Preliminaries:

 

††††††††††† I remember reading an article in Scientific American around 1978 regarding a hybrid car that used a flywheel as an energy storage device. The flywheel had an unusual geometry, with most of its mass near the rotation axis.

††††††††††† I suspected that we could discover in a lab situation what benefits a flywheel of this design would offer over a more standard design, which has the mass at some distance from the axis.

††††††††††† I have not been able to find the old article yet, although I will find it. I suspect that the geometry allowed the flywheel to rotate at a faster speed than a conventional flywheel without exploding. I donít remember what the deciding factors actually were.

††††††††††† The following lab experiment is intended to demonstrate the effect geometry has on flywheel performance, while focusing on rotational properties such as moment of inertia, energy storage and transfer, and mechanical power.

 

http://www.flywheeldesign.com/

 

 

 

Prelab Questions:

 

1.†††††††† Does the meter on the outside of your house measure energy or power?

††††††††††† (Energy. We will be discussing mechanical energy and power in this lab.)

 

2.†††††††† What factors contribute to energy storage in a flywheel?

 

3.†††††††† What factors contribute to the efficient transfer of energy from a flywheel?

 

4.†††††††† List as many ways you can think of that flywheels are used today.

 

5.†††††††† How can you mazimize the kinetic energy of a flywheel, without changing its rotational speed?

 

6.†††††††† What factors contribute to rotational momentum in a flywheel?

____________________________________________

 

 

Physics ### lab

Physics of Rotation - Flywheel

 

 

Purpose: The purpose of this experiment is to investigate the use of flywheels. We will measure mechanical power and observe energy conservation.

 

Equipment: For this experiment, you will need:

 

                    Flywheel apparatus with two flywheels (sphere and ring)

                    Electric drill

                    Stopwatch

                    CBL with timer

 

 

Background:

A flywheel is a recognizable component of any steam engine. This may lead to the belief that they are an obsolete technology.

 

It is also true that older (and generally smaller) metal presses operate with the help of a flywheel, (see pictures below).

http://www.lindsaybks.com/bks/patent/

http://www.universalpress.com/6250.html

 

However, research goes on worldwide for mechanical energy storage systems that utilize flywheels. Their many advantages include small size and quick recharging times, as compared to batteries. Below you see a commercially available model from GE Digital Energy (GEDE).

http://www.geindustrial.com/cwc/products?pnlid=8&famid=47&catid=211&id=aboutpm&type=Product+Information

 

 

Comparison of Rotational and Linear Physics

 

Moment of Inertia of a sphere about any diameter is (2/5)mr2. (m is mass, r is radius)

Moment of Inertia of a ring about its coaxial axis is mr2.

 

 

Procedure:

 

1)†††††††† Spin the different flywheels with the drill and then let them go to spin down on their own. Time how long they spin. Take three readings of each and average the times. Record your observations.

 

2.)††††††† Wind the yo-yo bob up on the spindle of the spherical flywheel and then release the weight. Time how long it takes to unwind once you drop it. Take three readings of each and average the times. Record your observations.

 

3)†††††††† Wind the yo-yo bob up on the spindle of the spherical flywheel and then release the weight. Measure how high it recoils after reaching the end of the string. Take three readings of each and average the times. Record your observations.

 

4)†††††††† Predict how long it will take, and how high the weight will recoil, while using the ring flywheel. Record your prediction. Include justification for your predictions.

 

5)†††††††† Wind the yo-yo bob up on the spindle of the ring flywheel and then release the weight. Time how long it takes to unwind once you drop it. Take three readings of each and average the times. Record your observations.

 

6)†††††††† Wind the yo-yo bob up on the spindle of the ring flywheel and then release the weight. Measure how high it recoils after reaching the end of the string. Take three readings of each and average the times. Record your observations.

 

7)†††††††† Explain the difference in the time and height readings between the two flywheels. Include calculations involving momentum, energy, and power.

 

 

Follow up Questions for Students:

 

1)†††††††† Why might it be advantageous to design a flywheel with its mass concentrated near the axis of rotation?

 

2)†††††††† If you double the speed of a given flywheel, how will its kinetic energy change? (Be specific.)

 

3)†††††††† If you double the mass of a flywheel, how will its kinetic energy be affected?

(Be specific.)

 

4)†††††††† Use the energy equations (mgh - Iw2/2) to calculate the rotational speed (w) of each flywheel when the weight reaches the bottom.

 

5)†††††††† Did the weights for both flywheels climb back to the same height after rebounding at the bottom? If not, why?

 

6)†††††††† During the yo-yo part of this lab, which of the two flywheels delivered more power? (Justify your answer.)

 

7)†††††††† If you wanted to store a maximum amount of energy in a flywheel of a particular mass, how would you shape it?

 

8)†††††††† List things you can think of that limit the amount of energy you can store in a flywheel.

 

 

 

__________________________________

 

 

 

Comments

 

Students generally assume that flywheels are an obsolete technology. They also assume that designing them with the mass at some distance from the rotational axis is best for all applications. This lab will follow a discussion that highlights recent achievements and challenges in applications of flywheels as energy storage devices.

 

The flywheel apparatus I used could easily be scaled down to desktop size.

 

I'll describe the flywheel apparatus I used.

 

 

A 2" ball bearing. Everybody knows its moment of inertia. It weighs 2.2 lb. (Interesting, because the one conversion factor I've never had trouble remembering is that there are 2.2 pounds in a kilo.)

 

I bent a ring. This flywheel has exactly (as close as I can tell with our scale) the same weight as the ball.

 

I made a weight. And, what the heck, it weighs 2.2 lb, too.

 

 

I drop the weight. As it falls, it unwinds a string around the axel. The ball starts fast and stops pretty fast. As the weight rebounds from the bottom of the string like a yo-yo, I record the height it attains.

 

Same deal with the ring. We calculate energy, etc, and compare with results.

 

Another thing I do it run the ball with an electric drill, (in reverse). I grab the chuck, which unleashes the axel. The thing spins to a stop, as I time it.

 

I do the same thing with the ring. Only difference is, it never stops. It runs until it's time to go home.

 

 

 

Sources

 

Flywheel Basics Tutorial

http://members.netjunk.com/flyrpm/basics.htm

 

Flywheel (Conservation of Energy)
http://demo.physics.uiuc.edu/LectDemo/scripts/demo_descript.idc?DemoID=451

 

Designing Safer Flywheels

http://www.testdevices.com/flywheel_article.htm

 

 

Moment of Inertia

http://hyperphysics.phy-astr.gsu.edu/hbase/mi.html

 

 

Moment of InertiaóRing

http://scienceworld.wolfram.com/physics/MomentofInertiaRing.html