# Thread: Angular momentum & rotation speed

1. Hello,

I've got a question and hope you guys can help me.
Does the effect of angular momentum reduce the speed of a rotating object?

For a good example please watch the following video from 4:10 to 5:10

Does the rotation of the wheel slow down due to the guy in the chair spinning around because of the angular momentum from the wheel?

Thanks a lot.

2.

3. Originally Posted by AjL
Hello,

I've got a question and hope you guys can help me.
Does the effect of angular momentum reduce the speed of a rotating object?

For a good example please watch the following video from 4:10 to 5:10

Does the rotation of the wheel slow down due to the guy in the chair spinning around because of the angular momentum from the wheel?

Thanks a lot.
No

Neglecting friction, the wheel will keep spinning at the same speed.

What is going on is that the vector angular momentum of the wheel, professor and chair remain constant. So when the professor rotates the attitude of the spinning wheel from horizontal to vertical, the chair and professor also spin so as to keep the total angular momentum constant.

But he has imparted no torque to the wheel about the axis of rotation which would be required to slow it down.

4. Thanks a lot DrRocket!

One more question, if you spin up a wheel it has a certain angular momentum direction, does this direction always stay the same? What if you flip it 90 degrees and then speed it up again?

5. Originally Posted by AjL
Thanks a lot DrRocket!

One more question, if you spin up a wheel it has a certain angular momentum direction, does this direction always stay the same? What if you flip it 90 degrees and then speed it up again?
The total angular momentum of a system will remain constant unless a torque is applied from outside the system.

When you flip it 90 degrees and speed itup again (or just flip it or just speed it up) you must apply a torque to do that.

Conservation of momentum is one of the most important principles of physics. There are no known violations and if one were found it would be a very big deal.

Conservation of angular momemtum is a logical consequence of conservation of momentum. Angular momentum about a point is nothing more than the cross product of the linear momentum vector with the vector that is the position relative to that point.

6. I am not entirely sure what you mean, but if the wheel were first parallel to the plane of the Earth and you flipped it so it was perpendicular then I should think that angular momentum would be lost quicker as the motion of the wheel is now having to contend against gravity for 180 degrees, whilst being helped by gravity for the next 180 degrees; if the wheel is not spinning at escape velocity then it will slow quicker than it would whilst having to contend with just friction/drag alone.

If the wheel WAS spinning at escape velocity then I should think that the radial g-force would rip the structure of the wheel to shreds.

Dr Rocket - Over to you!

7. Originally Posted by leohopkins
I am not entirely sure what you mean, but if the wheel were first parallel to the plane of the Earth and you flipped it so it was perpendicular then I should think that angular momentum would be lost quicker as the motion of the wheel is now having to contend against gravity for 180 degrees, whilst being helped by gravity for the next 180 degrees; if the wheel is not spinning at escape velocity then it will slow quicker than it would whilst having to contend with just friction/drag alone.

If the wheel WAS spinning at escape velocity then I should think that the radial g-force would rip the structure of the wheel to shreds.

Dr Rocket - Over to you!
Gravity has little to do with the question, and you did not include it in your original set-up of the question. You can do this thought experiment in deep space if you like.

Even in a gravitational field your wheel is symmetric so gravity will not be exerting a torque, unless your wheel is out of balance.

There is no such thing as "spinning at escape velocity".

Whether or not there are sufficient internal stresses in a real material wheel that are generated by spinning to cause structural failure is a completely different question from your original question.

You probably need to read a physics book on the subject of classical mechanics. There are a couple of good ones --Classical Dynmics of Particles and Systems by Marion is good at the advanced undergraduate level and Classical Mechanics by Goldstein is the standard graduate text.

Over and out.

8. erm. I wasn't the OP ?

but thanks on the book recommendations.....There is another BRILLIANT book thats free on the web as a .PDF. called "Motion Mountain" (yet to read it all though!)

9. This type of film shows that a classic windmill would start to rotate on a vertical axis if the millner would forget to lock the mill. It means a windmill influences the duration of a day as the torque on the mill is a torcque on the earth also. Minimum but principal it is. Maybe that,s why don quichotte was fighting against windmills.

10. Originally Posted by leohopkins
erm. I wasn't the OP ?

but thanks on the book recommendations.....There is another BRILLIANT book thats free on the web as a .PDF. called "Motion Mountain" (yet to read it all though!)
http://www.motionmountain.net/index.html

Interesting. And free.

