# Science questions not worth a thread I: I'm a moron!

Discussion in 'Science & Technology' started by The Imp, May 4, 2010.

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1. ### El_MachinaeColour vision since 2018Retired Moderator

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Link it! Good memes deserve propagation

2. ### Lohrenswald老任森林

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Why does the rotation velocity vector point parallell with the axis of rotation?

Am I stupid for not getting it?

3. ### Gigazcivoholic

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The rotation velocity vector is ultimately an abstract quantity and you could define it to point anywhere. Putting it along the axis of rotation is however rather intuitive because this vector is the same in your frame and in the frame of the rotating object.

4. ### AtticusDeityRetired Moderator

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Imagine you've got a shuttle in space and you want to speed up it a little, so you turn on the rockets. For the force that pushes the rocket forward there must be an opposite force, but since there's nothing in space, it must apply to the emissions from the rocket, right? Isn't that terribly bad way to accelerate, since the mass of the emissions must be very small?

Is there a workaround for this?

5. ### Flying PigUtrinque ParatusRetired Moderator

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If I understand it right, a rocket's propulsion is the mass of the emissions - the equal and opposite reaction of throwing something out the back is that the thing throwing it moves forwards. The mass of the emissions is actually quite large; fuel takes up the vast majority of the rocket's volume.

6. ### SamsonDeity

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Pretty much. The mass of the emissions is not nessaseraly small, watch a rocket taking off and the emissions are quite voluminous at least.

The work arounds are to have a fixed body to push against, hence the suggestions of rail guns to fire satellites into orbit, and the space escalator.

The problem can be reduced my maximising the velocity of of the emissions, hence ion drives.

7. ### Eran of ArcadiaStormin' MormonRetired Moderator

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Now that I'd like to see.

8. ### Gigazcivoholic

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With regard to the previous answers, the most efficient way to accelerate in space is in principle by emission of photons in the opposite direction. Nothing carries more impulse per mass. You can get a newton of force of the space ship for only 300 MW power and you would not actually need fuel because photons can be created from any kind of stored energy.

Unfortunately there is currently no way to easily turn anywhere near 300 MW power into unidirectional photons. Lasers are inefficient. Diodes would require too much space and material for their output. Infrared and X-rays are even harder to produce and manipulate.

9. ### uppiDeity

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Microwaves? Those could be generated fairly efficiently.

10. ### tokalaEmperor

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There is a quite interesting history behind the project to use giant guns as a first stage for space launches, and it looked to become actually feasible before getting killed by political in-fighting:
http://www.astronautix.com/articles/abroject.htm

11. ### AtticusDeityRetired Moderator

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Thanks everyone!

Yes, the total mass, but it's hard to imagine the mass emitted at any given second (aside the launch) would be that big. Although, I could imagine they don't need that big forces in space.

Another question: If we omit other planets, Sun etc., would the total gravitational force in the center of the earth be zero? The reasoning here is that there are equal masses in each direction of it. How about if we don't omit the other masses, were there still a point without gravitation somewhere near the center of the earth?

And third question, is there a measurable difference in the speed of things dropped at night and during the day, since in night the Sun also pulls the dropped item down, whereas in day it pulls it up? I tried to calculate the theoretical difference from the F= Gm1m2r-2 formula, but probably somehow screwed it up, since the 1N difference I got for 100 kg body sounds unbelievable.

I realize these are stupid and counterintuitive questions, but that's the reason they should IMO be asked. Besides, I'm a moron.

12. ### tokalaEmperor

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As long as you don't try to land or start from a massive body, indeed a tiny acceleration will do, e.g. provided by ion drives or light sails.
On and around massive bodies you will have to "waste" a lot of force just to avoid falling into it as long as you are on a sub-orbital trajectory.

Yes.
Imagine there was no such point, you could construct a perpetual motion machine, creating energy from nothing. Not very practical on earth, but feasible on some sponge-like moons and asteroids in the outer solar system.

You probably neglected that such an experiment happens in an accelerated frame of reference (earth orbiting the sun), which would cancel out most of the effect. What's left are higher order effects (see tidal forces), which are much smaller than that.

Naah, questions like that are actually asked as exercises in entry-level physics courses in universities, and unless you happen to imagine a suitable Gedankenexperiment or do the correct math, the solutions are indeed not necessarily obvious.

By the way, you might be interested in Kerbal Space Program, a really engaging rocketry simulator. Free demo available, and it's sale season at various gaming online stores at the moment.

13. ### AtticusDeityRetired Moderator

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Thanks!

Actually, I'm at the moment refreshing hasty physics (minor) studies from the last millennium. I had maths as major and went through the physics courses without understanding pretty much anything. We didn't have many of this kinds of problems, and that's why the things taught left open many questions or the feeling that there's something wrong with the reasoning, but you just can't put in words what it is.

I've heard the name Kerbal Space Program before, probably will give it a try!

14. ### hobbsyoyoDeity

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That's false, those concepts have nothing to do with having something to push against. Rail guns don't push against the earth, they just force a ton of energy into a projectile, it's more like an external propulsion system than it is like pushing against the earth. Note that a whole lot of gas is expelled out of the gun as it's fired - that is the reaction mass only instead of coming from the blunt end of the projectile (like in a rocket) it's coming from the gun.

The space elevator is also not really pushing against the earth. It's just a ladder that you climb up.

15. ### SamsonDeity

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Now I could be wrong, or just looking at it from a different point of view, but I think your criticism is false. Newton's Third Law says "For every action, there is an equal and opposite reaction". So in each case where a force is applied to a body being launched into orbit there is a equal and opposite force acting towards the centre of the earth. Also in each case there is a source of energy.

In the case of the rocket the source of energy is the chemical reaction in the fuel, and the opposite reaction is the gasses sent towards the earth from the rocket.

In the case of the rail gun the source of energy is electicity, and the opposite reaction is the rails / EM field generators pushing back against the earth.

In the case of the space elevator the source of energy is whatever is driving the cars, and the opposite reaction is the cars pushing down on the space elevator, which pushes down on the earth. Just like if you climb a ladder you are pushing down on the earth.

16. ### hobbsyoyoDeity

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EM launchers and guns (with suitable propellants) would work completely absent the Earth.

17. ### SamsonDeity

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Yeah, but if they were not attached to something they would end up travelling backwards with the same energy that the projectile has forwards. So if the gun was 10 times the mass of the projectile it would end up moving at 1/10th the speed of the projectile.

18. ### El_MachinaeColour vision since 2018Retired Moderator

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1/3.16th (1 over root10), but yeah.

19. ### uppiDeity

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No. Samson's number is correct (as is his analysis). This is not about energy conservation, but momentum conservation. Momentum is linear in mass and velocity, so 1/10 is the correct number.

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