Discussion Board

Topic: Geeky questions about centrifugal force

From: Randy Merkel
Location: Paso Robles, CA
Date: 09/27/2007

Howdy,

Feel free to post this on your blog if it has any redeeming value... I kind of doubt it ;)

I recently read EON which reminded me about some questions I had with centrifugal force used as artificial gravity in Clarks Ravenous with Rama. My first question is about jumping:

Lets say Im standing on the earth and I jump as high as I can. My legs accelerate me at more that 1G so I leave the ground. However two forces try to slow me down, resistance to the air (very small) and the Earths gravity which exerts a constant 1G at sea level. My attempt at a take off quickly comes to a stop and return to Earth.

But lets say Im standing on the inside of a really big, hollow, spaceship like Thistledown or Rama that spinning for artificial gravity. Since Im in contact with the inside of the ship, centrifugal (or is it centripetal?) force pushes me against the hull and I feel like I have weight. But what happens if I jump? Well it seems to me that as soon as I lose contact with the spinning hull Im no longer accelerated against it, and since there is no gravity, worth worrying about& Im in free flight! Sure the atmosphere will tend to slow me down but at something like 10 meters per second, thats not a lot of force. Seems like Id go for quite a flight! One wonders if it would be best to go about with a small jet pack or a collapsible set of wings just in case ;)

Note that in the Rama book a character took a chance by jumping off of a high cliff south of the big circular sea. My argument was I thought that he should arrive at the water going no faster than when he stepped off of the cliff because there would be no additional acceleration due to gravity. As a matter of fact, the atmosphere would tend to slow him down a little.

This leads to the air in the ship. I was never quite sure if you implied that the air in Thistledown grew thinner near the central axis, there was some sort of barrier, but I seem to recall that Clarke suggested near vacuum at Ramas access. Why? Well because gravity was pushing the air towards the hull& But what! There is no gravity, just spin. Id suggest that only the air molecules in contact with the hull would have weight. Air molecules not in contact with the hull would only be accelerated towards the hull by resistance with other molecules. Thus the gas would tend to fill the entire ship with more or less the same pressure.

So what do you think? Would one past time of the people of these ships be human powered flight or am I missing something?!??

Thanks for your time!

-- Randy in Paso

Re: Geeky questions about centrifugal force

From: Greg Bear
Date: 09/27/2007

I used Rendezvous with Rama as a test case when I was doing the equations for EON. While you're figuring this out (I'm not going to do it for you--Ask Marilyn vos Savant for that one!) don't neglect to take into account the Parable of the Spinning Plumb Bob. What happens if some mischievous imp decides to give the bob a little push toward you, as you spin it on the end of a string? (Just a single push--not a continuous force.) Does the bob keep moving until it hits you? If not, why not? (Hint: What are the bob's vector components?) The same effects will determine what happens to the atmosphere and everything else within the spinning cylinder. Objects at the axis are like a little bob on a much shorter string--or stuck to your finger. How would they move outward?

Re: Geeky questions about centrifugal force

From: Randy Merkel
Location: Paso Robles, CA
Date: 09/27/2007

You sly dog you! I havent checked with Ms. vos Savant, did I commit a logical fallacy somewhere? Well I wont be surprised&

I found this interesting link on circular motion (http://hyperphysics.phy-astr.gsu.edu/hbase/circ.html#circ) which shows the approximated vectors of our plumb bob.

I want to ask about driving an auto exactly counter to, and at the same speed as the ships rotation, but thinking about it is giving me a headache J

BTW I liked the book. Too bad the USSR broke up huh? I guess thats the risks of setting a story in the near future.

-- Randy in Paso

Re: Geeky questions about centrifugal force

From: Greg Bear
Date: 09/28/2007

Ah, but Putin seems intent on making any revision of EON to bring it up to date fairly minor! That car experiment by the way is quite good. It would have to go darn fast, of course, on a smooth circular track. That would be like putting a little rocket motor on the plumb bob, opposite the spin...

Now consider the diameter of a wheeled space station necessary to allow riding bicycles! Remember, the head is inboard and thus spins slower than than the feet... In the movie "2001," it seems very likely the astronauts in their lovely white squirrel cage--diameter approximately fifty feet--would become quite motion sick after a short time of standing up.

Re: Geeky questions about centrifugal force

From: patrick
Location:
Date: 09/29/2007

Um, isn't it referred to as 'inertial frames of reference'? In any case, there's probly all kinds of info on the NASA site with regard to things they've done in the shuttle, etc.

On Putin: well, yeah, given the recent story of that new bomb and that they've re-instated strategic bomber missions....and then, again, this news I caught from Bruce Sterling's WIRED blog about them becoming unified under a sort of mafia, perhaps distorted capitalistic, rule makes me wonder in an obscurely different direction.

Re: Geeky questions about centrifugal force

From: Greg Bear
Date: 09/29/2007

Of course, I'd have to take out all the Socialist rhetoric... wonder what will replace those five-year plans? Strangely, I feel almost nostalgic about this turn of events. Our good old adversaries, the Russians, back to keep us on our toes! They're even flying bombers of the same vintage we're using. Fifty years and counting... By the way, my guess is, a rotating space station is going to have to be almost a thousand feet in diamter to avoid nausea caused by coriolis. Which is why I suggested tethered crew quarters in MOVING MARS.

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