# Classroom Denomstration of Rotational Kinetic Energy

Is there an amazing way I can demonstrate Rotational Kinetic Energy in a classroom? It's for a presentation and I've looked and thought about some experiments but they involve Angular Momentum which another group has to present/explain.

Sorry if this is in the wrong section, it's my first post : )
Thanks.

How about bringing a top to class. When you toss a top down, and the top reaches the limit of the string, the potential energy that the top had when it left your hand, along with the linear kinetic energy of the toss has been converted to rotational kinetic energy. That stored kinetic energy is converted back to potential as the top returns to your hand.

Excuse me.. I meant yo-yo, not top.

Thanks jimvoit, I might use that one. Any other suggestions?

rcgldr
Homework Helper
Energy is the ability to do work, and it's energy that you've been tasked with. I can't think of a means to demonstrate angular energy without also involving angular momentum.

You could attach a weight to a string that was wrapped by differing amounts around the shaft of a horizontally mounted gyroscope to show a energy relationship between the distance the weight dropped and the speed^2 of the gyroscope, assuming you can figure out a cheap means to measure the rotational speed of the gyroscope.

jambaugh
Gold Member
Let's see... You could race two round objects down an inclined plane and then along a level surface.

Use say a car with light wheels and an identical car packed in a cardboard cylinder so that the whole thing rolls.

The question is why is the car rolling on its wheels traveling faster.

Point out that since they weigh the same the faster car has more kinetic energy due to its forward motion.

But since both dropped the same height they both got the same amount of potential energy from gravity. What happened to the missing energy in the car rolling inside the cardboard tube?

The answer is of course that part of the energy went into the rotation of the whole car where in the other case only the wheels are rotating...and even though the wheels roll faster they are small so have less rotational kinetic energy.

It would be ideal if you can measure the speeds and do the quantitative calculations on the potential, linear kinetic and rotational kinetic energies. Do you have one of those ultrasound rangers which hook into the TI calculators?

You could make an entire lab out of this after the basic demo.

[PS: To make it perfect let the car carry the cardboard cylinder in the one case. Also if friction comes up point out that (and carefully set things up so that...) there is no sliding so no frictional work is done.]

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jtbell
Mentor
Do you know how a mass oscillating up and down on a spring illustrates conversion of energy back and forth between potential energy and kinetic energy?

Some springs twist a little bit as they lengthen and contract, which causes the mass to start rotating in an oscillating fashion. With the right combination of spring and mass, there will be times when the kinetic energy is mostly translational (up and down), and so the mass bobs up and down more or less normally. At other times, the kinetic energy is mostly rotational, so the mass twists back and forth around its vertical axis while remaining almost stationary in its vertical motion. The system alternates between the two modes of oscillation. In between, the mass simultaneously bobs up and down while twisting back and forth. The total energy remains constant (ignoring friction): translational kinetic energy + rotational kinetic energy + elastic potential energy of the spring + gravitational potential energy.

Let's see... You could race two round objects down an inclined plane and then along a level surface.

Use say a car with light wheels and an identical car packed in a cardboard cylinder so that the whole thing rolls.

The question is why is the car rolling on its wheels traveling faster.

Point out that since they weigh the same the faster car has more kinetic energy due to its forward motion.

But since both dropped the same height they both got the same amount of potential energy from gravity. What happened to the missing energy in the car rolling inside the cardboard tube?

The answer is of course that part of the energy went into the rotation of the whole car where in the other case only the wheels are rotating...and even though the wheels roll faster they are small so have less rotational kinetic energy.

It would be ideal if you can measure the speeds and do the quantitative calculations on the potential, linear kinetic and rotational kinetic energies. Do you have one of those ultrasound rangers which hook into the TI calculators?

You could make an entire lab out of this after the basic demo.

[PS: To make it perfect let the car carry the cardboard cylinder in the one case. Also if friction comes up point out that (and carefully set things up so that...) there is no sliding so no frictional work is done.]

This is perfect! Thanks jambaugh! Yeah I won't be able to get my hands on one of that ultra sound rangers but the basic demo should be perfect for showing the rotational energy. Thanks again everyone!