Conservation of Rotational Momentum and changes in Rotational Speed

[physicsflunky]

Homework Statement

There is a record spinning when a blob of putty falls vertically on its edge; which is .10m from the center. What is the rotational speed of the record after the blob sticks on it?

additional information:

mass of blob = .040kg
mass of record = .10kg
rotational inertia of record before collision = .0005kg*m^2
rotational speed before collision = 4.5 rad/s

Homework Equations

I = m(r^2)
Iwi = Iwf

I = Rotational Inertia
wi = Initial Rotational Velocity
wf = Final Rotational Velocity
r = distance from point of rotation to point of applied force

The Attempt at a Solution

(calculate new rotational inertia)
I = m(r^2)
= .040(.10^2) = .0004

.0004 + .0005 = .0009

(set initial momentum equal to final momentum and solve for Wf)
4.5(.0005) = I(Wf)
.00225 = .0009(Wf)
2.5 = Wf

 

[vela]

What you did looks fine. Don't forget the units on your final answer.

 

[physicsflunky]

It was correct. I'll make sure to include units in the future. Thanks.