I looked through the material on classical mechanics. It seems like a nice discussion. I think that BRILLIANT is a bit of an exaggeration, but that is somewhat a matter of taste.

It is certainly worth reading, but it is no substitute for the detail in either of the books that I noted previously.

Thanks again guys! One quick followup question:

When the man in the video on the spinning chair holds the rotating wheel (which causes him to rotate) gets stopped by another person, does that mean his angular momentum that caused him to rotate in the first place is now zero or can he start rotating again when he puts the wheel vertically and then horizontally again?

12. Originally Posted by AjL
Thanks again guys! One quick followup question:

When the man in the video on the spinning chair holds the rotating wheel (which causes him to rotate) gets stopped by another person, does that mean his angular momentum that caused him to rotate in the first place is now zero or can he start rotating again when he puts the wheel vertically and then horizontally again?
Once he has stopped (due to forces and torques imposed by the assistant), the angular momemtum of the system of the man and wheel (relative to some arbitrary point in the laboratory) is whatever is determined by the spinning of the wheel. If he moves the wheel vertically he will start to spin again, and when he moves it back to horizontal he will again stop. Angular momentum is conserved, that is the key.

13. Another followup question that popped into my mind. Not sure whether there's a known answer for it though:

Why does the effect of angular momentum go in the direction that it does?
ie: Why does it follow the 'right hand' rule? Any particular "logical" reason why it's not the 'left hand'?

Thanks.

14. if you're asking why he spins to clockwise when the wheel is tilted counterclockwise. i believe the answer is that the part of the wheel farthest from the vertical axis of the chair is traveling counterclockwise relative to the vertical axis of the chair, while the part closest to the axis is traveling clockwise. the two points have different angular momentum around the axis of the chair.

the point going counterclockwise has more angular momentum, and in order to counteract that and maintain balance in the system the entire system must spin in the opposite direction(clockwise) with equal angular momentum to the difference between the two points.

15. Originally Posted by saul
if you're asking why he spins to clockwise when the wheel is tilted counterclockwise. i believe the answer is that the part of the wheel farthest from the vertical axis of the chair is traveling counterclockwise relative to the vertical axis of the chair, while the part closest to the axis is traveling clockwise. the two points have different angular momentum around the axis of the chair.

the point going counterclockwise has more angular momentum, and in order to counteract that and maintain balance in the system the entire system must spin in the opposite direction(clockwise) with equal angular momentum to the difference between the two points.
Can you elaborate perhaps a little more? I'm not following you completely.

The spinning wheel isn't really spinning counter- or clockwise when it's in a vertical position because it's a matter of which side you're looking from. I get that there needs to be a balance, but why is the direction of angular momentum the direction that is has and not the other way? Maybe it's a bit like asking "why is C, C".

16. okay well consider a person holding a weight at full arms length and sitting in that chair. if they throw the weight to the right(a tangent off of the circle that his hand is on) it pushes on his hand equally as it is thrown. this imparts momentum onto his hand. the hand is at a far distance from the middle of the chair so angular momentum is relatively high.

now consider if her were to hold a similar weight at half harms length and threw it to the left. the same effect occurs in the opposite direction, but the equal momentum imparted on his hand is a relatively lower angular momentum.

in this analogy your hands are the farthest and closest point on the wheel. this demonstrates that the farthest half of the wheel has more angular momentum than the closest half. and thus it has a net angular momentum of p1 - p2 where p1 is the farther half and p2 is the closest half. because of conservation of momentum, the entire system must spin in the opposite manner to make it go to zero. the equation that represents this is (p1-p2) + (p2-p1) = 0

if this isn't clear i'll just find a good article explaining angular momentum and post a link

17. [quote="AjL"]
Originally Posted by saul

The spinning wheel isn't really spinning counter- or clockwise when it's in a vertical position because it's a matter of which side you're looking from. I get that there needs to be a balance, but why is the direction of angular momentum the direction that is has and not the other way? Maybe it's a bit like asking "why is C, C".
Mathematically is has to do with a choice of an orientation for 3-space.

In physics this is usually described as the "right hand rule".

http://en.wikipedia.org/wiki/Right-hand_rule

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