This thread is inspired by the most recent radio episode of This American Life.
A “rotor” ride is essentially a spinning centrifuge. The passengers have their backs to the inside wall and the floor drops below. The spinning force keeps the riders in place, their feet dangling in the air.
During the radio-show, two separate people claimed that the operator of a ride entered the centrifuge and proceeded to “walk” over the heads of the passengers, his body essentially horizontal to the ground.
This struck me as an impossibility. As I understand it, to keep a person in place the outward force combined with the coefficient of static friction must be greater than the force of gravity. This works well when a person’s entire back is against the wall, but would be less effective if the surface were the only the sole of one’s foot. In addition, I think the walker would have a lot of difficulty remaining horizontal, rather, I think his body would fall head first towards the true ground.
I admit my physics isn’t top knotch, so I’m eager to hear the physics behind how this could be possible. Thanks dopers!
Edit: Here is the Episode mentioned. The rotor ride is discussed in act 2.
Sorry, I’m not quite edumacated enough to explain any of the physics involved.
But I can offer that it is possible to “stand on the wall” as the ride is spinning. Many moons ago, there was a ride such as you describe at Astroworld (Houston, Tx). It was possible, albeit with quite a bit of difficulty, to scootch ones legs up, rock forward and then stand straight up (out?) which put you in a horizontal position, in relation to the ground. I know because I did exactly that, my recollection of the experience is that it would have been possible but probably very difficult to do anything, other than stand in one spot. I recall that it took extreme concentration and very good balance to accomplish the act. (Not to mention, I nearly got thrown out of the park for the stunt.) I got the sensation that if I didn’t stay perfectly upright, centrifugal force would slam me down into the wall. :eek: Also, there was an incredible sensation of feeling way heavier than if I were standing on level ground.
With practice, I think yes it is possible, to do what was described. :dubious:
I appreciate the anecdote but I am still skeptical. What force is keeping your horizontal body from being carried by gravity to the true floor? The centrifuge can only act through the contact surface.
Unfortunately I am having trouble writing out the equation to determine just how fast the machine would have to be spinning in order to achieve the described effect. It would have to be so great that the force of gravity was negligible upon one’s orientation. Intuitively this seems unlikely, if not outright impossible.
I’ll attack this problem with more vigor tomorrow morning, if someone else here hasn’t already.
Actually, I can imagine a scenario in which the walker isn’t truly perpendicular to the floor, rather he is very carefully balanced at an angle towards the spinning wall.
So to start things rolling, how fast would a 10m in diameter room have to be spinning to secure a standing 100kg man to the wall? I’ll assume the surface area of his shoes is 50cm^2. The coeffecient of static friction of rubber on dry concrete is between 0.6 - 0.85.
It has been a while since I’ve done this stuff so I’ll have to sit down with a pad of paper for a while.
Whatever happens, real gravity doesn’t stop pulling, so the person is going to experience 1g in a downwards direction no matter what (I’m assuming this experiment is performed on Earth)
So there’s no way they can end up being in a totally horizontal position with respect to the ground - if the centrifugal force is equivalent to 1g, then they’ll be at a 45 degree angle to the ground. This angle can reduce as the centrifugal force increases, but so does their ability to stand up.
It is not impossible to walk on a 45 degree slope. 1 g of centrifugal acceleration (combined with Earth’s 1 g) is equivalent to a 45 degree slope on a 1.4 g planet. A little harder to walk, but no real problem for a fit person.
As the centrifugal force increases, the equivalent slope decreases but the total acceleration increases–I’d guess that actually walking is next to impossible at 2 gees or more.
I was on a Gravitron once that was probably around 3 gees, and empty enough (and with a apathetic enough operator) that we could crawl around the inner rim. There was no real perceptual slope–probably helped by the fact that the backrests themselves were at an angle. Lots of fun, but no way could I stand up at 3 gees.
Is this spam? For me, at least, those links redirect me either to web hosting company home pages, or to amazon.co.uk. (If it isn’t spam, what is going on?)
I would guess that the rotors produce more than 1g. However, standing up (or, rather standing sideways) in it your head and upper body would experience considerably less g force than your feet. I should imagine that this would feel weird and would mean that walking about would require some experience and skill.
