# Spinning Bycle wheel on string (precession)

• azaharak
In summary, the conversation discusses the dynamics of gyroscopic motion, specifically the behavior of a spinning bicycle wheel as it precesses about a vertical axis. The participants discuss the effects of gravity and torque on the angular momentum and speed of the wheel, and the challenges of accurately measuring and predicting these changes. They also mention previous discussions and resources for further understanding of the topic.
azaharak
Trying to analyze all the dynamics of gyroscopic type motion.

Assume the Z axis is vertical (which the wheel precesses about)

Have a spinning bicyle wheel, that is held up via string on one end, the biycle wheel begins to precess about the vertical axes via torque induced from gravity.

I'm trying to get an idea of the correct behavior of the system specifically when we analyze the angular momentum contribution in the z direction due to precession.

For a faster spinning disk, the precession speed is smaller. One can see this through derivation or by noting that the gravity induces the same torque in the same time, and a faster spinning disk signifies a longer angular momentum component perpendicular to the disk... which means that the change in angular momentum (from torque) would correspond to a smaller angular displacement.

"You can think of lifting/pivoting the ends of a meterstick and ruler through 1cm, the ruler since it is smaller in length will displace a larger angular distance."

Ok that's not my question yet...

If you attempt to make an increase to the Lz angular momentum manually by moving the spinning disk along with the precession, its angle of tilt increases, and it also "seems" that in doing so the speed of the disk decreases.

The angle of tilt increasing I can perceive through the torque introduced by myself, consquently the precession speed does increases too, however the change in the disks speed Is what puzzles me.

How do I know the disks speed changes? I tried/ performed the experiment with a bicyle wheel as described above, a piece of paper tickers past the spokes crudely informing me of the angular speed of the wheel. I notice that trying to torque the wheel so that its precession increases "seems" to reduce the wheels speed and tilt it more upright.

Thanks for you comments.

AZ

Last edited by a moderator:
azaharak said:
If you attempt to make an increase to the Lz angular momentum manually by moving the spinning disk along with the precession, its angle of tilt increases, and it also "seems" that in doing so the speed of the disk decreases.
[...]
How do I know the disks speed changes? I tried/performed the experiment with a bicyle wheel as described above, a piece of paper tickers past the spokes crudely informing me of the angular speed of the wheel.

AZ

To obtain a comparison I take the following setup:
A wheel is spinning, the axis is horizontal. Then the center of mass of the wheel is accelerated, in horizontal direction, perpendicular to the spin axis.

I expect that in the case of a spinning wheel the acceleration will be somewhat less than in the case of a non-spinning wheel. I expect that some of the work done will be in decreasing the spin rate of the wheel, rather than accelerating the center of mass. But even this case is tricky.

The case of a precessing wheel, with a torque applied that increases the rotation rate around the z-axis, is more complicated because of freedom to climb or slump down. Gravity is doing work. Right now I don't have an answer.

That's a really interesting problem you've raised. I'll be thinking about it a lot.

## 1. What is precession in relation to a spinning bicycle wheel on a string?

Precession is the phenomenon where the axis of rotation of a spinning object changes direction. In the case of a spinning bicycle wheel on a string, this means that the wheel will start tilting and changing direction as it rotates.

## 2. What causes precession to occur in a spinning bicycle wheel on a string?

Precession is caused by a combination of forces acting on the spinning object. In the case of a bicycle wheel on a string, the force of gravity pulling down on the wheel and the tension of the string pulling towards the center create a torque that causes the wheel to precess.

## 3. How does the speed of the wheel affect precession in a spinning bicycle wheel on a string?

The speed of the wheel does not have a significant effect on precession. The main factors that influence precession are the weight and distribution of the mass of the wheel, as well as the length and angle of the string.

## 4. Can precession be observed in other spinning objects besides a bicycle wheel on a string?

Yes, precession can be observed in many spinning objects, such as gyroscopes, tops, and even planets. This phenomenon is a result of the laws of physics and can be seen in various forms in different systems.

## 5. How is precession important in scientific research and applications?

Precession has many practical applications in various fields of science and technology. For example, it is used in the design of gyroscopes for navigation systems and spacecraft control. It is also studied in quantum mechanics and relativity, providing insight into the behavior of spinning particles and the curvature of space-time.

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