# Homework Help: Rotational motion of a spinning disk

1. Apr 15, 2012

### november1992

1. The problem statement, all variables and given/known data
A spinning disk is rotating at a rate of 30 rad/s in the counterclockwise direction. The disk is slowing down at a rate of 6 rad/s2. Find the angle through which the disk has turned after 3 s in radians, degrees, and revolutions.

2. Relevant equations

θ=θi + ωi*t + αt^2

3. The attempt at a solution

So when i plugged the variables into the equation I got 36.

θ = 0 + 90 - 54 = 36

I thought the units were in radians so that's what I entered for the radians part and I also got 2062 degrees, and 5.71 revolutions. But I got the question wrong.

(in degrees) = 3610.0000 degrees
(in revolutions) = 10.0300 revolutions

I don't understand what I did wrong.

2. Apr 15, 2012

### PeterO

I see the question as:

Initial ω = 30,
Final ω = 12 [loses 6 each second for 3 seconds]
Average ω = (30 + 12)/2 = 21

21 for 3 seconds gives 63 radians.

I think there should have been a 1/2 factor in the "αt^2" term of your formula !

[Note: I left the rad/s unit out throughout for clarity]

3. Apr 15, 2012

### november1992

aw man. I can't believe i missed that :/
thanks for the help.