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I Energy to turn a shifting mass wheel?

  1. Aug 23, 2016 #1
    Good afternoon and thanks for reading. I have been in a discussion with a friend and we are trying to get to the bottom of it. It all started when looking at overbalanced wheels and shifting mass wheels. I know that they will not turn themselves, but do they have a negative torque applying to them or is it totally neutral?
    I have a question regarding the energy to keep a wheel turning, specifically a shifting mass wheel compared to a wheel of identical mass without shifting weights.

    I understand the weight and diameter and placement of the weight of a wheel is needed to determine how much power is required to accelerate it, but I am trying to determine if there is a difference in the amount of energy input needed to keep a shifting mass wheel turning once it is rotating. I know at a point centrifugal forces start to apply, but for this question, I am focusing on a very slow rpm so that the only force to overcome is gravity and the friction of the bearings.

    My thought was since gravity is conservative and it is a wheel, both wheels would require the same amount of energy to turn regardless if the wheel was a solid wheel or if it had shifting masses.
    Any guidance here would be great.

    Thanks.
    Cameron
     
    Last edited by a moderator: Aug 23, 2016
  2. jcsd
  3. Aug 23, 2016 #2
    Can you draw a picture of all the pieces?
     
  4. Aug 23, 2016 #3
    as seen. a wheel with a part that would slide as the wheel rotated. All the research states that there is no positive energy gain or net torque gain, my question is does the wheel pictured require more energy to rotate than if the slide weight was fixed? I think that there is no difference but I was hoping to confirm.
     

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  5. Aug 23, 2016 #4

    Drakkith

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    I think it requires more energy, as you are expending energy to rotate the wheel and move the slide, but most of the potential energy added to the slide is lost as friction or from the impact when it slides down and stops. If the slide were fixed then all of the potential energy would be given back to the system once the slide gets to the top and starts to come back down.

    That's my guess anyways.
     
  6. Aug 23, 2016 #5
    that is kind of where I am confused. once the wheel makes one full rotation, wouldn't it balance out? you kind of get a bonus as the weight is on the falling side and getting the benefit of gravity??
     
  7. Aug 23, 2016 #6

    Drakkith

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    When the slide is locked? Sure. That's why you get all that potential energy back.
     
  8. Aug 23, 2016 #7
    so your saying if the slide can move, it would take more energy to turn than if it was fixed in one position? Wouldn't the fact that when it slides at @ 9 oclock and 3 respectively, it would balance itself? Maybe there would be a loss of energy in friction in the mechanics of the slide.
     
  9. Aug 23, 2016 #8

    Drakkith

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    The slide never reaches the 3-o'clock position. It falls soon after the 9-o'clock and never makes it over the top, so you can't reclaim all of the energy you spent lifting it.
     
  10. Aug 23, 2016 #9
    - take the wheel in a the first position, the slide is pulling the weight down, turn the wheel 90 degrees clockwise, the extended weight is at the 9 position and slides so that the opposing side is extending out toward 3 as it turned. that side would then fall. It would not exceed the energy to lift it, I agree. but what I am wondering is if we know the value of energy to turn that same wheel and the weight was fixed or even balanced, which would indeed need some sort of energy input to continue turning, and compared to the example with a slide mechanism,
     
  11. Aug 23, 2016 #10

    jbriggs444

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    Barring any irreversible losses (friction, impacts, damped vibrations, etc), if it takes extra energy to rotate the shifting mass wheel in the forward direction then time reversing the mechanism would result in a release of energy -- perpetual motion. Which is not allowed.

    So your initial thought was dead on -- since gravity is conservative, the thing will not require extra energy input. [Barring irreversible losses as above]
     
  12. Aug 24, 2016 #11
    Thank you all for your input. But my dilemma is the same. Which one is correct? You both make the same observations I make with 2 totally different results. I am trying to figure out a simple way to test / observe this to generate an answer.
     
  13. Aug 24, 2016 #12

    A.T.

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    In reality you will dissipate energy with that slide bouncing around. Assuming perfectly elastic collisions of the slide, the energy will be stored as kinetic energy in the bouncing slide, and might eventually be recovered.
     
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