My memory of riding in one of these is that it feels (and even ‘looks’) as though the cylinder has turned sideways, and you are lying on your back in a motionless cylinder. The weird part is that there is someone hanging above you on the opposite wall. (On the other hand, a friend of mine threw up in it after a few moments, so I guess it is more disorienting for some people.)
Nobody’s addressed this point yet. It turns out that empirically, the amount of friction between two surfaces is largely independent of the size of the contact area; this is occasionally called the second law of friction. So while there might be other reasons that one can’t walk around in a rotor ride, this isn’t one of them.
Equation for centripetal acceleration is a = V[sup]2[/sup]/R, where V is the tangential velocity at radius R from the center of rotation.
If the coefficient of friction is 0.75, then we need to generate a radial acceleration of 1/0.75 (= 1.33 g) in order to create enough friction to bear the weight of the person. So for 1.33 g (= 13 m/s[sup]2[/sup]) of acceleration at the rim of the device (5 m from center of rotation), the equation gives a tangential velocity, at the rim, of 8 m/s. With a 5-m radius, that means about 15 RPM, one revolution every four seconds. That seems to be quite plausible.
If you account for the fact that the person’s center of mass is about a meter inward from the wall, then the RPM needs to be a smidge higher (about 7%).
Just want to clarify that the effective slope is sideways. Not sure how fit you have to be to do that but you couldn’t just walk with your feet together normally. Imagine walking here. It’s steep. ETA: One of the steepest streets in the USis in San Francisco, and that’s only 17.5 degrees.
I have never experienced more than 1g, but I have carried my children on my back and at 2g it would be like carrying someone my own weight on my back, and to do it across a 26.6 degree slope would be damn near “forget it.”
Their numbers mesh fairly well with my calculated requirement of 15 RPM to achieve 1.33 g’s. Standing “horizontally” on the pads is inaccurate, as you would of course have to have your body pointing somewhere between horizontal and vertical.
Upon introspection of the incident I related, what Machine Elf has described is a fairly accurate description. IIRC I think I was at an angle of approximately 15 - 20 degrees from (above) horizontal. Also, the interior wall of the ride was lined with a rubbery material that had a very rough (kind of pebbly, actually) surface. I don’t know if it would’ve been possible to actually walk, as it was quite an effort just to maintain my balance and stand in one spot, and the ride operator was having a hissy fit over me just standing up. (It was strictly against the rules. Meaning, right before the ride started, everyone was admonished to NOT try to do, what I did. :D)
One other thing, the ride at Astroworld was unlike the Gravitron linked to in Machine Elf’s post. The one at Astroworld was a perfect cylinder about 12’ - 15’ (no more than 18’) in diameter (ie: the walls were perpendicular to the floor), once the machine had gotten “up to speed” (no idea of what that would have been) the floor literally dropped down approximately 2’. The ride was entered thru a door in the side of the cylinder and when you entered and stood with your back against the wall, the top rim of the cylinder was maybe 2’ above your head.
As a less technical aside: Most of these rides also spin at a tilt. i.e. once they start spinning, the entire tube tilts over so it’s no longer perpendicular to the ground. It’s entirely possible that the trick the OP describes was just a slightly angled human hamster wheel.
The one at Astroworld (circa. 1974-75) definitely did not tilt in any way. The line to ride the machine wound up and around to a platform that surrounded the top, so that you could observe the ride in progress. It was just a round cylinder that spun in place, inside of a frame.
The one at Six Flags over Texas in Arlington (between Dallas and Fort Worth) was exactly the same. It was called The Spindletop. I don’t know if it still exists or not.
The one at Astroworld was called the “Barrel of Fun” according to Google, and describes it as a “rotor ride” and cites 1971 as the opening date for that ride, no date given for the end of operation. IIRC it was taken down in the early 80’s.
Well, I just came back from a backpacking trip and we often had to traverse rock surfaces at 45 degrees, with a pack that was ~30% of my weight. 40% would be no problem, short-term. Yeah, you can’t have anywhere close to a normal stride, but it’s still doable. +100% weight would be difficult.
