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Follow up on Barlow's wheel

  1. Jun 21, 2015 #1
    1. The problem statement, all variables and given/known data

    In the Barlow's wheel experiment, let's say I want to find the torque of the forces that affect it. (Friction is ignored)

    I have W (weight) has no torque
    R (reaction) also has no torque
    F (the force that moves the wheel) does have torque

    I can calculate F's torque individually.


    Now my question is, if I use ΣΓ = I*α, I'll get 0, because the angular acceleration is 0 since the angular velocity is constant.


    Why do I get 0 when I do have torque?



    Thanks

    2. Relevant equations

    ΣΓ = I*α


    3. The attempt at a solution
     
    Last edited: Jun 21, 2015
  2. jcsd
  3. Jun 21, 2015 #2

    mfb

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    Without friction, the wheel is constantly accelerating (unless your current breaks down at some unrealistic speed).
     
  4. Jun 21, 2015 #3
    But alpha in this case is 0 because the velocity is constant, no?
     
  5. Jun 21, 2015 #4

    mfb

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    Why do you expect the velocity to be constant in the absence of friction?
     
  6. Jun 21, 2015 #5
    Because the force that's causing the velocity is constant? Neither the current, the radius or the magnetic field are changing.
     
  7. Jun 21, 2015 #6

    mfb

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    Which force?
    Without friction, you have an accelerating force (more precise: torque), so the wheel keeps accelerating.
     
  8. Jun 21, 2015 #7
    I don't know what you call it in English. The F = I*L∧B force.

    It's what's moving the wheel, and it's constant, and so the angular velocity is constant and the acceleration is 0, since the velocity isn't changing.
     
  9. Jun 21, 2015 #8

    mfb

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    No, a constant force leads to a constant acceleration.

    What you actually need is the integral ##\int_0^L I B r dr## for the torque, but that follows the same rules.
     
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