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[james_davis_nicoll]

Space drives the violation of conservation of momentum way!

I blame whoever taught this person in high school physics.

As I recall, John C. McLoughlin's now somewhat elderly The Helix and the Sword had living space ships that conserved reaction mass by recollecting it after it was ejected. It was something of an instant WSOD-killer when that little detail was introduced.

Comments

[galbinus-caeli.livejournal.com]

The washing machine in the example also has an outside input of energy through the power cord. No closed system there.

[james-nicoll.livejournal.com]

We could imagine an Oscillation Drive powered by solar panels, also an outside source of power, and the OD itself would still provide no net change to the system's momentum. The pressure of sunlight would provide a little force, though.

[galbinus-caeli.livejournal.com]

You might also be able to direct the waste heat out of the back end of space ship.

Someone might want to ask this guy to explain why every clocktower in europe isn't gliding across the landscape powered by its pendulum.

[bwross.livejournal.com]

He's not that stupid. Pendulums in clocktowers go back and forth symmetrically and so that obviously cancels out (besides, I'm sure most people who would believe in gliding clocktower potential, would just assume that the clocktower builders build towers strong enough to prevent it).

The trap he's falling into is one where he thinks he can get around this with an asymmetrical approach. Most people's intuition would probably get that one wrong (and hopefully none of those people are scientists).

[ross-teneyck.livejournal.com]

If I'm understanding him right, he's reasoning by analogy to the way that you can, for instance, sit in a rolling chair and move it across the room without ever touching the floor by throwing your body around in the right way. But there's stiction involved there which you don't get in space.

[fridgepunk.livejournal.com]

Well the trouble is that it's not entirely intuitive that throwing your feet forward while on a swing would also throw your shoulders backwards if your grip on the chains and the weight of your arse in the swing didn't stop it.

People don't think to hard about the fact that they don't float about near the ceiling most of the time and don't usually reach orbital velocity while out jogging.

[galbinus-caeli.livejournal.com]

Is a swing really a second law demonstration? I thought it had more to do with balance and changing the center of mass of the pendulum(swinger).

Thought experiment: The observer is watching a swinger from the side. The swinger is facing left. The swinger is sitting motionless directly below the bar. The swinger's feet are tucked up under the swing with heels near buttocks. This puts the swingers center of mass at about her hips. The whole system is at equalibrium.

The swinger then extends her legs straight out. (To the left from the point of view of the observer.) This moves her center of mass to the observer's left, no longer directly below the bar. The off center mass falls back toward the center, causing the swinger's body to move backward. Momentum causing the center of mass to pass the equilibrium point and swing the same distance to the right as the moved center of mass was to the left.

If the swinger freezes her posture, then barring any friction, she should continue this small oscillation forever. Instead, at the top of her right ward swing, she tucks her legs up under herself, this pulls the CoM to a point beyond the top point of the unaided swing. This adds potential kinetic energy to the swing and she drops through the equilibrium point even faster, Swinging higher on the left. Where she can extend her legs again and add yet more momentum to the system.


(All this assumes a swinger who can hold her torso perfectly rigid. I know that typically a swinger extends legs on the down swing, and tucks on the backswing, but when she does this she is also shifting her torso backwards, leaning back on the chains and shifting her CoM in that direction. So everything above is actually backward.)

I felt a change a-comin, soon...

[thesaucernews.livejournal.com]

Someone somewhat further down the list is suggesting using a (small enough) black hole to store the exhaust heat of a spacecraft as a form of stealth... the general arrangement of putting your entropy somewhere then taking that somewhere with you (and expecting to get somewhere) seems hampered by the same problem, at the very least.

[jaylake.livejournal.com]

Rather fond of The Helix and the Sword, myself.

Just sayin'

[timill]

Bother - Bob Shaw's glass&beer drive doesn't seem to be on the web anywhere.

[narmitaj.livejournal.com]

I liked Harry Harrison's rubber-band drive in Bill, The Galactic Hero, which he explains in a letter to New Scientist (http://www.newscientist.com/article/mg14319385.800-letters-warp-pollution-.html).

[avram]

See, there's the key to faster-than-light travel. Comedy. If we can make humanity funny enough, our universe becomes a comedy, and we can get those goofy comedic FTL drive ideas working.

[daev.livejournal.com]

James, you are a mighty one-man Garden Path Sentence generation machine!

[(Anonymous)]

IIRC, McLoughlin's THE HELIX AND THE SWORD ships were perfectly scientifically reasonable. They conserved reaction mass by always aiming their mass driver exhaust at various asteroid colonies. The colonies would collect the reaction mass.

Yes, as you point out, if the ship itself gathered its expelled reaction mass, one's WSOD circuit breakers would all blow.

The entire idea was that their space borne culture was energy-rich but matter-poor, the exact opposite of our planet bound culture. The amusing part was that the culture was matter-poor due to religious reasons, not technological reasons.

Maybe I'm an old fogie, but I am rather fond of THE HELIX AND THE SWORD.

[montedavis.livejournal.com]

And boomert... was so close. In the 7:28pm post just above your link he wrote: "...the very act of stopping the matter loses some of the momentum gained by throwing it in the first place."

Just do an arithmetic test on a few "high-v throw, low-v catch" values, and the zeroing-out (for any finite reaction mass) hits you in the eye. For "some" above, substitute "all," and Bob's your uncle.

(Who wrote the old story about the crippled starship that carried on by throwing a grapnel hook into [handwave] subspace [/handwave] and pulling on the rope?

[james-nicoll.livejournal.com]

I don't know that one but Robert Foward had a space ship that got around in gas giant moon systems by harpooning the moons.

[nyrath.livejournal.com]

"The Long Way Home" by Fred Saberhagen