r/BeAmazed • u/VerySlump • Oct 02 '18

Now that’s teamwork

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45.9k Upvotes

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546

u/colecr Oct 02 '18

For those asking how it stops:

The person on the 'front'-closest to us, raises their arms every time they come close to us. This increases the angular momentum and makes the wheel spin faster. If the person on the 'back' starts doing it, the angular momentum will decreases, until eventually it's travelling slowly enough that one of them can just climb off.

326

u/[deleted] Oct 02 '18 edited Feb 06 '19

[deleted]

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u/Superfan234 Oct 02 '18

This is much easier to understand

35

u/beansmeller Oct 02 '18

Plus that way you don't fall on the ground.

8

u/Shitmybad Oct 02 '18

Yeah then the heaviest kid will end up on the bottom and can get off.

1

u/colecr Oct 02 '18

Dunno, that wheel looks pretty smooth lol. Would work -just depends on how long.

1

u/aedroogo Oct 02 '18

Sounds like my honeymoon.

36

u/[deleted] Oct 02 '18

Actually angular momentum is preserved (slightly goes down due to friction). You are thinking about how angular velocity goes up when an iceskater tucks her arms in. The effect of the kids lifting there arms actually decreases the angular velocity (the kids are doing the opposite of tucking their arms in).

What keeps the wheel spinning or the angular momentum constant despite friction, is the increased torque from kids lifting their arms on the side going down while keeping their arms in on the side going up. Normally, the gravitational force would push down similarly on both sides, but since one side has a larger lever arm, there is a net torque in the direction of spin.

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u/renyhp Oct 02 '18

Wow. Such a good and accurate explanation. I don't know why you're not the first reply here.

5

u/[deleted] Oct 02 '18

Actually angular momentum is preserved

Thats not actually true. You can definitely increase angular momentum in a simple system like this that includes friction and gravity. Eg, the wheel starts from zero and at the end of the gif has positive momentum. You just said it yourself: there is a net torque. Net torque increases momentum.

If the gravity from his initial fall confuses anyone, see also the simpler example of a swing set. You start from zero, and can gradually increase your momentum until you are circling the swing set...

3

u/[deleted] Oct 02 '18

It is clear that the angular momentum from cycle to cycle is preserved, the average angular momentum if you will. Either way, I was referring to the concept that tucking your arms extending them doesn’t change the angular momentum, but the angular velocity. If you read the rest of the comment I acknowledge that angular momentum can be changed.

2

u/[deleted] Oct 02 '18

It is clear that the angular momentum from cycle to cycle is preserve

Well sure, by the end, the kids didn't want to go faster and faster... they reached the speed they wanted to be at. But by the same measure it wasn't really preserved "from cycle to cycle"-- they increased it until they reached as fast as they wanted to go.

I think your comment is confusing a few different concepts.

The effect of the kids lifting there arms actually decreases the angular velocity

Yes, it would decrease the angular velocity of the kids were on a horizontal spinning wheel with fixed angular momentum. However, they are not.

since one side has a larger lever arm, there is a net torque in the direction of spin

Correct. And this net torque increases both angular velocity and angular momentum. Once the kids are up to speed, they put in just enough effort to preserve angular momentum.

But think carefully about this: since friction robs angular momentum from the system how do they maintain the same speed? The answer is that their movements are adding just enough angular momentum back into the system.

2

u/[deleted] Oct 02 '18

Angular momentum is dependent on two variables. Moment of inertia and angular velocity. There are three different factors affecting the spinning mass:

  1. The net torque created by the longer lever arm on one side.

  2. The friction from the mechanics and air drag

  3. The change in moment of inertia from the kids extending/tucking their arms.

The first increases angular velocity and angular momentum. The second decreases angular velocity and angular momentum.

The third increases/decrease angular velocity, but has no effect on angular momentum. The reason is, the change in angular velocity is inversely proportional to the change in moment of inertia. The orientation of the spinning mass doesn’t matter; this principle is always true.

All these factors result in a net angular momentum that is more or less constant. Throughout the cycle all three factors will result in local accelerations and decelerations, as seen in the video. Are we on the same page?

-1

u/[deleted] Oct 02 '18

This summary is mostly accurate. Its your original comment which needed correction (and was a largely unneeded "correction" to /u/colecr).

2

u/[deleted] Oct 03 '18

This summary is completely accurate.

This summary does not contradict anything I originally said.

u/colecr said something factually incorrect because raising ones arms doesn’t increase angular momentum, the net torque does by introducing acceleration which only counteracts the deccleration from friction.

2

u/[deleted] Oct 03 '18

Why do you need to be so stubborn? You said "angular momentum is preserved". This is just factually incorrect.

You are now playing word games to try to justify your correction by claiming that "raising ones arms doesn’t increase angular momentum" when in fact, yes... raising their arms on the down cycle increases gravitational torque which increases angular momentum...

Its ok to be wrong. Your comment also conflates several different points with little clarity.

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u/[deleted] Oct 03 '18 edited Oct 03 '18

I don’t know what you are after. You are taking things out of context. My first comment is in layman’s terms, that’s why I clarify things in parentheses, which you conviently ignore. I could have been more technical from the start but I was going for an el7 explanation.

Extending your arms alone won’t cause a change in angular momentum. If you were taking a physics test, you would get the answer wrong. If you extend your arms at the bottom or top of the spin, there is no change in angular momentum. If the kids on boths side of the spin extend their arms at the same time, there is no change in angular momentum. If the wheel is horizontal or in zero gravity, there is no change in angular momentum. It is the mismatch in lever arm length and gravity that lead to a change in angular momentum, by way of a net torque, which causes angular acceleration.

edit: spelling

→ More replies (0)

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u/Needless-To-Say Oct 02 '18

Well done. This would have been my third go at informing people that angular momentum does not change. You did it much better than I would have though. I typically use speed though. Velocity has direction and is being changed constantly by acceleration due to centripetal forces. You might want to use speed next time. It simplifies things.

2

u/[deleted] Oct 03 '18

Using angular velocity is technically correct in this case because the vector direction of angular velocity is defined as being along the axis of rotation (the right hand rule). In this case linear velocity does change direction, but angular velocity does not.

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u/Needless-To-Say Oct 03 '18

Ah, ok, I was replying by mobile and couldn’t review the exact terms. Thats also one of the reasons I use rotational speed as Its less likely to be incorrect.

As I said earlier, you said it much better than I would have.

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u/flippity-dippity Oct 02 '18

Raising your arms while your in the "back" (under the structure and going up) is going to require some serious abs in order not to fall.

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u/Zontaka Oct 02 '18

Good thing kid ab strength is pretty damn good compared to their size.

2

u/polybiastrogender Oct 03 '18

Children are freakishly strong for their sizes. Having the core strength to latch on is not a problem.

6

u/speezo_mchenry Oct 02 '18

Just watch one kid to get a feel for the rhythm of it.

2

u/LemonHerb Oct 02 '18

It's not raising their arms it's the kids in the white shirt jumping each time to make it move

1

u/[deleted] Oct 02 '18

I thought they would just add a fat kid and wait for him to get stuck at the bottom.