Post by: Eternal Student on 18/05/2022 12:01:08
I was looking through some of the old Feynman lectures and came across this section which attempts to explain what makes a gyroscope start to precess instead of falling. Just to be clear, it does not suggest this is what sustains precession, just what starts it. You have a situation with a spinning gyro supported on a pedestal at the centre and you are holding the (soon to be) free end stationary with your hand until it is suddenly released.
gyro.JPG (28.45 kB . 583x312 - viewed 2394 times)This is what the lecture states:
Some people like to say that when one exerts a torque on a gyroscope, it turns and it precesses, and that the torque produces the precession. ........it does not fall under the action of gravity, but moves sidewise instead! .....( but ).........
.....The gyro actually does fall, as we would expect. But as soon as it falls, it is then turning, and if this turning were to continue, a torque would be required. In the absence of a torque in this direction, the gyro begins to “fall” in the direction opposite that of the missing force.....
[End of section 20-3, "Rotation in Space", Feynman lectures.
https://www.feynmanlectures.caltech.edu/I_20.html ]
https://www.feynmanlectures.caltech.edu/I_20.html ]
I can't be the only one who finds the idea of a "missing force" a bit odd. To say that things accelerate in the direction of a net force is one thing, to say that they would accelerate in the opposite direction to a missing force is much harder to live with.
There's a computer on my desk and it is missing a force to move it left but it does not accelerate to the right.
Anyway, is this explanation with "missing force" still popular? At the moment I'm inclined not to recommend that explanation. Maybe Feynman tried to oversimplify and has ended up with something that isn't all that usefull for understanding? i.d.k. I do note that he has quotation marks around "fall" as if he was just making an analogy to objects falling when they are missing a force to support them.
Best Wishes.
LATE EDITING: Fixed various spelling errors. Title changed from "in the direction" to "in opposition to" etc.
Post by: Zer0 on 06/10/2022 11:33:29
Perhaps Hal could have some useful additions.
Or maybe hoping this OP catches someone's eye.
P.S. - " If you think you Understand Quantum Mechanics, then You Don't Understand Quantum Mechanics ".
R.F.
🤭
Post by: Halc on 06/10/2022 15:39:47
To say that things accelerate in the direction of a net force is one thing, to say that they would accelerate in the opposite direction to a missing force is much harder to live with.It is pretty intuitive. You were holding the end and supporting the weight there. It wants to go down, at least at first. The steady-state is pure motion to the side, but at first it has to accelerate in that direction, and that acceleration produces a reaction force in the opposite direction which pushes the free end down. The falling axle produces a vertical torque on the gyro which gets it rotating in the normal direction, faster even than it normally precesses. This extra speed causes the upward motion, which slows the precession speed (almost to a halt for a moment). Since all forces are met with perpendicular motion, any off-center disturbance is going to perturb the axis location in a sort of circular motion, which decays with time (friction of some kind?) into a clean precession where the motion balances the forces on it and the precession can proceed in a stately line.
Consider the force exerted by the pivot on which the gyro rests. At first, when you let go, there will be a significant lateral force there accelerating the mass of the gyro left. That force pushes the gyro down. During steady-state later on, that force is pure down, taking the weight of the gyro and no more. The lateral forces are gone then. I don't see any missing force.
Great topic BTW, and glad to see you back, even if not at the prior rate of posting.
Very glad that you seem to be getting a grip on the health issues.
I (whole household actually) finally got covid for the first time last August, nothing severe.
Post by: Zer0 on 07/10/2022 03:27:55
🤞
You both are Great Contributors.
👍
P.S. - E.S. please employ a gardener, or bribe the neighbours kids with candies & toffees to mow your lawn.
(Btw, it's been 2 years, you must be having quite a jungle of sorts by now, isn't it)
🤭
Or simply buy yourself one of these...
& You can forget bout the neighbours botherin you.
Noone would Dare to mess around with ya!
😉
Post by: Eternal Student on 08/10/2022 23:49:03
I had forgotten about this post to be honest. It was written a while ago.
Consider the force exerted by the pivot on which the gyro rests. At first, when you let go, there will be a significant lateral force....... etc......Yes, I've always been interested in exactly what size (and direction of) force the pivot exerts on the gyroscope as time evolves. I think you're right, initially the pivot (the place where the central pedestal touches the tip of the gyroscope shaft) could very well be exerting a significant force laterally. I've never seen an experiment where something like a set of torque gauges were added to this point of contact that reveal what forces are being exerted initially and in the steady state.
Thanks for everyone's time, best wishes and bye for now.