Spin an LP turntable

1. Oct 22, 2006

mikefitz

An LP turntable must spin at 3.51 rad/s to play a record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 1.8 revolutions, starting from rest? The turntable is a uniform disk of diameter 31 cm and mass 0.26 kg.

I know that 1.8rev is equal to 648 degrees. This means that the disc must spin 648 degrees to reach the required 3.51 rad/s for the record.

But I don't know how to begin to calculate the torque required to achieve that angular velocity. Where do I begin> Thanks

2. Oct 22, 2006

OlderDan

Begin by expressing everything in radians rather than degrees, and by reviewing the equations of rotational kinematics. They are directly analogous to liner kinematics.

3. Oct 22, 2006

mikefitz

so, total rotation (1.8 rev) is equal to 11.3097 rad. It must spin at 3.51 rad/s.

How do I use this information to calculate a torque? thanks again

4. Oct 22, 2006

OlderDan

Look at your rotational kinematics equations. Find one that relates a change in angular velocity to an angular displacement and a constant angular acceleration. Find another one that relates torque to angular acceleration.

There are other paths to the solution (ther usually are) but these two equations will be a fairly direct route.

5. Oct 22, 2006

mikefitz

equations my teacher has given me:

KErot = .5 I w^2
I = .5 m r^2
Torque = F r sin(theta)

I guess I am unsure which equation relates all these different elements together?

6. Oct 22, 2006

OlderDan

Look here and scroll down to the table showing the linear equations and their angular analogs.

http://online.cctt.org/physicslab/content/PhyAPC/lessonnotes/rotationalmotion/kinematics.asp

You should have seen all of these before. If you have not ssen them, you are seeing them now. Give particular attention to the last one. There is also a rotational analog of Newton's second law

F=ma <> Torque = I*alpha

See what you can do with these.

7. Oct 22, 2006

mikefitz

great, thanks for the help OlderDan - no I had not seen that last equation until I clicked your link.