Rotation Problem: Angular Displacement & Spin Time Calculation

[KTiaam]

Homework Statement

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?

 

[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.

btw answer is supposed to be 924 Rad.

 

[haruspex]

5 rev per second = 10 pi radians per second

$\alpha$ = 0.4 r/s²

Is it getting faster or slower?

 

[KTiaam]

Is it getting faster or slower?

slower. that does not make velocity negative though?

 

[haruspex]

It'll make alpha negative.