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Post by lemgo on Feb 23, 2015 16:36:56 GMT
I was playing around with the new sliding blocks and found a strange bug that occures when they are spinning. This is defenitely not what is supposed to happen in a physics simulation. www.youtube.com/watch?v=9PV6A03viy8 |
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Post by SuburbanSB on Feb 24, 2015 3:10:50 GMT
... So.. you put down a wooden wheel, then a spinning block on top, with sliders either side, then started it, and the spinning block would spin, then it slows down, and spins in the opposite direction?
And it only seems to happen with sliders attached to the sides which is very odd...
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Post by lemgo on Feb 24, 2015 7:56:57 GMT
Exactely, it only happens with sliders as the moving masses. The wooden wheel can be replaced by a swivel joint or unpowered cog and the same thing happens, so that doesn't seem to play a role here as long as it can spin freely. It even happens when it's suspended by contractable springs in midair. The starting block may also be replaced by anything else that has equal connection properties.
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Post by SuburbanSB on Feb 24, 2015 8:22:56 GMT
Hmmm it's a very odd behaviour indeed. But I don't think theres anything I can further add to this discussion... As in theres no physical properties that could cause this sort of behaviour due to the spinning force...
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Post by lemgo on Feb 24, 2015 8:40:48 GMT
Yeah, this behavior is just physical wrong...
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Post by rockyspears on Feb 24, 2015 22:15:32 GMT
Is it Wrong?
If the friction in the joint between the starter block and "the other" connection is less than that between the starter block and the spinning block, then the spinning block will slow down and the spin transferred to the "other" joint, so the spinners slows, then stops, then goes in other direction. Same applies if the starter block has larger rotational inertia than spinning block.
Initially the spinner goes as expected, but slowly drags the starter block along, then as the starter block catches up it begins to "spin" the spinner, all the energy comes from the spinner. This is the same as the propeller effect whereby the propeller is turned by an engine, but due to friction in the engine, the aeroplane is being "twisted" in the opposite direction (real life example)
And to complete the picture if you build this and look closely, you will see that the lower sliders are in fact further apart, so that starterblock-slider combination will have a higher moment of inertia than the slider-spinningblock combo. So even were the friction the same top and bottom of the starter block, the bottom component takes more effort to spin, but once going, is harder to stop
Rocky
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Post by lemgo on Feb 25, 2015 0:20:43 GMT
You proofed it wrong with your real life example  For shure the part which "creates" the momentum (e.g. airplane engine, in this case spinning block) does cause itself to being twisted the other way around but this has in any case to be the opposite direction of the propeller - simple rule of momentum: it has to stay constant in any closed system, but it doesn't with this bug... To make this easier to see i made a version that is only supported by contracable springs and turns in midair without a way to transfer any momentum to the outside. youtu.be/fzQiHEFgjWs |
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Post by rockyspears on Feb 25, 2015 17:07:55 GMT
This isn't a closed system. The spinning block keeps adding energy. Your example just proves the point, getting faster and faster. The spinning block reverses direction because the wooden block has a higher rotational inertia. If you were brave enough to grab a propeller, the plane would spin in the opposite direction, which is what the videos show is it not?
So all is good? No?
Rocky
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Post by lemgo on Feb 25, 2015 19:43:10 GMT
Conservation of energie =/= conservation of angular momentum =/= conservation of momentum and in terms of angular momentum and linear momentum (except for gravity at the start) it is a closed system, but you are right - it is an open system in terms of energy with the spinning block adding more to overcome (air-)friction. Yes, if you would grab a propeller the plane would spin the other way around, BUT if you startup the planes engine in midair and watch it from outside they will never ever start spinning in the same direction, doesn't matter how long you watch it. Different rotational inertia don't has this kind of effect - it would just cause different angular velocities in OPPOSITE direction Again the plane is a good example for that, with the propeller which has very little rotational inertia spinning incredible fast and the planes body with its big rotational inertia spinning very slowly. Maybe you get my point with this collection if free spinning things - as you can see, only the ones with sliders reverse their spinning direction (at about 00:10) which is physical wrong... youtu.be/eaycz37AmWgHowever: As in every scientific topic - if you are able to proof me wrong, please do so - i would love to learn new stuff. But as far as my knowledge goes at this time, the spinning sliders are some kind of perpetuum mobile creating global angular momentum out of nowhere  |
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Post by rockyspears on Feb 26, 2015 18:41:12 GMT
lemgo,
You have the better of me, but I realise also that this is an aeroplane WITHOUT a propeller! In the example there is only one spigot used on the spinning block, this has made it bloody obvious that they cannot both spin in the same direction.
Playing with this more, you will find it only need 1 set of sliders, and it matters not which, put them on the spinner or the block, so long as the block has something on its faces, you get this odd result. (oddly, a bare spinner does not exhibit this behaviour, even just bracing one set of spinners together works too)
The reason for the strike through? I am experimenting and typing so my thoughts have changed: (maybe about to make a further fool of myself, but prepared for that)
Aeroplane model:
in mid air we have an engine turning a planes airframe with no propeller. the engine spins one way, the airframe in the other
Then we massively increase the inertia of the engine, that slows the engine down, but the crankshaft still turns the same way, so o matter what the frame and engine go opposite directions,
Damn, that means you are right, Damn
Fun playing with blocks though!
Rocky
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Post by lemgo on Feb 26, 2015 19:06:00 GMT
Hey rocky,
okeeyy, haven't tried it with only one pair of sliders but this further shows that there is something messed up with the inertia of them.
It's great that you are so open minded and experiament around to find out whats going on - that just makes arguing and discussing something so much more fun than what you mostly get on the internet (people sticking to their oppinon like superglue no matter how many arguments you throw at them^^)
So much fun to play - can't wait to see how this is going to evolve.
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