Help with precession experiment

[icarolo]

I want to produce precession by the use of a bycicle wheel and a spinning chair.
Whats the best position of the wheel both with respect to the vertical and horizontal to cause the maximum precession?

 

[Chi Meson]

I am not sure what it is you want to show. The best demonstration of precession would be:

Spin the bicycle wheel, hang the bicycle wheel from a string attached to ONE side of the axis. Since the weight of the wheel causes a torque in a plane that is perpendicular to the angular momentum of the wheel; conservation fo angular momentum dictates that motion in the third plane must result.

This new rotation (in the third plane, the plane that is parallel to the Earth's surface) is the "precession" of the wheel. TO get "maximum precession" (I assume you mean maximum angular speed inthis third plane) you would apply the maximum torque on the wheel. YOu could do this by also pulling down on the other side of the axel (opposite where the string is attached.)

So if you are sitting in a chair holding the spinning wheel, start with the wheel axel directly left-to-right (as you look at it). FOr maximum rate of transfer of angular momentum to you, you must try to apply a torque on the wheel that perfectly clockwise (or counterclockwise) from your viewpoint AND another torque that is in the plane parallel to the ground.

This second torque on the wheel is to STOP the precession of the wheel, and in doing so , the wheel exerts a counter torque on you which causes you to start spinning. The relative moments of inertia of you and wheel will determine the final rates of spin, so "maximum precession" is not very meaningful.


So if you started with the wheel situated so that the top of the wheel spins away from you, then you exerted a clockwise torque, you would also have to push with your right hand (and pull with your left) to keep the wheel axel always right-to-left in you (now rotating) perspective.

Keep in mind that you are in a system that includes the earth, and torques are exerted on the Earth as well. (You could not do this while balancing on a thin pole, for example.) Total angular momentum for you, the wheel, and the earth, stays constant.
This is all so much simpler to show with 3-D diagrams and demonstrations.

 

[Chi Meson]

What a freakin waste of time!

 

[pete worthington]

What's up with Chi ? This is really great stuff !

My experience demonstrating precession has shown that keeping the vertical and horizontal distance of the bicycle wheel minimized keeps the friction on the platforms bearing minimized. You can draw a force diagram and see how the angular momentum vector and distance of it to the platform bearing plane are critical. A larger distance creates a larger orthogonal loading on the bearing.

I use a $12 lazy susan bearing sandwiched between two pices of plywood. When a student stands centered on the platform with a spinning wheel in hand (holding onto axle) they have total control over precession by applying a torque to the shaft (either in or out). By staying centered (wheel close to vertical) and keeping wheel as low as possible (wheel close to platform) they can turn a few revolutions. I lubricate the bearing with a thin oil before assembly.

I suppose a chair will work but you would need a larger platform and the loading on the bearing may not be as uniform. This is just one way. I would not call it the best. Create your own and discover. This is where the beauty and art of Physics lie !

 

[Chi Meson]

My comment was referring to the original poster. I realize, too late, this question was posed by the same guy what was previously known as "aviator." THis guy had an obsession with misunderstanding physics and seemed to be on his way to finding "free energy." No matter what I said, the guy would not understand it, then he would argue, then he would condemn me for not saying how brilliant he was. My response itself, not the experiment, was a freakin waste of time.

 

[T@P]

actually what's the deal with having icarolo's username crossed out in the first post?

this is a cool experiment, my physics teacher did it for us ( i think)

 

[Doc Al]

actually what's the deal with having icarolo's username crossed out in the first post?

The crossed out name indicates that the user is banned. The thread remains because it would be a shame to lose Chi Meson's excellent description of a simple precession demonstration.

 

[pete worthington]

Chi Meson's post is a great description. This will help me process, understand and communicate the physics of precession to my h.s. physics studentsmuch better.

I did a co-op at draper labs in cambridge mass when i was studying engineering at NU. I worked with a group of MIT engineers on math modeling of the gyroscopes that are in nuclear missiles. I was in the midst of taking Fortran 77 at the time. As a hands-on type of person, i had a strong desire to see what the gyroscope itself looked like. "show me the gyroscope". After 3 months of working there I never saw the physical gyroscope. My supervisor was content with the 45 page 17 parameter mathematical model. I spent most of my time at drapers library reading a paper back book entitled "Fortan for dummies".

The most memorable experience i had at draper was crossing nuclear free picket lines each morning on the way to work. I couldn't resist looking at the picketers faces and eye's. This was life altering for me since the following semester, conflicted with my future, i dropped out of NU.

 

[capullosois]

It interesting this experiment though I understand the original poster dindt mean to hold on the bycicle wheel to produce torque but to let the precession do the work.
The correct answer is that the maximum precession is achieved by setting the wheel 45º with respect to the vertical plane and 0º in the horizontal plane.
By the way i have just completed an experiment that showed that a gyroscope that precesses takes the same time to end its torque than a gyroscope that doesn't have precession.
So where does the work that produce precession come from?
In fact in a powerball by beating the gyro momentum you not only slow it down but accelerate it

 

[Chi Meson]

Again.

Is it so likely that three different people have the same obsession with and misunderstanding of rotational motion?

I'm "outie."

PS.
Thanks DocAl

 

[pete worthington]

Yeah, I have a better picture of my misunderstanding. Giancoli (our text) had no discription of precession. Halliday/Resnick/Walker did. Precession occurs when you hang the wheel by a string off the axle and the wheel rotates about the string. When you hold the shaft and create rotational motion on a platform it is conservation of angular momentum at work, not precession. This thread has been helpful for me. This is a great resource for a lone physics teacher up here at a Maine high school.