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Old 2015-08-22, 10:07 AM   #46
leo
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Originally Posted by MuniAddict View Post
I remember seeing a video of a girl doing a still stand on a ping pong net!
I interpret that exclamation as indication you think that's more impressive.
Although it's far less sideways swing than a real slack-robe; the swing it allows makes it easier than on a solid surface.
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Old 2015-10-18, 04:20 PM   #47
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Physics of stillstanding

So (asking here because this is the most recent thread on stillstanding!)...how is stillstanding actually possible? Can anyone give me a scientific/physics explanation?

Balance is all about keeping your centre of mass above your contact point with the ground. Generally, on an unicycle, this involves moving your contact point on the ground - because, with only a single contact point, you can't really change the position of your centre of mass (well, apart from jumping!); and gravity constantly acts to move your center of mass away from your contact point.

But, in a still stand, is this still the case? Sideways, you have some tiny amount of movement of your contact patch, by rocking the tyre/wheel from one side to the other (maybe 1cm to 1 inch with a really big fat tyre). Is that really what's going on? When I'm practicing, and I fall off (push against the wall), this is because I've reached the limit of how far I can lean the wheel / bend in the middle ...and it always seems that I'm leaning the wrong way to be consistent with my contact patch hypothesis. (Yet, I manage some stillstands of 5-10s, albeit only occasionally!)

So, is there some other physics involved (I've heard a variety of arguments that it's to do with rotational inertia and momentum)? Obviously, actually *doing* it is very different from understanding it, but I feel it'd help to know what was actually going on....
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Old 2015-10-19, 04:41 AM   #48
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Quote:
Originally Posted by acl View Post
So (asking here because this is the most recent thread on stillstanding!)...how is stillstanding actually possible? Can anyone give me a scientific/physics explanation?

Balance is all about keeping your centre of mass above your contact point with the ground. Generally, on an unicycle, this involves moving your contact point on the ground - because, with only a single contact point, you can't really change the position of your centre of mass (well, apart from jumping!); and gravity constantly acts to move your center of mass away from your contact point.

But, in a still stand, is this still the case? Sideways, you have some tiny amount of movement of your contact patch, by rocking the tyre/wheel from one side to the other (maybe 1cm to 1 inch with a really big fat tyre). Is that really what's going on? When I'm practicing, and I fall off (push against the wall), this is because I've reached the limit of how far I can lean the wheel / bend in the middle ...and it always seems that I'm leaning the wrong way to be consistent with my contact patch hypothesis. (Yet, I manage some stillstands of 5-10s, albeit only occasionally!)

So, is there some other physics involved (I've heard a variety of arguments that it's to do with rotational inertia and momentum)? Obviously, actually *doing* it is very different from understanding it, but I feel it'd help to know what was actually going on....
Surely you just move your centre of mass by moving your upper body/arms/hips to keep the centre of mass over the tyre instead of letting it fall outside the contact point of the tyre which would result in your falling too far to one side/forwards/backwards causing you to have to dismount.

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Old 2015-10-19, 04:46 AM   #49
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Originally Posted by acl View Post
So (asking here because this is the most recent thread on stillstanding!)...how is stillstanding actually possible? Can anyone give me a scientific/physics explanation?

Balance is all about keeping your centre of mass above your contact point with the ground. Generally, on an unicycle, this involves moving your contact point on the ground - because, with only a single contact point, you can't really change the position of your centre of mass (well, apart from jumping!); and gravity constantly acts to move your center of mass away from your contact point.

But, in a still stand, is this still the case? Sideways, you have some tiny amount of movement of your contact patch, by rocking the tyre/wheel from one side to the other (maybe 1cm to 1 inch with a really big fat tyre). Is that really what's going on? When I'm practicing, and I fall off (push against the wall), this is because I've reached the limit of how far I can lean the wheel / bend in the middle ...and it always seems that I'm leaning the wrong way to be consistent with my contact patch hypothesis. (Yet, I manage some stillstands of 5-10s, albeit only occasionally!)

So, is there some other physics involved (I've heard a variety of arguments that it's to do with rotational inertia and momentum)? Obviously, actually *doing* it is very different from understanding it, but I feel it'd help to know what was actually going on....
Surely you just move your centre of mass by moving your upper body/arms/hips to keep the centre of mass over the tyre instead of letting it fall outside the contact point of the tyre which would result in your falling too far to one side/forwards/backwards causing you to have to dismount.



My assumption is that you have two ways to balance. Either moving your centre of mass around so that it is over your base of support or moving your base of support so that it is under your centre of mass.

With unicycling you do a bit of both. Sometimes you speed/slow your pedaling rate to move your wheel further forward/back if you are falling a certain direction. I.e. moving your base of support in relation to your COM. This is how we generally ride I think.

Other times you may suddenly shift your upper body backwards or forwards to get your COM back closer over the base of support. This usually happens for me when I am suddenly surprised and unbalanced by a bump I didn't see.

