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Angular impulse

  1. Jun 10, 2017 #1
    1. The problem statement, all variables and given/known data
    The rotor of an electric motor weighs 10 pounds and is 4 inches in diameter. What is the length of time required for the motor speed to increase from 0 to 1800rpm, assuming a constant electrical torque of 20 in-lb and zero external load during this period? Assume the rotor is a homogeneous cylinder.

    2. Relevant equations

    3. The attempt at a solution
    Moment of inertia (I) of cylinder according to google is $$I=1/2 MR^2 = 1/2*10*2^2=20\\\\\
    1800 rpm = \frac{1800}{60} \frac{rotations}{second} * \frac{2\pi rads}{rotation} = 188.5\\\\
    The book says the answer is 0.49 seconds. I got 188.5 seconds. Which is correct?
  2. jcsd
  3. Jun 10, 2017 #2


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    0.49 s is correct.

    Your overall approach is fine. But you are not giving enough consideration to the units of the various quantities. For example, the rotor weighs 10 pounds. But does M in the formula I = (1/2)MR2 represent weight? You should rework the problem plugging in units for each quantity in the calculation. If the units in your calculation do not reduce to seconds when you solve for t, you know you made an error.
  4. Jun 11, 2017 #3


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    A slightly different take on the error.... Not sure which is more helpful.
    The lb there does not represent mass. What does it represent?
  5. Jun 11, 2017 #4
    I got it. There were 2 problems. They gave me weight but I thought it was mass, so I had to divide by gravity. The other was they gave me torque in inch pounds, I had to convert it to foot pounds. Thank you both.
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