# Homework Help: Momentum and circular motion

1. Dec 13, 2004

### mikep

A 2g bullet hits a 5kg wood block, which hangs from a 1.5m long string. This causes the block to swing through an arc of 5°. What was the speed of the bullet before it hit the block?

I know that I need to use the momentum and circular motion equations. Can someone tell me what I need to do with the 5°?

2. Dec 13, 2004

### Tide

You can use that angle to determine by how much the block rose from its intial position which will come in handy when you try to find its speed immediately after it was struck by the bullet. Think energy conservation!

3. Dec 13, 2004

### Tjl

What you are describing is a ballistic pendulum. A simple device used to measure the speed of a projectile.

$$v = \frac{m+M}{m}v'$$

$$= \frac{m+M}{m}\sqrt{2gh}$$

H will be as Tide mentioned the distance it rises. You can find that by finding the y component of the angle.

P.S. I skipped the first step of the process, because in order to understand the problem, you need to understand that first step. What help would it be if I told you everything :p