Conservation of Angular Momentum

Conservation of Angular Momentum explained through student demonstrations.

Category:

Education

Tags:

• geddes
• physics
• momentum
• sequoyah

[Rosco0284]

I would like to make a closed system with her. Unless she is under 18 years old, in which case - nice video, very informative.

[physilicious]

@Rosco0284 She's 14 and my daughter, so thanks for the kind comments about the video being nice and informative :-)

[Gintable]

No she isn't a closed system, she is breathing in air and exhaling human breath.

And also, there ARE external forces on her, they are called gravity and normal force.

The correct statement is there is no external torque on her.

[physilicious]

@Gintable Any contributions from the act of breathing are insignificant, and the forces due to gravity and the normal force are equal and opposite, thus cancelling each other. It would have been more accurate to report that the net external force and torque are zero. Thanks for your input.

[frasierface]

what is the bottom thing that spins?

[physilicious]

@frasierface: the bottom thing spinning in the angular momentum video is a rotating platform. You buy them at science/lab equipment supply stores.

[JDaveD]

lol..... I*(butt) is equal to i *(bigger butt)

[TheMisterytour]

@frenchsaber burn her lol

[frenchsaber]

She's a witch.

[surajcena91]

The girl and the wheel in her hand forms a closed system. initially the total angular momentum is from the wheel only and it is in horizontal direction. Now when the girl is tilting the wheel she is tilting the angular momentum of the wheel form the horizontal. The vertical change in the angular momentum is balanced by the rotation of the girl in opposite direction. How does the the angular momentum in horizontal direction maintained constant especially when the wheel is kept parallel to ground?

[inqubeko]

@physilicious

? The total torque of her and the disc she stands on is not zero! Otherwise she would continue to spin forever. Due to friction. She loses energy in this way, there would be more energy lost in the case when she holds the wheel horizontally because then she spins as well and then total friction is greater then when only the wheel is turning when she holds it vertically.

There should be a seperate term for 'conservation' of momentum for macroscopic systems, it is confusing.

[puncheex]

To expand: When the girl holds the wheel vertical all the angular momentum is concentrated in the only free object in the vertical direction: the wheel. It is applying torque to the girl but it is being resisted (unconsciously; it's not that big). When she turns it horizontally, the torque it applies is now free to turn her in the opposite direction the wheel is turning until the same momentum is in her as in the wheel; her momentum gain is equal to the loss in the wheel, which slows.

[puncheex]

@Gintable: What is meant by "closed system" in the context of the experiment is a system in which no other unbalanced torques are being applied to her or the wheel, excepting the inevitable friction at the bearings (which are indeed slowly converting the rotational energy into heat). The fact that she is moving the wheel, that the rubber in the tire is oxidizing, entropy is increasing and she's giggling are all irrelevant to the "closedness" of her system.