Homework Help: Rotational motion of a turntable

1. Jul 15, 2010

mizzy

1. The problem statement, all variables and given/known data
The turntable of a record player rotates at 33.33rev/min and takes 20.0s to reach this speed from rest. Calculate: a) its angular acceleration, b) the number of revolutions it makes before reaching its final speed.

2. Relevant equations
angular acceleration = angular speed/ time
angular displacement = angular speed x time

3. The attempt at a solution
part a:
I converted the angular speed to rad/s. I solved for angular acceleration, which is delta angular speed divided by time.

part b:
I used the equation, angular speed = angular displacement/time.

therefore, angular displacement = angular speed x time

I then converted rad into revolutions --> 11.1 revolutions. Is that correct??

Thanks

2. Jul 15, 2010

Gear.0

Part A is correct.

For part B you're on the right track, but remember that would only work if the record were spinning at a constant speed of 3.49rad/s. But it is always changing as it goes from 0rad/s up to 3.49rad/s.

You want to do basically the same thing you did, except in place of 3.49rad/s you want to use the average angular velocity.

3. Jul 15, 2010

mizzy

Thanks.

So for part b, average angular velocity will be 3.49/2 = 1.75 rad/s.

therefore, angular displacement = 1.75 x 20.0 = 35 rad --> 5.57 revolutions.

4. Jul 15, 2010

Gear.0

Yup, that's it :D

Although, technically if you round here it would be 1.74 rad/s because of the round even rule, that many people are unaware of.
But don't mind that lol, the answer is right.