When you still stand I think you are just doing all the balancing from the moving your upperbody/arms/hips to keep your COM over your base of support, without the cycling bit.
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Old 2015-10-19, 05:42 AM   #50
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Is there a tutorial? My thighs get fatigued. Am I doing something wrong?
Yes, using your thighs. Part of the learning process, just like when learning to ride, is learning to relax the areas you don't need to tense up. So for your legs, you're only making tiny movements forward and back. All of your weight should be on the seat and with time, you'll get your legs relaxed so you don't wear them out.
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I also find my stillstands are better when they're done because of terrain (like riding muni and setting up to hop over an obstacle) rather than setting out to still stand...
I think this is beause you rode into those stillstands. It's much easier to ride into one than to start one "cold", like the guy in the YouTube vid who does it up on his high rail. When riding into it, you get to line yourself up as much as possible before you start the difficult balancing part.
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Originally Posted by acl View Post
how is stillstanding actually possible? Can anyone give me a scientific/physics explanation?
I never took a physics class, but I know how to stillstand (some). But I think it's easiest to see the mechanics of it by watching someone else do it. Like the guy on his wooden rail in that YouTube vid. The tire can't move side to side. But the rest of him can. When he moves his arms and upper body, you see that movement pivoting around his waist area, which is his center of mass. If I understand it correctly, his balance is maintained by moving that center point slightly to the left and right with those upper body movements.

But how is that possible? Because he's not made of sticks. He can make S curves or other shapes, to adjust his center slightly differently from what his arms are pushing his upper body to do. It's not much, but it's enough.
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Old 2015-10-19, 06:37 AM   #51
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The guy in the video, still standing on the rail is our very own "Juggle Addict"

Well done Juggle Addict
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Old 2015-10-19, 07:26 AM   #52
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I don't think you can move your centre of mass by waving your arms - unless the air resistance from that is significant (!). As pinoclean's diagram nicely shows, all you can change is the distribution of your mass around the same central point. To move your center of mass, you have to exert force against an outside object...(and there is only one outside object available: the ground!)
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Old 2015-10-19, 08:56 AM   #53
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Originally Posted by acl View Post
I don't think you can move your centre of mass by waving your arms - unless the air resistance from that is significant (!). As pinoclean's diagram nicely shows, all you can change is the distribution of your mass around the same central point. To move your center of mass, you have to exert force against an outside object...(and there is only one outside object available: the ground!)
Possibly centre of gravity would be more appropriate than centre of mass? Moving your arms (and upper body and hips) around will move where the centre of gravity lies. As long as the centre of gravity is over the base of support you will not overbalance
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Old 2015-10-19, 09:29 AM   #54
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Quote:
Originally Posted by acl View Post
I don't think you can move your centre of mass by waving your arms - unless the air resistance from that is significant (!). As pinoclean's diagram nicely shows, all you can change is the distribution of your mass around the same central point. To move your center of mass, you have to exert force against an outside object...(and there is only one outside object available: the ground!)
Your assumptions only work in zero-G and are about maintaining impulse, not centre of mass. You are not an isolated system, but subject to an external force, gravity.

By lowering your centre of mass you can leverage your potential energy to change your impulse and shift your centre of mass horizontally. If you do that right you can let gravity pull you back over the contact patch. You can straighten your body again over the patch, to restore potential energy without disturbing the centre of mass horizontally. In theory you can standstill infinitely this way. At least until your body runs out of fuels that you expand mostly, restoring potential energy (or waste on internal friction, because you are tense).
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Old 2015-10-19, 03:29 PM   #55
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It's not all about moving your centre of mass. Or at least it's not about moving your centre of mass in any conventional linear way. Because by waving your arms about you can generate rotational inertia which results in your centre of mass moving up rather than down - and hence moving it back over the contact patch.
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Old 2015-10-20, 04:53 AM   #56
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Your assumptions only work in zero-G and are about maintaining impulse, not centre of mass.
Again I'm not a Physics student, but do you mean zero-G and also vacuum? In zero-G with atmosphere, pushing against the air does a lot more than it does in heavy gravity.

Suffice it to say that stillstanding is possible. You've seen people do it, hopefully you've also worked at it yourself. So we're trying to figure out how all that flailing about can "fix" your balance without any side-to-side motion of your wheel. Not whether we can or not; that's been established. At least for the greater "we", who have put a lot of time into it!
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Old 2015-10-20, 07:31 AM   #57
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I think the reason we sometimes flail our arms about is to provide counter balance which offsets our body moving out of line as we negotiate difficult terrain etc.

The experienced rider will generally wave his arms less, because he responds quicker using his torso muscles to bring his body into line.

When I'm riding on my road uni my hands are gripping the handlebars most of the time. This requires quick responses from the torso muscles.

But if I come to some rough terrain then my arms are used. I believe extending the arms provides more leverage as gravity forces pull on them.
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Old 2015-10-20, 08:44 AM   #58
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Again I'm not a Physics student, but do you mean zero-G and also vacuum? In zero-G with atmosphere, pushing against the air does a lot more than it does in heavy gravity.
I don't think the effect is significant. Especially if you're just waving around your arms.
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Old 2015-10-20, 12:04 PM   #59
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conservation of angular momentum

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I don't think the effect is significant. Especially if you're just waving around your arms.
Actually, waving your arms is the most effective tool you can use in a stillstand. Try this experiment. Get into a quick stilllstand and then quickly rotate your extended arms in a clockwise direction. You will find it rotates your body about the wheel in the opposite direction. The effect is quite large. The physics principle at play here is called "conservation of angular momentum". The first time astronauts went on a space walk to perform repairs they bumped into this principle. They tried to turn a nut with a wrench and found they could not do it. When they tried to turn the nut counter-clockwise their body would rotate clockwise instead. Since then NASA has installed handles that the astronauts can grab to brace themselves.
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Old 2015-10-20, 04:37 PM   #60
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Actually, waving your arms is the most effective tool you can use in a stillstand.
But it's not effective because you push air.
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