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Two separate flywheels

  1. Jan 18, 2011 #1
    Alright, so let's say there are two separate flywheels. They're both the same design and material except that one is a scaled down version of the other (let's say 50% smaller).

    To get them both spinning at the same RPM does the one that weighs less take less energy?

    Thanks :D
  2. jcsd
  3. Jan 18, 2011 #2
    Re: Flywheel


    Edit. Oh wait, I may have misunderstood the question.
    Are we talking spindle RPM or outside edge of the flywheel RPM?
  4. Jan 18, 2011 #3
    Re: Flywheel

    What do you mean by spindle RPM but I was thinking about the flywheel's circumference RPM.

    Another question... let's say you have two identical flywheels but one is in deep space and is considered pretty much weightless how much energy would that take to get spinning compared to one on Earth (let's say the only force effecting the one of Earth is gravity giving it weight).
  5. Jan 18, 2011 #4
    Re: Flywheel

    And another question for someone that is really smart :) Let's say that a flywheel spinning at 1000 RPM weighing 50 Kilos "magically" weighed 500 kilos suddenly. Would the stored kinetic energy in the wheel jump by 10x?
  6. Jan 18, 2011 #5
    Re: Flywheel

    Ok. I understand your Q now (my bad, not yours); the 50% scaled down version WILL require less energy, as there is less mass to accelerate.
  7. Jan 18, 2011 #6
    Re: Flywheel

    The energy to accelerate a spinning mass on earth versus deep space is the same(essentially)
    Remember that earths gravity has nothing to do with the forces required to overcome radial acceleration. Another view: The energy to push a bowling ball 1-foot in deep space is the same as on earth.
  8. Jan 18, 2011 #7
    Re: Flywheel

    That scenario is identical to tapping energy from a flywheel(the "sudden" 500 kilos is the tap load)
    The kinetic energy would be REDUCED by 10x, though as potentially useful energy.
  9. Jan 18, 2011 #8
    Re: Flywheel

    Ah OK thanks for clearing that up!
  10. Jan 18, 2011 #9


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    Re: Flywheel

    So what you really mean is the surface speed at the edge of the flywheel. The smaller flywheel will spin at twice the speed but have 1/2 the mass and 1/2 the radius. Comparing the two flywheels with the same surface speed:

    Ilarge = 1/2 m r2 = I
    Ismall = 1/2 (1/2 m) (1/2 r)2 = 1/16 m r2 = 1/8 I

    ωlarge = ω
    ωsmall = 2 ω

    Elarge = 1/2 (I) ω2 = 1/2 I ω2
    Esmall = 1/2 (1/8 I) (2 ω)2 = 1/4 I ω2

    The smaller flywheel will have 1/2 the energy of the larger flywheel at the same surface speed.
  11. Jan 18, 2011 #10
    Re: Flywheel

    Two new questions!

    Is there a beginners book or better yet an interactive learning tool so that I can get into physics? I don't really want to go to a college for this (already have a degree and enough debt) but I find this all extremely fascinating and I'm so glad I found this forum!

    Alright, so let's say there is a flywheel spinning at 1000 RPM and a mass (not weight) of lets say a unit of 1. If there were a way to make the mass into a unit of 10 would the energy increase by a unit of 10x or decrease again by 10x?
  12. Jan 18, 2011 #11


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    Re: Flywheel

    It would depend on how the mass was added. If the mass was simply 9 non-spinning flywheels and they were suddenly connected to the spinning flywheel with a lossless coiled spring, the total energy and momentum would remain the same, but it would cycle through various states with some of the energy stored in the spring, except at two points in the cycle where the spring would have zero energy. There would be times when the original flywheel would be spinning backwards with the group of 9 flywheels spinning forwards. If a clutch was used, there would be friction losses due to energy converted into heat. If the 9 flywheels were already spinning at the same rate as the original flywheel, they would already have 9x energy and would make the total 10x when connected to the original flywheel.
    Last edited: Jan 18, 2011
  13. Jan 18, 2011 #12
    Re: Flywheel

    Cyclic motions of this nature would reduce the RPM's.
    Perhaps one can think of it this way: Push your child on a playground swing. Back and forth, back and forth.
    Then, suddendly, another child jumps on the same swing.
    What happens?

    a) the swing slows down and does not move as far.
    b) it takes more energy from you to push the swing to what it was previously.

    Both happens.
  14. Jan 18, 2011 #13


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    Re: Flywheel

    Depends on when, where, and at what speed the other child jumps on the swing. If the second child jumped from a platform above the peak hight of the swing there would be an energy increase. Example video of this from an aerobatic act:

    Last edited by a moderator: Sep 25, 2014
  15. Jan 18, 2011 #14
    Re: Flywheel

    This is what I was really thinking:

    Let's say there is a special hollow flywheel. It's spinning at whatever RPM and then a heavy liquid is injected through a tube to its center and the spinning causes the liquid to travel through more tubing to the outer rim of the flywheel.

    Let's say the original weight of the flywheel is 100 kilos and it's spun to 1000 RPM. After the injection the flywheel is now 500 kilos. Does this slow down the wheel? If so would it take less energy to get the wheel back up to 1000 RPM after the injection than just having a wheel weighing 500 kilos and spinning that weight from the start?
  16. Jan 18, 2011 #15
    Re: Flywheel

    Please remember this: It requires MORE energy to enable or sustain an energy storage system than that which can be extracted.
    There are no exceptions.
  17. Jan 18, 2011 #16
    Re: Flywheel

    Haha you realized what I was thinking about. I'm just stretching my brain trying to think of how a flywheel could go from storage to generator.
  18. Jan 18, 2011 #17


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    Gold Member

    Re: Flywheel

    Sounds like you read a post of mine, from a few years ago. If not, I'll say what was on my mind, might help your thoughts.
    If your flywheel rim has a large mass of steel, a cavity of some size, a heat element, then dumping a weight of mercury will increase kinetic energy and drive a generator as speed slows. The mercury boils and a vapor moves out to a condenser and becomes liquid again, returning to the hollow axel, the process repeats.

    A liter weight wheel is increased to speed, then a heavier weight wheel passes energy to the generator. A 3600 rpm flywheel might fluxuate by no more than 2 or 3 hundred rpm.
    Everything will be based on how long to boil whatever weight of mercury is removed at the low speed.

    You'll likely have to come up with something other than mercury, I know I'll never try to go anywhere with the idea, too many easier things to look at.:smile:

  19. Jan 18, 2011 #18


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    Re: Flywheel

    The inevitable miscalculation and conclusion of perpetual motion will come from assuming you can "magically" increase the mass of a flywheel somehow. Even if you're just filling a hollow flywheel with a liquid, you still have to give that fluid energy to make it spin with the flywheel. There's no magic to be had here. TANSTAAFL!
  20. Jan 18, 2011 #19
    Re: Flywheel

    No magic? Man, that ruins everything.

    My initial idea was to have many flywheels made from a dense material in a relatively weightless environment (somewhere in space).

    Those many small flywheels are connected to one large flywheel's outer edge. The smaller wheels are spun quickly with little effort and then the bigger flywheel is spun creating artificial gravity on the smaller wheels. I was guessing that you could just turn on a generator for the smaller wheels and get energy from them and turn on a generator for the bigger wheel until it all comes to a halt. Then start it over again.

    I wish I could find an equation or experiment explaining why this doesn't work.
  21. Jan 18, 2011 #20
    Re: Flywheel

    Nope didn't see yours... couldn't sleep last night and thought this up. It's cool though that someone else has been thinking about these things too.
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