# Comparing Flywheels: Energy Requirements

• D9 XTC

#### D9 XTC

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

Yes.

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

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).

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?

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.

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).

The energy to accelerate a spinning mass on Earth versus deep space is the same(essentially)
Remember that Earth's 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.

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?

No.
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.

Ah OK thanks for clearing that up!

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

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 let's 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?

Alright, so let's say there is a flywheel spinning at 1000 RPM and a mass (not weight) of let's 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?
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.

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Alright, so let's say there is a flywheel spinning at 1000 RPM and a mass (not weight) of let's 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?

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.

Then, suddendly, another child jumps on the same swing.
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:

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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?

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.

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.

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.

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.

Ron

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!

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.

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.

Ron

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.

I wish I could find an equation or experiment explaining why this doesn't work.
The equation is:

Eout = Ein

If you analyze your system correctly, you'll always end up being able to reduce the equations to that form.

The scientific principle is conservation of energy: http://en.wikipedia.org/wiki/Conservation_of_energy
It's cool though that someone else has been thinking about these things too.
People have been trying to find perpetual motion for thousands of years. They wasted their time: it doesn't exist. Please don't waste yours!

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!

Russ, If I could give infraction points, I would, I think you deserve a couple in this case.

I did not mention perpetual motion. If you understood mechanical things better, and I could do math and use proper words, we might be a good pair to promote better energy efficiency.

What makes you think the mercury needs to be given spin energy?

If a spoke and rim flywheel is in motion, then mercury is being held in the hollow shaft, a valve release action will let a measured quantity of mercury move through the hollow spokes and at the rim the liquid will move to the outer surface of the cavity, then spread in two directions until equilazition of mass is reached. This would all take place in microseconds most likely.
I don't think your mind is seeing the small volume of space that, say 5 pounds, of mercury will require.
You may or may not know that Harley Davidson Motorcycles, have in the past and maybe still use mercury in the flywheels, but only for a balance function.

D9 XTC has not been back as far as I can tell, I hope his spirit of an enquiring mind is not diminished and that he will continue to learn how to evaluate and present his ideas.
My comments are based on the fact that his thinking is in line with what I have considered for some time, the energy balance is delicate as in all systems, the liquid/vapor/liquid process will best involve a cascade design of two or more systems. If efficiency can no longer be improved toward 100%, why is anyone here ? and why promote more education ?

P.S. Sorry if the use of gender and age implications are wrong.

Ron

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The problem is that as the mercury moves outward in the wheel, the wheel spin slows down. This is true no matter what mechanism is used to move mass into the flywheel. As the wheel spin slows down, kinetic energy is actually lost.

What makes you think the mercury needs to be given spin energy?
The mercury requires a tangental force in order to increase it's tangental speed as it moves outwards from the center of the wheel. If the mercury was just released unbounded, it would flow in a straight line, and not follow the vanes of the wheel.

Enough. Now that we know what the OP was on about, the thread is in violation of PF rules. I'm not going to allow a debunk-my-perpetual-motion-machine thread.

Locked.