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Question about a flywheel

  1. Sep 7, 2008 #1
    Hello everyone,

    First of all may i say i am a layman to physics. Unfortunately, i have difficulty with maths.

    However, my question is this...

    Supposing i wish to rotate a car wheel (by hand) with the brake engaged upon it; but am not quite strong enough to turn it.

    Supposing i were to spin up a flywheel with a mass twice that of the car wheel (by the same hand) and then engage the braked wheel to be driven by the flywheel at a ratio of one to one

    Would the flywheel give me that little extra strength (energy) needed to not only turn the braked wheel but also maintain it at a constant rpm for a given time?

    Thankyou for your time
  2. jcsd
  3. Sep 7, 2008 #2


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    It depends on how much braking force is applied and if you can vary the gear ratio of the flywheel (flywheels generate energy by decelerating).
  4. Sep 8, 2008 #3
    Hi Russ, thanks for your reply.

    Ok, lets say im using "X" amount of force to maintain a flywheel 300mm x 25mm with a weight of 10lbs spinning at 100 rpm.

    Lets say we had another flywheel half the size and half the weight.
    To maintain it spinning at 200 rpm would require a certain amount of force which is proportional to "X"

    Lets call the force needed to drive the big flywheel indipendantly X1 and the force needed to drive the small flywheel indipendantly X2

    Lets add X1 and X2 together and call the result "Y"

    Now, lets connect the two flywheels in such a way that one rotation of the big flywheel is equal to two rotations of the small flywheel.

    Now lets spin up the big flywheel to 100rpm.

    Im trying to understand the principle here more than anything; so can we ignore potential friction losses to bearings and air please.

    Now we are spinning the big flywheel at 100 rpm which is in turn driving the smaller flywheel at 200 rpm.

    In this situation; would the force needed to drive the big flywheel be more, less or the same as "Y"?

  5. Sep 8, 2008 #4


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    Gold Member

    If you're ignoring friction and losses to air resistance, you require zero force to keep your flywheels spinning at a constant speed.
  6. Sep 8, 2008 #5
    Hi Brewnog, thanks for your reply.

    Ok, when you say zero force; do you mean that once the flywheels were at speed in a frictionless environment then all driving force could be removed and they would not start to slow down? - perpetual motion?

  7. Sep 8, 2008 #6


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    Staff: Mentor

  8. Sep 9, 2008 #7
    Thanks Russ - i think i get it now
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