beausoleil

Ho hum.

Spinning on its axis, not orbiting the Earth's old position. It's spinning on it's axis now, why would it stop?

Tangential velocities have nothing to do with it. The children don't keep orbiting the centre of the merry-go-round. They do move off on tangential velocities, but that's not the point. As they move off, they are still rotating on their axes, which is what the guy was getting at.

I've read the other thread now, and one person answers what's-his-face's question about the planets correctly (ie he gives the currently accepted answer). In general, planets inherited the rotational direction of the solar nebula in both their orbits and their rotational directions. However, the last accretionary events were massive impacts, and the nature of these determined the precise angle of the spin axis of each planet. Mercury and Venus have spins aligned with their orbits, Uranus is almost 90 degrees off.

[ December 04, 2002: Message edited by: beausoleil ]</p>

Iasion

Greetings,

fando said :
Actually not quite immediately, but 1.3 seconds later, the time it would take for the "information" that the Earth was gone to arrive - i.e. the time it takes for the ripple of space "unbending" from the Earth's mass to reach the Moon.

QuentinJ

Vorkosigan

Actually, while we're on the topic, I watched part of the unwatchable film Pitch Black last night. In the beginning, a spaceship crashes on a desert planet. As it descends through the atmosphere, it is spinning on its axis. Long and thin, it appears to spin about a point on the center between the long ends, like baton, not like a drill. In order to slow the spin, they dump weight, specifically, by dumping pieces off the end of the ship to make it shorter. Shouldn't they speed up due to CoAM? Or is angular momentum lost when they give up mass (let's ignore the friction of the air). I'm embarrassed to admit that I taught a semester of HS physics, and still can't answer this question.

Vorkosigan

Mr. Sammi

Making the ship shorter if it is spinning about itz long ends will reduce itz angular velocity when the ship attains itz new shortness. I cannot imagine how the ship becomes shorter unless it was modularily constructed.

There is a period of instability just as some mass, m1, leaves the collective. The resulting momentum which the shorter ship retains will be distributed after the departing mass takes some momentum away. The total effect which is the gravitational force acting on the ship plus the effects of the jettison should dictate the resulting stability/equilibrium. I think 2 simultaneous differential equations should solve this nicely (if you don't add more effective parameters).


Sammi Na Boodie ()

Bumble Bee Tuna

Angular velocity should increase as the ships ends are flung off. Just as a skater spins faster when they bring their arms in (Of course this situation is really like a skater whose arms are amputated mid-spin).

If they really wanted to make the ship slow down, they would need to get mass and launch it perpendicular to the ship, in the direction of the ships rotation. This would create a torque against the direction of motion, which would slow it down. How their ship is easily dismantled and its pieces launched around all while it's spinning rapidly, I have no idea. The ship would have to be designed for solving this kind of failure, which would be somewhat silly. And why would the design not just be rocket boosters (which are the same concept of ejecting material to give momentum, except much more efficient then just giving random objects the heave-ho)?

-B

beausoleil

If there's no torque driving the spin then losing the ends will have no effect on the rate of spin - those pieces take their angular momentum away with them and the remainder continues to spin at the same number of rpm. The analogy with the figure skater is incorrect since the figure skater still has her arms, jsut changes her moment of inertia. Now if you chopped them off and let them fly away it would be closer...

If you move things from the centre to the end and jettison them, the spin will slow as you move things (moment of inertia change) but not when you jettison.

In the case of something like a merry-go-round, where the spin is driven, losing mass makes it speed up because the same torque is now operating on a lower moment of inertia.

Vorkosigan

The ship was modular, even had blasters to blow off the sections, as if it was expected to enter the atmosphere while spinning Thanks for the explain, Beau.

Vorkosigan

Mr. Sammi

I guess that solves it then...

Who was doing the math, solving the equations, knowing when to push the buttons?

Sammi Na Boodie ()

Vorkosigan

No idea. They just pushed them on the assumption that it would work. The whole movie was unbelievable, I got out midway through it. It was just awful in every aspect.

Jayjay

Thanks for the answers.