Is the Spinning Wheel's Rotation Maintained? | Investigating Kinetic Energy

[SpaceThoughts]

Hi.

I searched and found no answer to this simple question:
Is the spinning wheel in this videoclip keeping the same rotation (kinetic energy) when flipped upside down and back again?
(if we forget about friction)

 

[A.T.]

(if we forget about friction)

If the bearings are friction-less they have no means to apply any torque along the spin axis. So if there is also no drag on the rest of the wheel, it will maintain its kinetic energy.

 

[vanhees71]

In the movie it's correctly explained with conservation of angular momentum. When turning the wheel upside down the guy has to so work which gets converted in the additional energy to make him and the wheel rotate as a whole, keeping angular momentum constant.

 

[SpaceThoughts]

Thanks for both answers.
I’ve been thinking a lot about the experiment done in the video above, and to me it seems we are losing energy every time we recycle the experiment, questioning conservation of energy.

Think of the laboratorium as an empty isolated system only containing what we need to make the experiment.
Imagine the guy in the video is a robot, that gets the energy for the experiment from two separate stretched springs in his body.

The robot performs two actions during the experiment:
1. Turning the wheel upside down, using the potential energy from the first spring to perform this action.
2. Turning the wheel back again to its original position, using the potential energy of the second spring to perform this action.

The total energy in the system before the two action starts, is the kinetic energy of the spinning wheel, and the two times potential energy of the springs.
At the end of the experiment, the spinning wheel has the same kinetic energy, but the potential energy of the two springs are missing.

Every time we want to repeat this experiment, we would need to stretch the two springs again, converting energy to potential energy in the springs, that we won’t get back.

I am most curious to hear your thoughts about this.

 

[anorlunda]

I’ve been thinking a lot about the experiment done in the video above, and to me it seems we are losing energy every time we recycle the experiment, questioning conservation of energy.

Rest assured, energy will be conserved. You should depend on that to form the next logical question --- where does the energy go?

 

[SpaceThoughts]

Rest assured, energy will be conserved. You should depend on that to form the next logical question --- where does the energy go?

That is what I am asking here. There seems to be no heat loss. Comparing the start scenario with the end scenario shows a difference.

 

[anorlunda]

That is what I am asking here. There seems to be no heat loss. Comparing the start scenario with the end scenario shows a difference.

What about the hands of the man? Work done by or on the hands. Have you accounted for that?

See @vanhees71 answer in #3.

 

[A.T.]

The robot performs two actions during the experiment:
1. Turning the wheel upside down, using the potential energy from the first spring to perform this action.
2. Turning the wheel back again to its original position, using the potential energy of the second spring to

This is way to vague to analyse energy and work done.

 

[SpaceThoughts]

What about the hands of the man? Work done by or on the hands. Have you accounted for that?

See @vanhees71 answer in #3.

Nice expression, and thanks for spending your time on me:-)
Yes, I should say work done by the hands in both cases. Force is used in both flipping the wheel upside down and back. If we started the experiment with the wheel upside down, it would be the same cycle, only the direction of the guy/robot tuning around would be opposite. Where is the trace of the energy used? If there were considerable heatloss because of friction, the guy/robot should not be able to stop himself completely from turning in the end of the cycle. Are you saying between the lines, that the energy goes to making energy disappear? The netto result of that must be loss of energy. From looking at the isolated system from the outside.

 

[SpaceThoughts]

This is way to vague to analyse energy and work done.

In what way? The setup is quite simple ...