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

[sweetwilliam]

Homework Statement

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

Homework Equations

angular acceleration =ω/ t

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

 

[gneill]

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.

 

[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 that's in the book...thanks

 

[gneill]

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 that's in the book...thanks

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

 

[sweetwilliam]

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

 

[sweetwilliam]

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

 

[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?

 

[gneill]

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?

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

 

[sweetwilliam]

OOOHHH yes ok gottcha thanks