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Two separate flywheels 
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#1
Jan1811, 04:35 PM

P: 28

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
Jan1811, 04:40 PM

P: 2,292

Yes.
Edit. Oh wait, I may have misunderstood the question. Are we talking spindle RPM or outside edge of the flywheel RPM? 


#3
Jan1811, 04:49 PM

P: 28

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). 


#4
Jan1811, 05:08 PM

P: 28

Two separate flywheels
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?



#5
Jan1811, 05:11 PM

P: 2,292

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.



#6
Jan1811, 05:16 PM

P: 2,292

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 1foot in deep space is the same as on earth. 


#7
Jan1811, 05:19 PM

P: 2,292

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. 


#8
Jan1811, 05:23 PM

P: 28

Ah OK thanks for clearing that up!



#9
Jan1811, 05:43 PM

HW Helper
P: 7,179

I_{large} = 1/2 m r^{2} = I I_{small} = 1/2 (1/2 m) (1/2 r)^{2} = 1/16 m r^{2} = 1/8 I ω_{large} = ω ω_{small} = 2 ω E_{large} = 1/2 (I) ω^{2} = 1/2 I ω^{2} E_{small} = 1/2 (1/8 I) (2 ω)^{2} = 1/4 I ω^{2} The smaller flywheel will have 1/2 the energy of the larger flywheel at the same surface speed. 


#10
Jan1811, 05:56 PM

P: 28

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? 


#11
Jan1811, 06:13 PM

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#12
Jan1811, 06:14 PM

P: 2,292

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. 


#13
Jan1811, 06:19 PM

HW Helper
P: 7,179

http://www.youtube.com/watch?v=GPWJc8sLhjo 


#14
Jan1811, 06:27 PM

P: 28

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? 


#15
Jan1811, 06:35 PM

P: 2,292

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. 


#16
Jan1811, 06:38 PM

P: 28

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.



#17
Jan1811, 07:41 PM

PF Gold
P: 707

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. Ron 


#18
Jan1811, 07:49 PM

Mentor
P: 22,315

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!



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