What happens to precession when torque is removed?

[R Power]

Hi friends
I read about gyros and read their working and analysis based conservation of angular momentum. I understood it as I was made to by the analysis but i didn't find the real reason why gyro shows such a behaviour. I mean what forces it to do the magic it does? Is there any analysis based upon forces? Can someone explain me this fascinating device?
Thanx in advance for good or any reply

 

[lugita15]

If you have an hour of time on your hands, you might want to watch this video lecture:

http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/detail/embed24.htm

Walter Lewin explains quite nicely the apparent "magic" of a gyroscope, and he does touch briefly on the subject of forces between roughly the 28 minute mark and the 30 minute mark, but I strongly suggest you watch the whole thing if you want to understand how gyroscopes work.

Basically, the answer is that gravity is never the only force acting on a gyroscope.

 

[Cleonis]

Hi friends
I read about gyros [...] but i didn't find the real reason why gyro shows such a behaviour. [...] Can someone explain me this fascinating device?

A brief version of the explanation is in the following physicsforums post (from me), about https://www.physicsforums.com/showpost.php?p=2697666&postcount=3".

More extensive coverage is in the article about http://www.cleonis.nl/physics/phys256/gyroscope_physics.php" on my website.


By the way, the demonstrations by Professor Walter Lewin that Lugita15 mentions are by far the most impressive that I know of. He uses motors to spin up the gyros - so fast that it's scary.

However, Lewin makes no attempt to explain the gyroscopic effects. What he does is that he shows that the mathematical formula correctly describes the behavior of the spinning gyroscope.
That is useful in itself, but it doesn't constitute explanation.

 

[GRDixon]

Hi friends
I read about gyros and read their working and analysis based conservation of angular momentum. I understood it as I was made to by the analysis but i didn't find the real reason why gyro shows such a behaviour. I mean what forces it to do the magic it does? Is there any analysis based upon forces? Can someone explain me this fascinating device?
Thanx in advance for good or any reply

Consider an increment of mass in the gyro's spinning rim. When there is no torque applied to the axle, only a centripetal force toward the axle acts on the mass. The gyro does not precess. But now consider the additional force, experienced by the increment of mass, when a torque is applied to the axle. This is manifest as a force perpendicular to the rim and parallel to the axle. Ask yourself where the increment of mass will now end up after a quarter revolution. Note that neither of the force components has a component in the direction of the increment's velocity, and hence neither force component does any work. The gyro's total kinetic energy is unchanged by precession. And of course the moment the torque is taken away, the precession ceases.

 

[Cleonis]

And of course the moment the torque is taken away, the precession ceases.

Out of curiosity: do you predict that after taking away the torque the precession ceases instantaneously?

What I have in mind is the following:
- A gimbal mounted gyroscope wheel.
- A rig that can clasp the axis of the wheel, with a mechanism in place that can unclasp as fast as you want.

I predict that if you unclasp fast enough then on footage shot with a high speed camera you will see a time interval during which the gyroscope wheel adjusts to the new situation. During that interval there is change of direction of the wheel axis
After that very short adjustment phase the gyroscope wheel will keep the same orientation.


By the way, it seems to me that the inverse, instantaneous start of a torque on a wheel, is much more difficult, if at all possible. To unclasp you can have a mechanism that opens up very fast. But it would be very difficult to obtain a rig that can clasp extremely fast, and yet not introduce unwanted side effect, arising from small inaccuracy in aiming.
Therefore I focus on the case of unclasping extremely fast.

 

[GRDixon]

Out of curiosity: do you predict that after taking away the torque the precession ceases instantaneously?

To the extent there is no such thing as a truly rigid body, I would say time delays must be inevitable. The "poor man's" method of determining the sense of precession is meant to ignore such nuances.

 

[Cleonis]

Out of curiosity: do you predict that after taking away the torque the precession ceases instantaneously?

To the extent there is no such thing as a truly rigid body, I would say time delays must be inevitable. The "poor man's" method of determining the sense of precession is meant to ignore such nuances.

Actually, I dawned on me that what I had in mind is not possible.

A wheel is a body with an axis of symmetry, and I was only thinking about spin around that axis of symmetry. But a wheel can spin just as well around any other axis that passes through its center of mass. When a wheel spins around an arbitrary axis then the axis of symmetry sweeps out a cone.

If you take away the torque very quickly then you don't end up with a spinning wheel that has its axis of symmetry pointing in a steady direction. Instead you get the spinning motion described above, with the axis of symmetry sweeping out a cone.

If you take away the torque gingerly, you are applying dampening. It feels natural to remove the torque in such a way that you end up with the axis of symmetry pointing in a steady direction.

So unfortunately the setup that I had in mind is not possible.

By the way, I wasn't thinking at all about the rigidity of the wheel.