# Homework Help: Conservation of momentum

1. Oct 5, 2004

### In a "moment"um

If I have a solid disc with a diameter of 30 cm and a mass of 700 grams with a speed of 45 rpm, and a mass of 350 grams falls 6 cm from center what is the speed of the disc. There are also no external torques.

I have no problem solving the angular momentum of the disc before the mass falls on it.

L=Iబ which is L=1/2(.7 *.15^2)*4.9
L=3.85*10^-2

From there I do not know how to incorporate the 350 grams or the 6 cm into the problem.

2. Oct 5, 2004

### In a "moment"um

Never mind I found out what I was doing wrong.

Instead of: L initial = L final I used the: I initial + I gum=I final

I then did: బ initial * I initial + బ gum*I gum=బ final*I final

Since బ gum had no angular speed I ended up with:
బ initial * I initial =బ final * I final
and since I figured up I final I was able to divide it out and get బ final by itself.

బ initial * I initial/I final=బ final

and arrived at the answer. :rofl:

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook