Gyroscope speed change question

by penek
Tags: angular momentum, gyroscope
 P: 2 Hi, i tried to figure out and made some research but with no luck to my problem: Let us supouse that we do have gyroscope with no friction (of course theoretically). We can imagine something like spinning rotor rotating around but not touching spin axis (magnetic levitation) and this enclosing in a boll with perfect vacuum inside. So if the gyroscope is spinning and we do not touch it it will theoretically run forever with the same angular speed. My Question is - is it possible to slow down the angular speed of the giroscope(rotor) by simply changing direction (more precisely plane) of the angular movement of the rotor ( supposing again that we can change direction with no friction)?
 P: 114 You might want to look at something called a 'powerball'. With this you can increase the gyro spin speed by forcing precession on the other two axes.
P: 2
 Quote by pumila You might want to look at something called a 'powerball'. With this you can increase the gyro spin speed by forcing precession on the other two axes.
Thank You, as you suggested I read about "powerball" a little, and it gives me some better understanding now. I was surprised about its behaviour. Must say that gyros are very fascinating. I even ordered a "powerball" to better see and feel its working, and strenghten my arms by the way :). Of course they are some movies on youtube but you can only watch, not feel

Thanks once again

PF Gold
P: 693

Gyroscope speed change question

 Quote by penek So if the gyroscope is spinning and we do not touch it it will theoretically run forever with the same angular speed. My Question is - is it possible to slow down the angular speed of the giroscope(rotor) by simply changing direction (more precisely plane) of the angular movement of the rotor ( supposing again that we can change direction with no friction)?
No, in a zero-friction-scenario you cannot change the angular velocity of the rotor.

The rotor shaft is suspended in bearings. (Any kind of bearing; mechanical bearings, or air cushion bearing, any form of supporting the rotor shaft.)

The bearing encircles the rotor shaft, so the support of the bearing must be thought of as being exerted upon the geometric center of the rotor shaft.

That geometric center of the rotor shaft is a pure line in space. Since you can only grip that line, you have no way of exerting torque around an axis that is parallel to the rotor shaft. Absence of torque around an axis parallel to the rotor shaft means that after each reorientation of the rotor its angular velocity is the same as before the reorientation move was started.

Powerball
A powerball uses friction. By the feel of it I think the rotor shaft of the powerball rotor is about 5 milimeter thick. That rotor shaft runs in a groove. With the string you get a starting velocity. With the slow initial angular velocity the gyroscopic effect is weak, the rotor shaft presses only lightly agains the sides of the groove, so it's tricky to increase the rotor speed.

But once you manage to get past that the gyroscopic effect gets stronger and stronger, so you can make the rotor shaft press hard agains the sides of the groove, giving you more grip, and you can pack on velocity rapidly.
 P: 114 I am not convinced that friction has any role to play here other than in limiting the maximum rate of spin. If friction was the cause it is difficult to identify the exact process whereby the powerball could speed up. You have to understand that there are three axis to any gyroscope. Spin on one axis creates a moment that enables torque on one of the other axes to cause a rotation on the third axis. However there is only the driving mechanism that defined each axis. We can force torque and a spin rotation on the second two axes, to create changes in the rotation of the original spin axis. In other words if we define the original spin axis as 'a', the second as 'b' and the third as 'c', we start the powerball gyro by spinning up axis 'a' with a string. Then we can force motion with our wrist that rotates 'b' to create a new spin axis, and at the same time put torque on 'c', so that we change the rotation rate on 'a' which is now the third axis of this new motion. The spin rate on 'b' must be low to keep the new moment low, so that the changes in rotation rate on 'a' are high for low torques. The wrist action must be precise.
PF Gold
P: 693
 Quote by pumila I am not convinced that friction has any role to play here other than in limiting the maximum rate of spin.
I did some additional reading, and I came across the following statement in the Gyrotwister FAQ:

"[...] If oil enters the inner mechanisms, then the axle can no longer roll. Instead, it would skid. The rolling process is totally fundamental, because it allows the rotor to build up speed. If the axles skid, they never get a chance to build up any speed."

So, you can verify it for yourself. If you lubricate the groove of your powerball, and you find that after that you can no longer get it going then you know that friction is essential for the powerball.
 P: 114 I think you may have misunderstood what they are trying to say. The friction must be high enough that there is no slip at all, the axle must roll in the casing like a tyre rolls, without any slip against the casing. The oil causes slip and that destroys the ability to create the necessary torque through the casing into the axle. The friction is not a slip or loss mechanism, it is static friction - that is, it must be high enough that there is no slip to cause loss of traction between the rolling axle and the case. It can be much higher than this level without affecting the mechanism, but if it is too low to maintain rolling static friction (perhaps through the addition of oil) then the mechanism fails.
PF Gold
P: 693
 Quote by pumila The friction is [...] static friction
Indeed it is.

In retrospect, when I wrote "a powerball uses friction" it would have been clearer to write "a powerball uses static friction", as I was thinking of static friction. It's about grip, which is stated a couple of sentences later: "[...] make the rotor shaft press hard against the sides of the groove, giving you more grip, [...]

Summerizing:
- The powerball is about grip of the rotor axis, rolling along either side of the groove.
- A technical term for grip is static friction.

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