Seems like this poster isn't interested in the law of conservation of energy.
Intuitively I think of examples like a hula-hoop where you can accelerate or decelerate its rotational speed without exerting any direct rotational torque. I have a feeling this example is masking a similar concept.
please point me to the frictionless bearings and the part where anything spinning in air has no frictional losses .... my credit card is ready ...
... but I believe that a machine CAN be made, with frictionless bearing, where a torque CAN be transmitted to a rotor, and the rotor be thus made to rotationally speed up. And without cheating like using magnetic forces (electric motor) or wind forces (turbine wheel) or any other cheating nonsense where something mechanically touches the rotor, or where a magnetic or electric or any other sort of field produces a force on the rotor. And no use of gravity, either. Just by appropriate machine design, and appropriate forced motion of the axle. Anyone care to comment on that?
I'm listening.
it's not built up from nothing - the friction in the system allows the swing user to impart momentum ... and therefore build up the swing oscillation ...
Quite so. We are completely in agreement, but noting that the "opposing force" is at right angles to your movement of the axle, so no work is done. Indeed, strictly speaking, the gyroscopic effect does not "oppose" your motion of the axle at all, because there is no component of opposing torque in the direction of your angular movement of the axle.
We can carry out the experiment in vacuum to eliminate wind drag. Of course there is no such thing as a perfectly frictionless bearing. I'm not suggesting that. But in Physics, and in though experiments, we often visualise an idealised case without friction to help us understand the basic concepts.
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I'm not claiming anything that would contradict well established physics. Just some interesting (at least to me) examples. I have 3 such examples, but will start with a child's swing. This is a rotational system. The ropes or rigid bars form the "spoke", and the axle is the pivot at the top of the swing. The rotational bearing at the top can be considered frictionless, and nothing touches or exerts a force on the rotating object, being the seat, the rods from the seat to the pivot, and the person on the swing.
But here is the curious part. We all know that the child (and even an adult!) can get the swing into rotational motion just with appropriate body movements and muscle forces. One can build up the swing's motion without limit. And if rigid rods are used rather than ropes, then it would be possible to build up the motion so that the seat and person on the seat went around and around, true rotary motion, built up from nothing, and without any external torque being exerted on the system.
Does anyone else find this example quite curious? Where has the torque come from to get the swing rotating, given that the only thing that physically touches it is a frictionless bearing at the top of the swing. Does this mean that a bicycle wheel on frictionless bearing can after all be rotationally sped up, without an external force or torque acting on the wheel? But I said in previous postings that that was impossible.Is it? We would appear to have a contrary example.
What say you to all of this, Kajunbee, as I sense that you understand what I'm talking about. Or anyone. Is the principle of conservation of angular momentum in trouble?
The movement is never perpendicular to the external force.
Would you still be able to swing if the ropes were replaced with rigid rods.
Dear precision99, apart from bare assertion - can you offer proof that friction does not assist in getting a swing started - ...?
The swing example is inappropriate for the original gyroscope problem. Swing involves periodical transfer between potential and kinetic energy. Work is done by weight shift which dynamically changes the moment of inertia.
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I have however learned that friction plays a role in the energy transfer between gyroscope rotation and external work in a gyroscopic exercise tool, see https://en.wikipedia.org/wiki/Gyroscopic_exercise_tool
Yes. In fact it's even better.
With ropes or chains, then you are limited to winding up the swing until the ropes become horizontal, and after that they lose their tension and you can't wind up any further. Hey, every kid knows that! When I was a kid, I sure as heck tried to see how high I could swing, and can assure you that if you try too hard then the chains become slack at the top of the stroke, and you can go no further. And it also gets a bit scary once the chains start getting slack as they approach horizontal.
But with the ropes replaced by rigid rods, this limitation is removed, and it is possible to crank the swing up and up until you start doing full circles. I have not personally done or witnessed this, but have read that this is the case, and has been observed.
the bend angle in the chain does indeed help establish motion - have you ever thought how one is able to start a swing from zero motion - without friction .?
it should be clear to even a casual observer that without friction, getting a swing started is a near impossible scenario - wind resistance will assist too...
the bend angle in the chain does indeed help establish motion - have you ever thought how one is able to start a swing from zero motion - without friction .?
it should be clear to even a casual observer that without friction, getting a swing started is a near impossible scenario - wind resistance will assist too...
I find your posting confusing. First you say that the bend angle in the chain helps establish motion. And then you say that without friction, it is effectively impossible to get a swing started. That's a contradiction.
I have however learned that friction plays a role in the energy transfer between gyroscope rotation and external work in a gyroscopic exercise tool, see https://en.wikipedia.org/wiki/Gyroscopic_exercise_tool
No contradiction at all - I realise there is friction in a real system that assists starting ... I suggest a primer in logic ... the bend angle assists imparting momentum ONCE you have started swinging - the friction also assists ...
You have ignored the fact that there is net zero momentum at rest, this is what requires friction to form an offset force - have you a degree in mechanical engineering by any chance ... ? if not please find a fellow who has and they can explain the nuances of frictionless systems ...
A swing can be started without friction.
We can move some part of the mass off-center (some other part will go off-center in the other direction).
If we then move this mass up/down with correct timing, we will add energy to the system, and the swing is going.
I don't understand your sentence, so don't know if I agree or not. Can you elaborate? And we are all still waiting to know exactly how you would use a (very large) spinning "bicycle wheel" to harness the rotational energy of the earth.
But I do stand by everything that I have said re forces (or torques if you prefer) being at right angles when you change the orientation of the axle of a spinning bicycle wheel. But I might not have made it clear, so will explain in more detail.
Hold the axle of your spinning bike wheel (one hand supporting each end) so that the axle is horizontal. Now, change the orientation of the axle by rotating it in the horizontal plane. A hefty reaction force will be produced, trying to lift one end of the axle, and lower the other end. These vertical forces on the ends of the axle are at right angles to your horizontal movement of the ends of the axle, and therefore you do no work in rotating the axle in the horizontal plane. If you don't believe me, try it. And actually, this must be the case, because if the axle genuinely resisted your altering of it's orientation (rather than producing a force at right angles) then you would be doing work against that resistance. And then physics would be in real trouble. Where would the work done end up? As explained previously, it can't end up increasing the rotational speed of the wheel, because no torque can be transferred through the bearing to the wheel to speed it up.
So, do you agree with what I wrote above?
OK. So grab the pendulum bob, and lift it so that the rod or string is say 30 degrees off vertical. Hold the raised bob perfectly stationary. The angular and linear momentum is zero. Now let it go. It will gain angular momentum
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