# Homework Help: Angular acceleration and displacement problem

1. Nov 25, 2012

### Lotus93

I've listed the whole problem because I know I'll have more questions, but all I want to know atm is whether I'm plugging the right values in to the equation. Thanks in advance for your help.

1. The problem statement, all variables and given/known data
A compact disc speeds up uniformly from rest to 5200rpm in 620rad. The disc’s diameter is 5.0cm.

a. What is the angular acceleration in rad/s^2?
b. How long does it take to reach this speed?
c. What is the total angular displacement in degrees?
d. How many revolutions does the disc make in this time?
e. What is its centripetal acceleration at maximum speed?

2. Relevant equations

θ = (1/2)αt2
ωf2 = ωi2 + 2αθ

3. The attempt at a solution
For part A, first I would convert angular velocity to rad/sec
So 5200 rot/min = (5200*2pi) rad/(60 sec) = 544 rad/sec
Since I don't know time yet, I would use the second equation above.

The first thing I want to ask is if my values for ωf, ωi, and θ are correct.
ωf, = 544
ωi = 0
θ = 620 radians

2. Nov 25, 2012

### haruspex

Yes. As a matter of technique, I always keep things in algebraic variable form (i.e. using the labels for the quantities) until I have the final equation, and only then plug in the constants. This makes it easier to find algebraic errors, easier for others to follow, and avoids propagation of rounding errors.

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