# Rotation Problem

## 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?

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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.

haruspex
Homework Helper
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5 rev per second = 10 pi radians per second

$\alpha$ = 0.4 r/s2
Is it getting faster or slower?

Is it getting faster or slower?
slower. that does not make velocity negative though?

haruspex