# Homework Help: Angular momentum preservation problem

1. Jan 19, 2012

### Guille_L

1. A rod (A-B) with the length L falls straight down with only translational velocity. The angle between the ground and the rod is O. When the rod hits the ground, the end A hits a step which stops A from sliding. Determine the angular velocity right after it hits the ground. http://oi43.tinypic.com/2a7zxpz.jpg

2.
I = moment of inertia
v = velocity
m = mass
w = angular velocity

I = (mL^2)/3

Angular momentum:
LsinO*m*v = Iw = m*L^2*w/3

w = 3v/L

OR

T = kinetic energy = Iw^2/2 becomes
V = potential energy = mgLsinO

Put them equal and you get

w^2 = 3gsinO/L

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I know I am not getting this 100% just dont know how to combine it.
I have used the following as help:
http://www.mech.kth.se/~hanno/LosnTent5C1140Mar05.pdf
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jan 19, 2012

### rcgldr

I'm not sure momentum is conserved (unless you include momemtum of the earth). In the case where the rod is vertical, the rod just stops at the time of collision. The collision is inelastic, so energy is not conserved either.