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Force, torque with a motor + a wheel + a belt

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  1. Dec 12, 2014 #1
    1. The problem statement, all variables and given/known data

    A motor accelerates more and more a wheel without friction. It is supposed the belt gives forces F1 and F2 at only one point.

    http://imageshack.com/a/img674/1437/Uh0XIp.png [Broken]

    2. Relevant equations

    None

    3. The attempt at a solution

    I would like to know if I drawn all forces:

    http://imageshack.com/a/img674/2809/o4uI79.png [Broken]


    1) Is it correct ?

    2) The motor is turning clockwise at ω, so the wheel is turning at ωR1/R2 ?

    3) The motor needs to give F1R1ωt, so the wheel receives F2R2ωR1/R2=FR1ωt because |F1|=|F2|, correct ?

    4) The sum of forces on the belt is F3-F4, if the motor accelerates this could say the belt is accelerating too, then F3 must be higher than F4, so F1 must be higher than F2 (the absolute value not the vector) ? I'm not sure, if F2 is lower the torque on the wheel is lower too, so the last calculation is false ?
     
    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Dec 12, 2014 #2

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    Looks pretty good.
    Correct.
    Appears to be.
    Are you absolutely certain?
     
  4. Dec 12, 2014 #3
    The belt is accelerating, like F=ma, for me there is a little difference, the motor gives an energy for accelerate the belt and the disk, a little part must go to the belt and the main part for the disk, no ?
     
  5. Dec 12, 2014 #4

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    Have you been given masses and moments of inertia? Ordinarily (not absolutely guaranteed) for problems like this, you're given a "massless" belt.
     
  6. Dec 12, 2014 #5
    Yes, it's for that, if the mass of the belt is 0, |F3|=|F4| but if the belt has a mass |F3|>|F4|, correct ?
     
  7. Dec 12, 2014 #6

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  8. Dec 12, 2014 #7
    ok, thanks :)
     
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