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Spin an LP turntable

  1. Oct 22, 2006 #1
    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. jcsd
  3. Oct 22, 2006 #2

    OlderDan

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    Begin by expressing everything in radians rather than degrees, and by reviewing the equations of rotational kinematics. They are directly analogous to liner kinematics.
     
  4. Oct 22, 2006 #3
    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
     
  5. Oct 22, 2006 #4

    OlderDan

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    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.
     
  6. Oct 22, 2006 #5
    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?
     
  7. Oct 22, 2006 #6

    OlderDan

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    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.
     
  8. Oct 22, 2006 #7
    great, thanks for the help OlderDan - no I had not seen that last equation until I clicked your link.
     
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