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Angular momentum to Power

  1. Feb 22, 2015 #1
    As you can see, solid disk is rotating at steady angular speed, without any external force beeing applied.

    If i stop it gradualy within 60 second, by applying some linear load, how many kWh can i extract from this moving object?

    inertia.jpg
     
  2. jcsd
  3. Feb 22, 2015 #2

    mfb

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    At most the total kinetic energy. You just have to convert J to kWh.
     
  4. Feb 22, 2015 #3
    If i double angular speed,it doubles angular momentum but but why it quadruples Kinetic energy ?

    If i apply load in this case and slow down the wheel by 50%, it looses 75% of kinetic energy, so i think total amount of J which i can extract is more complex to determine or am i missing something?



    inertia2.jpg
     
  5. Feb 22, 2015 #4

    A.T.

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    Because of how they are defined.
     
  6. Feb 22, 2015 #5
    ok so in real life, to stop wheel in 60 seconds i need some load,
    to stop wheel with double speed in 60 seconds, do i need double load or quadruple load ?

    i would say , i need just double load

    so what does this KE represent in real life?
     
  7. Feb 22, 2015 #6

    A.T.

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    This:
     
    Last edited: Feb 22, 2015
  8. Feb 22, 2015 #7
    my point is, to accelerate wheel from zero to some speed it takes 1 unit of J

    to double the speed it takes another 1 unit of J

    so 2 unit of J total, linear relationship, when it comes to power input


    but somehow power output, KE is not linear but exponencial ?
     
  9. Feb 22, 2015 #8

    A.T.

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    No, it takes additional 3J to double the speed that was achieved by 1J.
     
  10. Feb 22, 2015 #9
    if i set torque to fixed value, to accelarate from zero to some speed it takes 1 unit of time, to accelarate to double speed from zero with same torque, it takes 2 units of time

    so i see pretty linear input, not 1J vs 3J as you said
     
  11. Feb 22, 2015 #10

    A.T.

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    Input of what?
     
  12. Feb 22, 2015 #11
    torque * time
    or
    force * time
     
  13. Feb 22, 2015 #12

    A.T.

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    Is that energy?
     
  14. Feb 22, 2015 #13
    no its, not

    but how do you calculate it in this case,

    In rotational systems, power is the product of the torque τ and angular velocity ω,

    9c7474f3d90aa327b5ef37257a11343a.png




    this is linear relationship, just tell me how did you get your exponencial

    is it sum of energies from zero to some speed vs sum of energies from some speed to double speed?
     
  15. Feb 22, 2015 #14

    A.T.

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  16. Feb 22, 2015 #15
    ok so according to this

    if i rotate the wheel with some electric motor and measure electricity consumption
    from zero to speed x ,it will take lets say 10 seconds and i will use lets say 1 kwh

    during another 10 seconds i accelerate from x to 2x, and use additional 3 kwh ?

    is this correct according to facts above?
     
  17. Feb 22, 2015 #16

    A.T.

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    Assuming 100% efficiency, yes.
     
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