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Torque and angular speed

  • Thread starter cstout
  • Start date
  • #1
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Homework Statement



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.

Homework Equations



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

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.
 

Answers and Replies

  • #2
Doc Al
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What's the most basic definition of acceleration? That's what you need here.
 
  • #3
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change in distance / change in time
 
  • #4
Doc Al
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change in distance / change in time
That's the definition of velocity. What's the similar definition of acceleration?
 
  • #5
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oh yea, sorry, change in velocity / change in time
 
  • #6
Doc Al
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oh yea, sorry, change in velocity / change in time
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.)
 
  • #7
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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?
 
  • #8
Doc Al
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but what is the average speed, is it simply the given speed of 3.53 or something else?
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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