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Author Topic: What is the simplest explanation of gyroscopic precession?  (Read 35240 times)

Offline JP

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What is the simplest explanation of gyroscopic precession?
« Reply #50 on: 14/04/2011 00:55:45 »
The reason friction is important is because, if Mr "Know-it-all-theoretical-physicist-JP" was actually able to find a frictionless surface and position it normal to the force of gravity AND spin a spherical spinning-top (of uniform density) on that surface, the spinning-top wouldn't precess.

I'm very confident it would.  I can show it with math, but I don't know if you'd trust that without a frictionless surface to back it up.  You can experimentally check it by testing a gyroscope on a series of surfaces with decreasing coefficients of friction.  The precession rate should decrease towards zero as you reduce the friction.  Of course, without friction, it will never slow down, and slowing down makes the precession more evident, so you'd have to start it off at a large angle with respect to the surface to begin with...

If I can find my toy gyroscope around, I might give this experiment a shot.  In an oiled glass pan, I should see very little precession.

Ah! But you may have overlooked a couple of teensy details. The top is completely spherical, and it has uniform density. Therefore, there is no couple.

That's where your mistake is, Geezer!  Perfect spheres are called cows, not tops!
 

Offline Geezer

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What is the simplest explanation of gyroscopic precession?
« Reply #51 on: 14/04/2011 01:06:30 »
The reason friction is important is because, if Mr "Know-it-all-theoretical-physicist-JP" was actually able to find a frictionless surface and position it normal to the force of gravity AND spin a spherical spinning-top (of uniform density) on that surface, the spinning-top wouldn't precess.

I'm very confident it would.  I can show it with math, but I don't know if you'd trust that without a frictionless surface to back it up.  You can experimentally check it by testing a gyroscope on a series of surfaces with decreasing coefficients of friction.  The precession rate should decrease towards zero as you reduce the friction.  Of course, without friction, it will never slow down, and slowing down makes the precession more evident, so you'd have to start it off at a large angle with respect to the surface to begin with...

If I can find my toy gyroscope around, I might give this experiment a shot.  In an oiled glass pan, I should see very little precession.

Ah! But you may have overlooked a couple of teensy details. The top is completely spherical, and it has uniform density. Therefore, there is no couple.

That's where your mistake is, Geezer!  Perfect spheres are called cows, not tops!

Typical! A standard theoretician's deflection technique.

First of all he introduces "frictionless surfaces", then he has the audacity to object when I introduce perfect spheres.
 

Offline JP

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What is the simplest explanation of gyroscopic precession?
« Reply #52 on: 14/04/2011 01:59:02 »
Well, I tried it out on an oiled glass surface, a table cloth and a wooden cutting board and it precessed on all of them noticeably, and didn't seem to be significantly less on the glass.  I guess putting it in a Teflon pan would be best, since that has a coefficient of static friction of 0.04, but I'd rather not scratch up my cookware with a gyroscope.  ;)
 

Offline Geezer

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What is the simplest explanation of gyroscopic precession?
« Reply #53 on: 14/04/2011 02:10:53 »
Well, I tried it out on an oiled glass surface, a table cloth and a wooden cutting board and it precessed on all of them noticeably, and didn't seem to be significantly less on the glass.  I guess putting it in a Teflon pan would be best, since that has a coefficient of static friction of 0.04, but I'd rather not scratch up my cookware with a gyroscope.  ;)

We could have saved you the trouble! Obviously, the centre of mass of a top can only go down rather than up. But the centre of mass of a sphere will not go down (or up), and if there is no friction between the sphere and the surface on which it rests, there is no reason why it would precess.
 

Offline JP

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What is the simplest explanation of gyroscopic precession?
« Reply #54 on: 14/04/2011 02:57:25 »
Well, my gyroscope isn't quite spherical, but maybe we can neglect any deviation from sphericity as being minor.  Then precession is experimental error!

If not, then perhaps syphrum would find this useful.  :)
 

Offline Geezer

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What is the simplest explanation of gyroscopic precession?
« Reply #55 on: 14/04/2011 06:43:29 »
Well, my gyroscope isn't quite spherical, but maybe we can neglect any deviation from sphericity as being minor.  Then precession is experimental error!

If not, then perhaps syphrum would find this useful.  :)

I'm just glad we were able to get it all sorted out.
 

Offline yor_on

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What is the simplest explanation of gyroscopic precession?
« Reply #56 on: 14/04/2011 07:29:08 »
It would indeed look somewhat suspicious if you had a gyroscope spinning in your pan instead of food JP. Good thinking there, we wouldn't want anyone thinking of scientists as odd, it might reflect on us us physics lovers too.

But I think I found a answer to your and Geezers exchange of thoughts.

"An ultrahard carbon film coating many times slicker than Teflon has been developed by Argonne researchers. The new material's coefficient of friction is less than 0.001 when measured in a dry nitrogen atmosphere--20 times lower than the previous record holder molybdenum disulfide."

The nitrogen atmosphere might be slightly harder to arrange though? At least there won't be any observers around, alive that is :)
 

Offline Geezer

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What is the simplest explanation of gyroscopic precession?
« Reply #57 on: 14/04/2011 08:27:07 »
Well, my gyroscope isn't quite spherical, but maybe we can neglect any deviation from sphericity as being minor.  Then precession is experimental error!

