# Homework Help: Grindstone angular acceleration

1. Oct 23, 2007

### Vanessa23

[SOLVED] Grindstone angular acceleration

1. The problem statement, all variables and given/known data
Object: a disk-shaped grindstone of mass 1.69 kg and radius 7.95 cm that operates at 707 rev/min. When the power is shut off, you time the grindstone and find it takes 32.1 s for it to stop rotating. What is the angular acceleration of the grindstone? (Assume constant angular acceleration.)

2. Relevant equations
W=.5mv^2
V=distance/t
W=Wo+at
a=alpha, which is angular acceleration

3. The attempt at a solution
V=[707*(2pi/60)]/32.1 = 2.31m/s
W=0
Wo=.5(1.69)(4.50^2) = 4.50J
0=4.50+a(32.1)

however, this is the wrong answer according to the program. It didn't have a problem with the units though, so I don't know what I am doing wrong. Please point out any wrong assumptions or completely wrong or silly mistakes. THANK YOU!

2. Oct 23, 2007

### hage567

You seem to have W representing two different things here. I don't know what you are trying to do with the first equation you have, but it's not for rotational motion. Also, Wo is your initial angular velocity, I don't understand why you are trying to make it into energy?

All you need for this is your third equation. Change the 707 rev/min into rad/s, that is your Wo. Solve for the angular acceleration.