# Bullet hitting a door problem

sp3sp2sp

## Homework Statement

A 10 g bullet traveling at 370 m/s strikes a 8.0 kg , 0.80-m-wide door at the edge opposite the hinge. The bullet embeds itself in the door, causing the door to swing open.
What is the angular velocity of the door just after impact?
Express your answer to two significant figures and include the appropriate units.

## Homework Equations

use conservation of momentum for slab about edge: linear momentum = angular momentum
mv = Iw
I = (1/3)ML^2 (for a slab)
w = (3mv) / (ML^2)
(w = omega)

3. The Attempt at a Solution

w = (3mv) / (ML^2)

w = (3* 0.01kg * 370m/s) / (8.0kg * 0.80kg^2)
w = 2.167 = 2.2 rad/s
= wrong

cant see what I am doing wrong, thanks for any help

Mentor
use conservation of momentum for slab about edge: linear momentum = angular momentum

Linear momentum and angular momentum are different quantities with different units. They cannot be set equal. In this case, only one of those is conserved.

Hint: Use conservation of angular momentum. (What's the angular momentum of the bullet just before impact?)

sp3sp2sp
bullet doesn't have any angular momentum before impact So Iw = 0?

Mentor
bullet doesn't have any angular momentum before impact
Sure it does (about the door hinge axis). Read this: Angular Momentum of a Particle

So Iw = 0?
No.