If not, then perhaps syphrum would find this useful.  :)

On a slightly more serious note (only slightly mind you) in the case of the spinning top at least, I think precession is a consequence of its inherent instability. It's really just a pointy thing falling over.

The top's "planar inertia" is large, so it takes a while to fall over, and while it's falling and rotating, friction between its point and the surface that supports it causes the point to travel across the surface. I'm not sure if that's really precession or not.

If there was no friction there, the top would still fall over, but it's center of mass would head straight down and the point would not wander all over the place.

By making the top into a sphere, it can achieve a state of equilibrium and always have a "point" to spin on.
 

Offline JP

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What is the simplest explanation of gyroscopic precession?
« Reply #58 on: 14/04/2011 15:54:22 »
I agree, Geezer.  A top won't precess unless so long as it's center of mass is in a direct vertical line with it's point of contact with whatever surface it's on.  This always happens for a normally-shaped spinning top because it doesn't want to stand up on it's tip--it wants to fall sideways, which brings it's center of mass out of line with the point of contact. 

If you had this magical "frictionless surface," what you'd expect to see is that the center of mass of the top stays in one spot, while the body of the top moves around due to precession and spin.  The point of contact with the table would freely slide around.  This is because there is a net torque about the center of mass, due to the normal force, but no net force.  No net force means the center of mass doesn't move.  The net torque leads to precession.

If you had friction, as the tip moved about on the surface, the force of friction would exert a net force on the top as well as an additional torque.  The net force would mean that the center of mass of the top would move about and the additional torque would change the precession somewhat.  Interestingly, both this net force and additional torque are proportional to the normal force multiplied by the coefficient of kinetic friction.  This means that their effects should vary continuously as you "dial up" friction in your problem.  So for low friction surfaces, you should see only a tiny change from frictionless surfaces: the center of mass will only move a little and precession will be almost as described for frictionless tops.
 

Offline Geezer

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What is the simplest explanation of gyroscopic precession?
« Reply #59 on: 14/04/2011 18:33:32 »
If you had this magical "frictionless surface," what you'd expect to see is that the center of mass of the top stays in one spot, while the body of the top moves around due to precession and spin.  The point of contact with the table would freely slide around.  This is because there is a net torque about the center of mass, due to the normal force, but no net force.  No net force means the center of mass doesn't move.  The net torque leads to precession.


Thanks JP. I'm still struggling with the bit above. Am I right in thinking gravity is responsible for producing the net torque about the center of mass? If so, I'm wondering how a torque is produced when there is no friction.

Is it because the plane of rotation cannot be perfectly normal to the gravitational forces?
 
 

Offline JP

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What is the simplest explanation of gyroscopic precession?
« Reply #60 on: 14/04/2011 19:48:40 »
Torque occurs when a force is applied to some other point than the axis of rotation in a rigid object.  For the top, you can think about it rotating about axes passing through it's center of mass.  If you draw all the forces on a top on a frictionless surface, the only two are gravity, pulling down on it's center of mass and the normal force pushing straight up at it's point of contact with the floor.  Since the normal force is the only one not acting on the center of mass, it causes a torque.

You can imagine that if you had a gyroscope in space and tied a string to it's tip and then pulled that string straight "up", it would rotate about it's center of mass as well, in just the same way.  It's just a factor of any force not acting on the center of mass that will cause it to rotate.
 

Offline Integza

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Re: What is the simplest explanation of gyroscopic precession?
« Reply #61 on: 23/08/2015 20:08:14 »
Try watching this video, i hope it helps ;)

 

Offline alancalverd

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Re: What is the simplest explanation of gyroscopic precession?
« Reply #62 on: 23/08/2015 22:27:24 »
You can define the rotation of a body with a single vector, the angular momentum of that body.

If we spin a top or a gyroscope in "ideal" conditions then the angular momentum vector lies along the geometric axis of the body.

Angular momentum is conserved in all interactions, so if an external force now tilts the spin axis, the spin axis rotates so that the sum of the spin and rotational angular momenta equals the original angular momentum.

This is not to be confused with the apparent precession of a gyro compass! A free gyroscope points in a fixed direction in space, but the earth is spinning at 15 degrees per hour so an uncompensated gyro compass appears to drift at 15 deg/hr, which can lead to serious navigational errors!
« Last Edit: 24/08/2015 13:47:08 by alancalverd »
 

Offline ritchie

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Re: What is the simplest explanation of gyroscopic precession?
« Reply #63 on: 28/10/2015 18:23:31 »
I realise that this is somewhat off topic, but having recently re-studied the excellent and most impressive works of Prof. Eric Laithwaite, I noticed that in all of the filmed 'one armed lifts' of 40lb rotating flywheels above the head, (all done with one arm) the 40 lbs + weight always rotated around the body and the head to the wielders right.

I also tried to move myself whilst sitting on a swivel chair, (using only whatever arm gestures would work) and found that I can only move (slowly rotate the chair and turn myself ) to my left. (or with great difficulty using any and all gestures)  a very little amount to my right.

Why is this? and would it make any difference if both experiments were conducted in Australia?  (seriously :-))
 

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Re: What is the simplest explanation of gyroscopic precession?
« Reply #63 on: 28/10/2015 18:23:31 »

 

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