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How do I convert mechanical work into torque on a suspended object

  1. Sep 2, 2012 #1
    Note: given many questions rg spinning objects, this is NOT related to perpetual motion or other similar dreams…

    Q: How do I convert mechanical work (ie: energy from a battery) into a spinning force (torque) on a self-contained object suspended from a rope.

    I am looking for an alternative method to using a gyroscope with a single horizontal axis pivot, where one applies a force twisting the axis of the gyroscope causing a pivoting force on the system.

    I was thinking a possible solution would be to find the CG (center of gravity) of the object and moving a weight in a horizontal “D” pattern with the center of the straight portion passing through the CG. If this works, then I speculated that an off balance wheel would approximate the same effect having a weight on the wheel pass through the CG and would be much simpler to implement.
    http://img401.imageshack.us/img401/4561/torquedevice.jpg [Broken]

    Is this the best solution or is there a better way of doing this, and what formulas would I use to predict the net torque created from each revolution of the wheel so I can predict correct RPM and weight needed on wheel to produce a given resulting torque.

    P.S. Sorry if I have incorrectly used terms like torque as my physics classes were so long ago I struggle to remember much of the finer details.
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 3, 2012 #2


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    As I understand it...

    It's not really the gyroscopes that allow (for example) Hubble to be pointed in different directions. Hubble uses reaction wheels to do that. Hubble spins a wheel one way and the telescope moves the other way.

    Your proposal sounds like some sort of off set reaction wheel?

    I'm not sure what you are trying to achieve but how about having two "objects" each spinning in opposite directions? One connected to the shaft of a motor and the other to it's case.
  4. Sep 4, 2012 #3
    Thanks CWatters for your reply. Let me give an illustration which may help clarify what I am trying to understand.

    Every experienced rock climber knows that when dangling from a rope you are able to control the direction you are facing. I have done this many times with much flailing of arms and legs you are able to instinctively control the direction you are facing much like a cat is able to flip over and land on their feet. Despite being able to preform this trick, I have no idea of the physics behind it and want to build a device that accomplishes directional control on a hanging object by mimicking the motion of the arms and legs

    FYI: the practical application for me is as a safety device for paragliders to help exit a situation called "riser twist" <See you tube vid for example> the riser twist occurs at 30 seconds, and at 1:08 the pilot initiates the flailing of legs to start the unwinding.
  5. Sep 5, 2012 #4


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    Essentially that's what hubble does to change the direction it points. Acrobats and divers also rotate their arms in one direction to cause their body to spin/twist in the other.

    How about a motorised disc under the seat? Spin it one way and you rotate the other way? Too heavy?
  6. Sep 6, 2012 #5
    I think the legs are doing something totally different then what the hubble does as they do the same repeated cycle without needing to change the rate of the cycles. The hubble uses a reactionary gyro, where the desired change occurs only while the gyro is being accelerated in the opposite direction which results in an ever increasing rpm of the gyro. When a rock climber is hanging from a rope he does not need to increase the speed of flailing but gets a noticeable result from each cycle.

    I'm hoping someone on this site is cleaver enough to explain the cycle that is happening in the example of the rock climber hanging from a rope and being able to effect the direction he is facing. Only with a clear understanding of this can one proceed to reproduce this cycle with a mechanical device. I suspect it is a trick of changing the moment of inertia, thus my original drawing where the wheel is moving a weight to the center and back out, thus actively changing the moment of inertia of the whole system.
    Last edited: Sep 6, 2012
  7. Sep 7, 2012 #6


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    Reaction wheels don't have to be accelerated to work. They work by conservation of angular momentum.

    If you sit on a rotating stool/office chair you can turn around by swinging your legs in a circle. You will see that your legs rotate one way and you body the other. Thus preserving angular momentum. It does not matter that the center of rotation of your the legs is offset from your center of rotation. (Beware this may not work on a chair that you spin around to adjust the height).
    Last edited: Sep 7, 2012
  8. Sep 7, 2012 #7
    CWatters thanks for the reply.
    This does not agree with all the reading I have done regarding reaction wheels with zero degrees of freedom. Can you please give a deeper explanation or citations so I can investigate this.
    Can you explain HOW this works, I have already stated in the first post that it is possible (thanks for reaffirming this), but I am looking for someone smarter than me who can show HOW THIS WORKS.
    Last edited: Sep 8, 2012
  9. Sep 8, 2012 #8


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    As I understand it this results from a combination of..

    a) Newtons third law...



    b)Conservation of angular momentum

    Before rotating your legs you have zero angular momentum (you are stationary). While you are rotating your legs they have +ve angular momentum. Therefore the rest of you must have negative angular momentum eg the oposite direction. The total is still zero angular momentum. When you stop rotating your legs your body stop rotating so you still have zero angular momentum. Only now you face the other way.

    The reaction wheel is similar. Spin it one way and the spacecraft must spin the other way so that there is no net change in angular momentum.

    Falling cats also use the same trick as climbers. Dropped upside down they rotate their head and feet one way and their body rotates the other. Result is they land right way up. Sounds impossible but ..

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