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# conservation of angular momentum

Fri 1 Aug, 2008 01:28 pm
If one detatches a bicycle wheel and axle, holds the axle and spins the wheel, the axle resists changing its orientation, that is the axle is very stable. The same holds true for a gyroscope. Is this explained by the conservation of angular momentum? Does the higher concentration of mass away from the center make the axle orientation stronger?
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ebrown p

1
Fri 1 Aug, 2008 01:57 pm
Let's work this through... I am not sure what level of physics you are taking.

Please write how you understand (from your class) what conservation of momentum is (this could either be a description or a mathematical function).

Then we can talk it through.
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Fri 1 Aug, 2008 02:49 pm
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Fri 1 Aug, 2008 02:57 pm
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coluber2001

1
Tue 5 Aug, 2008 11:16 am
ebrown_p wrote:
Let's work this through... I am not sure what level of physics you are taking.

Please write how you understand (from your class) what conservation of momentum is (this could either be a description or a mathematical function).

Then we can talk it through.

Thankyou for your reply. I'm sorry it took so long for my response, but I don't have a computer and have to go to the library once a week.

I have a pretty good understanding of conservation of angular momentum in that the rotational velocity times the moment of inertia (distance of mass from center) equals the momentum. I understand that moving the mass away from the center must slow the velocity, as a ballet dancer doing a pirouette. But the book I'm reading (Isaac Azimov) doesn't mention the stabilizing of the axis with the increasing moment of inertia. I think this is an important concept with many applications, and I'm struggling to understand it. I can see that a bicycle becomes stable as it moves, but I don't know why.

I'd appreciate it if you can help me with this.
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ebrown p

1
Tue 5 Aug, 2008 01:00 pm
The key point is that angular momentum is a vector quantity. This means it has a direction as well as a value. (If you want me to go deeper, please ask).

For a gyroscope rotating counter clockwise on a table (i.e. it is spinning parallel to the table), the angular momentum vector points directly up.

To change the angular momentum, and this includes changing the direction of the angular momentum (which you do when you change the way the top of the gyroscope is pointing) you need to apply a torque.
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