Fun with Less Kilowatts— Build A Flywheel

By Vernon Trollinger, November 15, 2017, Energy Efficiency, Family, Green

Welcome to Fun with Less Kilowatts! We believe that science experiments at home can be a creative way to engage kids in learning while having fun. They can be educational AND great activities to keep your kids busy and away from the television. Each month, we’ll feature a new science experiment that can be a great resource for parents and teachers.

Build A Flywheel

Flywheels work because they are mass in motion. Once all their huge mass is spinning, they can store an enormous about of energy —which makes them even cheaper and more efficient to store energy than batteries. James Watt incorporated the use of a flywheel into his first steam engines in order to maintain momentum between the piston strokes. As time went on, larger and larger flywheels were built to provide more energy. A prime example is the big wheel on display at the Tredegar Ironworks National Park in Richmond, VA. It measures 12 feet across, weighs several tons, and once powered the rolling mill that pressed and stretched hot steel.

Flywheels, like the example at Tredegar, were made of bronze or steel. Although their axles were mounted in oiled bearings to reduce friction, friction was always present. This limited flywheel speed to a few thousand RPMs. Unless the flywheels were manufactured with exact precision, they could become dangerously unbalanced at high speeds and fly to pieces. At Tredegar in 1905, a 25 foot wide flywheel exploded, killing one worker, injuring two others, and tearing up a 100 foot long building.

Flywheels are still used in a host of ways, including everything from smoothing out electricity from wind farms to providing battery-like energy storage for power plants and even vehicles.

How to Build a Flywheel


Fun with Less Kilowatts— Build A Flywheel | Bounce Energy Blog

Building a flywheel is really easy. All you need is a wheel (mass) and a method to get it spinning. In this experiment, we’re focusing on how to assemble a wheel at home that’s easy to put into controlled motion. For that, you’ll need:

  1. Two 16 oz plastic soda bottles.
  2. One wine bottle cork (Natural cork is preferred as plastic ones have a smaller diameter.)
  3. 3-6 feet of Nylon string with a weight attached at one end. (I happen to have a very nice plumb bob for my weight.)


Fun with Less Kilowatts— Build A Flywheel | Bounce Energy Blog
1. Assemble the your flywheel by the sticking one end of the cork into one soda bottle about 1/3 of the way in. Stick the other end of the cork into the other soda bottle so that it’s also 1/3 of the way in. You want to have a nice space of exposed cork between the bottle openings.

Fun with Less Kilowatts— Build A Flywheel | Bounce Energy Blog

2. Wrap your nylon string ONCE around the cork. Nylon cord is strong but it’s also slippery enough so that it won’t stick to the cork.

3. Place the weighted end of the string on the floor.

4. Hold the other end of the string up so the string is straight.

5. Making sure that the string is still wrapped once around the cork between the two bottles, let the bottles go.

The Result

Fun with Less Kilowatts— Build A Flywheel | Bounce Energy Blog

The bottles will spin as they descend the string. If there’s enough room at the bottom, the bottles may briefly keep spinning on the floor.

The Science

As mentioned, flywheels store rotational energy (momentum) just by spinning. Once a flywheel starts spinning, its momentum makes it very hard to stop. However, once a flywheel is in motion, it takes less energy to add to or maintain the flywheel’s motion.

How much energy is that? The amount of energy stored in a flywheel is proportional to the square of its rotational speed or RPMs. By doubling the mass of a flywheel, you can double the energy capacity. Doubling rotational speed quadruples energy capacity…which is why those big old monster flywheels that got loose in the past could do so much damage.

Today’s flywheels are made from by stacking carbon-fiber-reinforced-plastic (CFRP) rotors. Different weights (or mass) can be built to order by varying the number of rotors per flywheel.  Superconducting magnetic bearings are used, and once refrigerated they create a magnetic field strong enough to levitate the stack of rotors. This allows a 4 ton flywheel to spin with virtually no friction. The flywheel is also connected by its shaft to a motor/generator that can both keep the flywheel spinning and generate electricity. Finally, the entire assembly is sealed in vacuum inside a big can to eliminate any possibility of air-drag that might possibly slow the flywheel.

While today’s flywheels are designed to spin up and keep spinning with very little energy, they are NOT perpetual motion machines. Even under the best conditions, a flywheel could not work as a perpetual generator because motor brushes, contacts, magnetic fields, etc used for generating electricity would slow it down.

Alternative Hacks

Try the experiment again by increasing the mass:

  • Switch from the two 16 oz bottles to a pair of 2-liter bottles. The cork should fit. The speed rate should be the same but notice how much the bottles continue spinning at the bottom.
  • If you’re braver and a little stronger, try the experiment by filling the two 16 oz bottles with sand. They will spin up with even more energy and will be harder to stop at the bottom. Because these bottles will be heavy, perform this experiment variation outside in case the cork or a bottle breaks!
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