Simple angular problem but answer is wrong

1. Oct 20, 2008

maniacp08

When a turntable rotating at 33 1/3 rev/min is shut off, it comes to rest in 26 s. Assume constant angular acceleration.
(a) Find the angular acceleration.
rad/s2

(b) Find the average angular velocity of the turntable.
rad/s

(c) Find the number of revolutions it makes before stopping.
rev

For Part A I did:
omega = omega initial + alpha * t
omega = alpha * t
omega/t = alpha

(33.3 rev/min) / (26s) * (2Pi rad) / (1 rev) * (1 min) /(60s)
= .1341218402 rad/s^2 = approx .13 rad/s^2
but this is wrong.

For Part C:
displacement = omega initial + 1/2 alpha * t^2
= 1/2(.1341218402 rad/s^2)(26s)^2
=45.33318199 approx = 45 revolutions.

For Part B is it taking the average of initial omega velocity and final omega velocity?

2. Oct 20, 2008

LowlyPion

Isn't acceleration negative? (V goes to 0.)

Your equation for c) yields radians not revolutions.

b) yes. Initial angular velocity over 2.

3. Oct 20, 2008

maniacp08

Hmm, I put -.13 rad/s^2 but it was still wrong and as for part C I calculated to approx 71 revolutions and it was wrong. what else I did wrong?

Initial angular velocity / 2
initial angular velocity is = angular acc. * 1s correct?

4. Oct 20, 2008

LowlyPion

33.33 = 100/3
My calculation for angular acceleration is w = 2*π*f = 2*π*100/(3*60)

Since w = a*t => a = w/t = 2*π*100/(3*60*26) = -.13426 r/s2

Total radians = 1/2*a*t2 = 45.38 radians

Revolutions = radians/(2*π) = 7.22 revolutions

It couldn't have been 71 revolutions because it only was going 33/min. At full speed it can't be over 33/2 since 26 sec is < 1/2 min.

w = 3.49 rad/s and half is 1.75 rad/sec

5. Oct 20, 2008

maniacp08

Oh, I guess I should've put more decimal places rather than just -.13 rad/s^2
and for part B, you are right it was 7.22, I saw it as 72.

Thanks for the clarification!

6. Oct 20, 2008

LowlyPion

Not sure what the number of significant digits are for your teacher. Perhaps there is something from class?

They ask for rad/sec2 and that has to be a small fractional number.

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