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Torque and angular speed

  1. Mar 17, 2008 #1
    1. The problem statement, all variables and given/known data

    An LP turntable must spin at 3.53 rad/s to play a record. How much torque must the motor deliver if the turntable is to reach its final angular speed in 1.9 revolutions, starting from rest? The turntable is a uniform disk of diameter 30.7 cm and mass 0.24 kg.

    2. Relevant equations

    T=rF
    F=ma
    Theda = (Angular Acceleration (t^2)) / 2

    3. The attempt at a solution

    I tried using the above equations but i'm not sure about finding the angular acceleration to find the the final Torque.
     
  2. jcsd
  3. Mar 17, 2008 #2

    Doc Al

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    Staff: Mentor

    What's the most basic definition of acceleration? That's what you need here.
     
  4. Mar 17, 2008 #3
    change in distance / change in time
     
  5. Mar 17, 2008 #4

    Doc Al

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    Staff: Mentor

    That's the definition of velocity. What's the similar definition of acceleration?
     
  6. Mar 17, 2008 #5
    oh yea, sorry, change in velocity / change in time
     
  7. Mar 17, 2008 #6

    Doc Al

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    Staff: Mentor

    Good. So the angular acceleration is the change in angular velocity over time.

    But I just realized that they didn't give you the time--I misread the problem. (D'oh!) So you'll need to use another approach to find the time.

    Try this: What's the average speed of the turntable while it's accelerating? Use that--and the distance--to calculate the time. (Change revolutions to radians.)
     
  8. Mar 17, 2008 #7
    Ok, so the Time = Distance / Average Speed, the distance is 11.932 radians, but what is the average speed, is it simply the given speed of 3.53 or something else?
     
  9. Mar 17, 2008 #8

    Doc Al

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    Staff: Mentor

    No. 3.53 rad/s is the final speed and 0 is the initial speed. So what's the average speed?
     
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