# Homework Help: Angular velocity?

1. Aug 9, 2011

### khurram usman

A disk of radius R and mass M is mounted with its axis vertical.It is inially at rest.A bullet of mass m and velocity v is fired horzentally and tengential to disk .It enters the disc at its perimeter.What angular velocity will be acquired by disk?
1=mv/(m+M)R
2=mv/(2M+m)R
3=Mv/mR
4=2mv/(MR+2mR)
5=(m+2M)v/MR

plz explain how u compute the answer

2. Aug 9, 2011

### cepheid

Staff Emeritus
Have you considered applying the conservation of angular momentum?

Hint: what is the angular momentum of the bullet around the disc's centre just prior to impact? What is the angular momentum due to the combined rotation of the disc+bullet around the disc's centre after the collision?

3. Aug 9, 2011

### khurram usman

ok...i see the approach
the angular momentum of bullet at tangent to disk will be mvR
now how to calculate velocity?

Last edited by a moderator: Aug 9, 2011
4. Aug 9, 2011

Good.

5. Aug 9, 2011

### khurram usman

well as i see it the combined angular momentum after collision will be (m+M)Rv'
correct??

6. Aug 9, 2011

### Staff: Mentor

No, not quite. How would you express the angular momentum of a rotating disk? How does the presence of the bullet stuck onto the disk change that?

7. Aug 9, 2011

### khurram usman

maybe the bullet changes mass distibution
i am not quite getting it

8. Aug 9, 2011

### Staff: Mentor

Sure it does.

But before worrying about disk + bullet, how would you express the angular momentum of a rotating disk alone?