If std_match would come on board, then maybe there would be no dissent at all.
One thought experiment:
I think I now agree with you.
One thought experiment: the spinning bicycle wheel suspended on only one end of the axle (as seen on several videos on Youtube). We agree that the precession force/energy comes from gravity, and the other end of the axle will slowly be lowered.
If you are correct, the unsupported axle end will fall down as if the bicycle wheel wasn't spinning if we block the precession movement without friction.
I was sure that it would still go down slowly, but now I see that everything fits together better if it would go down as fast as a non-spinning wheel. The precession force would be very high.
On a related topic, we presumably all agree that with frictionless bearings, it is impossible to speed up or slow down the bicycle wheel by movement or rotation of the axle, because it is impossible to transfer torque through the bearings to the wheel. That was certainly the case in our gyroscope examples, where no work is done when rotating the bicycle wheel axle, and the rotational speed of the wheel remains constant. But I hinted several times that perhaps that may not always be true. What do others think?
- rotational speed goes up yes - but energy is not gained - your grasp of basic physics is tenuous at best ...
Also, Foucault's pendulum if at the poles, rotates or precesses at a maximum speed -
Precision99 is correct.
For an outside (stationary) observer, the angular momentum is conservered, not the rotational energy.
By pulling the arms inward, the skater does "work", which ends up as an increase of the rotational energy.
There is a calculation example with a skater a bit down on this page: https://philschatz.com/physics-book/contents/m42182.html
Now you just have to get your head around the swing with no friction is difficult to start from rest ( at bottom ).
He escapes by gyrating his legs back-and-forth, so that he swings a small distance, growing to a large distance...
But this effect, not really mysterious, is actually due to air friction. If you have an open umbrella in your hand, it is trivial. It is the same air friction that (mainly) is responsible for the pendulum to stop...
More like a weather vane picks up speed when the air moves.
Hmmm. Well yes, it is certainly possible to exploit air drag to start (and even pump up) the swinging of a person hanging on a rope. As you say, with an open umbrella this effect could be made very significant.
But you seem to imply that this is the only mechanism, and that it would be impossible to start and amplify the swinging motion without air drag. Is this what you are saying? And do you believe this is also true of a child's swing? Did you read this somewhere on the internet?
Definitely some more discussion required here.
Another mystifying real-life demonstration. A heavy disk is mounted on bearings at one end of a bar. When it's spinning a man can easily lift it high in the air by one hand holding the free end.
The key to making it possible is for him to start it precessing as he begins lifting it. So then we wonder if gyroscopic precession is doing the work?
As no one else seems keen to express an opinion (or present evidence) on this, then I'll stick my neck out and say that air drag is most certainly not necessary for starting or pumping up a swing or a person swinging on a rope.
And while it is possible to contrive situations where air drag can be exploited, such as with using an open umbrella, it is my belief that that in normal operation of a swing, any effect from air drag in starting or pumping up the swing is negligible. I base this statement on detailed experimenting and observation using my backyard swing at home, where I found quite clearly that the dominant mechanism is weight shift, not air drag.
To be precise, all you need to do is to start out sitting in the upright position, and then lean right back, so that your upper body ends up being almost horizontal.
This lowers your centre of gravity (COG),
What it also does is shift your COG backwards.
Thus, the line of the chain no longer passes through your COG...
You are not being honest in your experiments.
If you lean back sitting on a swing so that your centre of gravity shifts outside the seat, you will fall off.
You need to hold on to the ropes to lean back. Then the seat will also move in the opposite way. Note that the seat will move around the point at which you are holding the ropes (or whatever it is supported). The rope above the point you are holding on should stay vertical.
You need to raise your centre of gravity a bit (like what you do when you climb a stairs) and use the gravitational potential energy.
This potential energy need to be transferred to the swing. Loosen your grip on the ropes and bring your centre of gravity back to the seat. You have transferred some energy (equivalent to climbing a height equivalent to the swing vertical rise).
This is all there is in physics.
Sitting on a stationary swing, raising or lowering the centre of gravity an be performed only on a vertical axis.
If you are not holding on the ropes (holding on the seat is not good enough), you will fall off.
The top part of the rope still stays vertical.
It is a very simple exercise and you need to draw a diagram and see the forces acting at various points.
I agree that it can get confusing sometimes.
You are not being honest in your experiments.
If you lean back sitting on a swing so that your centre of gravity shifts outside the seat, you will fall off.
You need to hold on to the ropes to lean back. Then the seat will also move in the opposite way. Note that the seat will move around the point at which you are holding the ropes (or whatever it is supported). The rope above the point you are holding on should stay vertical.
You need to raise your centre of gravity a bit (like what you do when you climb a stairs) and use the gravitational potential energy.
This potential energy need to be transferred to the swing. Loosen your grip on the ropes and bring your centre of gravity back to the seat. You have transferred some energy (equivalent to climbing a height equivalent to the swing vertical rise).
This is all there is in physics.
Sitting on a stationary swing, raising or lowering the centre of gravity an be performed only on a vertical axis.
If you are not holding on the ropes (holding on the seat is not good enough), you will fall off.
The top part of the rope still stays vertical.
It is a very simple exercise and you need to draw a diagram and see the forces acting at various points.
I agree that it can get confusing sometimes.
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