# Rotation Problem

1. Mar 13, 2013

### KTiaam

1. The problem statement, all variables and given/known data

A coin is spinning on its edge at 5 rotations per second.
Friction slows down its spin rate at .4 r/s2

a) what angular displacement does the coin have by the time it's slowed down to half its original angular velocity.

b) how long before the coin stops spinning?

2. Mar 13, 2013

### KTiaam

5 rev per second = 10 pi radians per second

$\alpha$ = 0.4 r/s2
ωi (angular velocity initial) = 10 pi r/s
ωf (angular velocity final) = 5 pi r/s

Δθ = angular displacement

Known equations

ωf2 = ωi2 + 2$\alpha$(Δθ)

I tried plugging the know variables in but i keep getting a negative answer.

3. Mar 13, 2013

### haruspex

Is it getting faster or slower?

4. Mar 13, 2013

### KTiaam

slower. that does not make velocity negative though?

5. Mar 13, 2013

### haruspex

It'll make alpha negative.