded in the block and the block is found to have a velocity of 0.5 m/s after collision. What is the speed of the bullet?

Bullet Speed= 2.5 m/s

Mv1i + 0 = (m +M)V
Mv1i + 0 = (5gm +10 kg) .5 m/s
m1 u1 + m2 u2 = (m1 + m2) V
(5gm u1) + (10kg *0m/s) = (5gm + 10kg) .5m/s
(5gm u1) + (0) =7.5
u1 = 2.5 m/s

2. Feb 3, 2004

### dlgoff

Think of the conservaton off momentum. Remember the bullet is embeded in the block.

added by edit: Never mind. I didn't look at your work well enough. You've got the idea. But (5gm + 10kg)=1005gm not 15gm.

regards

Last edited: Feb 3, 2004
3. Feb 3, 2004

### Crashdowngurl

So... :)

So would this be more on track:

Mv1i + 0 = (m +M)V
Mv1i + 0 = (5gm +10 kg) .5 m/s
m1 u1 + m2 u2 = (m1 + m2) V
(5gm u1) + (10kg *0m/s) = (5gm + 10kg) .5m/s
(5gm u1) + (10kg *0m/s) = (1005gm) .5m/s
(5gm u1) + (0) = 502.5
u1 = 100.5 m/s

Bullet Speed= 100.5 m/s

Thank you so much for your help! :)

Emma

4. Feb 3, 2004

### dlgoff

Any time. And welcome to the forums.