# Magnitude of angular acceleration- my first post ever in physics forum

1. Dec 1, 2011

### sweetwilliam

1. The problem statement, all variables and given/known data

CD is playing for 74 min, starts at angular speed of 480 rpm then slows to 210rpm. whats the magnitude of the angular acceleration

2. Relevant equations
angular acceleration =ω/ t

3. The attempt at a solution
i converted the rev to rad then min to sec then divided by sec and got 50.3 rad/s and 14.7 rad/s ... 14.7-50.3=-35.6/4440s= -.0080 my book says its wrong....fyi this is my first pot in physics forum so i hope i did it all correct

2. Dec 1, 2011

### Staff: Mentor

Hi sweetwilliam , welcome to Physics Forums.

Your initial angular velocity value looks okay at 50.3 rad/s. You might want to check your value for the final angular velocity.

3. Dec 1, 2011

### sweetwilliam

Hi, thanks...i re-did the final rev/min and got 29.43 but when i put that back into my formula I am still not getting the answer thats in the book...thanks

4. Dec 1, 2011

### Staff: Mentor

That value, 29.43, still doesn't look quite right. Can you expand on your calculation?

5. Dec 1, 2011

### sweetwilliam

i took 281rev/min and multiplied it by 2pi then divided by 60 sec...thanks

6. Dec 1, 2011

### sweetwilliam

oh sorry i meant 210 rev.min x 2pi then divide by 60 which =21.99

7. Dec 1, 2011

### sweetwilliam

ok after using that number i got -.0064 rad/s^2 which is partially correct by why in the book does it say .0064, where did the neg. go?

8. Dec 1, 2011

### Staff: Mentor

They asked for the magnitude of the acceleration. Magnitudes are always positive.

9. Dec 1, 2011

### sweetwilliam

OOOHHH yes ok gottcha thanks