Catch the Mistake! Conservation of Angular Momentum

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made with love at MIT
Creative Commons: CC BY-NC-SA

Written and produced by: Elizabeth Choe
Directed by: George Zaidan
Editing and animations by: Per Hoel
Camera: Adam Morrell
Hosted by: Ryan Robinson

[DiscoFang]

At 1:35 after he has already explained the right hand trick, he then explains it again BUT he's mistakenly spun the wheel clockwise for the demonstration but says it's spinning anti-clockwise. In the final shot of him on the pedestal he's spun in the correct way but actually has a different wheel without the white marker. So the mistakes are several.

[markandrews]

All other demonstrations I have found on this topic show that when the direction of torque is reversed by a change in direction of spin, the person rotates in the opposite direction, not stopping as in this demonstration. It seems viable that the system should stop for a moment as the change in direction of the force could result in a moment of zero velocity until time and force in the new direction results in a new net force and momentum.

[guangshuobulian]

Cool video Ryan!! Clear explanation and great camera performance.

[chandapandya]

The error in this video is at the last demonstration when he says the wheel has an angular momentum of positive L, actually it is negative L ( if u observe the wheel close ly) and so when he flips it, the system rotates with an angular momentum of -2L.

[kshitizsingh]

when you flipped the wheel in clockwise direction you were also rotating in clockwise direction. how?

[vinayseth]

Is this a standard pedagogical tool used in the US- Because the recommended videos section is showing me a lot of videos on other physics-related channels with the same demonstration- is this demonstration also used in American schools to clarify the concept?

[popayed]

What makes l to l, not -l. How does wheel know it is turning right or left.? And if you take wheel when it is upside down, i mean -l does it act like l?

[auntyIdontknow]

Does the same happen with out holding wheel, simply by tippy toeing?

[sarthaksharma]

when you spin for the first time same thing happens right?

[rommelnb]

great video-- I will try it with my class

[tusharkantdeo]

Make a video on MOTIONAL EMF AND its real life

[samanthaknepp]

Is the mistake that you say angular momentum measures both how hard it is to start and how hard it is to stop spinning? We know momentum doesn't measure how hard it is to start the motion.

[carlosaf]

(2:06) PLEASE I need a rapid response... Regarding you are spinning clockwise... wouldn't that be -2L ??? Furthermore, you don't spin the opposite direction of the wheel at any time, do you ????

[adosar]

when you are in the ground angular momentun is conserved?

[philsnell]

Ryan defines positive angular momentum as spinning counter-clockwise.  When he flips the wheel through 180 degrees, he has changed the angular momentum of the wheel to -l.  He says that he needs to acquire angular momentum of +2l in order to conserve the angular momentum of the system.  Yet he spins clockwise, not counter-clockwise.  Is this not the opposite direction from that predicted?  He does not describe his motion with the labels and illustrations he used on the wheel, to show how he is spinning with angular momentum of +2l.  What a pity.  I am now thoroughly confused.

[markandrews]

Two months has past Ryan and I was hoping you could provide some more information as many people seem to be confused. When I watched this video a couple months ago it put my brain into a spin because your demonstration was the only one that I have seen where the one direction, you nominated (+L) no rotation is observed. When in the (-L) direction rotation is observed.

This link below from Hyper-physics provides a theoretical backup of what your demonstration showed.

The only explanations I can think of to explain this scenario now and to rebut my earlier thoughts are as follows:
(i) The Angular Frequency (Velocity of the wheel) is low therefore not a great deal of FORCE is generated
(iB) The Moment of inertia is constant in this example since the wheel has a constant circumference and no other mass was displaced in either an outer or inner position like in some similar experiments. (Yes...stating the obvious)
(ii) In the +L position the Angular Momentum is pointing up so my QUESTION is: Were you stationary because Gravity was stronger than L and therefore cancel out the momentum in the anticlockwise direction?

(iv) I understand that The Laws of Conservation of Momentum dictates -L (requires )+2L (to get to) = L
I think the confusion for some of us comes from the associating -L in the clockwise direction of spin of the wheel, and now you are spinning in that direction yet you have a spin of 2L in order to add up to L

Please jump in and correct anything I have said that may be wrong.

[hitren]

The explanation in this video is wrong. After flipping the wheel, he spins in a clockwise direction, implying that his angular momentum is negative. Since this angular momentum is in the same direction as the total angular momentum, the initial angular momentum of the wheel has to also be negative, which means that he misstated the initial direction of the rotation of the wheel. It's very difficult to tell since the wheel he is using does not have a white marker (it should!), but if you look closely, you can see that the wheel is initially spinning clockwise.

[markandrews]

Lecture 24: Rolling Motion, Gyroscopes l 8.01 Classical Mechanics, Fall 1999 (Walter Lewin) watch from the 23rd minute if you want to fast track to the same demonstration. Totally different outcome....

[TheAnimammal]

The mistake is that angular momentum is not conserved. Angular energy is conserved.

[cyrusIIIII]

Is this really at MIT? This was a wrong explanation. At the end, based on his explanations +2L which is upward, he should rotate clockwise but he rotates counterclockwise!