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# A question about physics.

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posted on Oct, 25 2015 @ 12:53 AM
If space is a vacuum (relative to our atmosphere), then how does thrust work in space?

Let's say I'm sitting in a rolling office chair while facing a wall. I then use my arms to push myself away from the wall. That's thrust.

But now I'm in the same chair, and resistance has caused me to stop rolling dead in the middle of the room. So once again, I push my arms out to generate more thrust, only there is nothing to push off from, therefore I'm stuck.

There must be an explanation. What is it?
edit on 25-10-2015 by BELIEVERpriest because: typo

posted on Oct, 25 2015 @ 01:08 AM
a reply to: BELIEVERpriest

And if you had some sort of engine attached to that chair you would be able to move.

www.polaris.iastate.edu...

Found this explanation.

posted on Oct, 25 2015 @ 01:19 AM
a reply to: Sremmos80

In the parable of the rolling chair, the thrusting arms are the engine. Within the atmosphere, I always figured the engine used the resistance of the atmosphere for thrust.

posted on Oct, 25 2015 @ 01:21 AM

originally posted by: BELIEVERpriest
a reply to: Sremmos80

In the parable of the rolling chair, the thrusting arms are the engine. Within the atmosphere, I always figured the engine used the resistance of the atmosphere for thrust.

No, it doesn't. This is high school level Newtonian physics. There are LOTS of good middle school/high school sophomore level books that will walk you through basic Newtonian mechanics.

posted on Oct, 25 2015 @ 01:21 AM
a reply to: BELIEVERpriest

Was mistaken.
edit on thSun, 25 Oct 2015 01:55:36 -0500America/Chicago1020153680 by Sremmos80 because: (no reason given)

posted on Oct, 25 2015 @ 01:23 AM
a reply to: BELIEVERpriest

When you push off the wall, you're exhibiting Newton's 3rd Law "Every action has an equal and opposite reaction" - you push on the wall and thus the wall pushes on you.

A rocket in space does the same thing with propellent - combusting fuel in a tight space, i.e. increasing it's pressure and forcing it out one end. As the propellent is pushed out, it "pushes back", thus moving the rocket forward.

This is actually easier to do in a vacuum than anywhere else because there is no drag to oppose this force, whereas your office chair will roll to a stop due to friction.

posted on Oct, 25 2015 @ 01:24 AM

originally posted by: Bedlam

originally posted by: BELIEVERpriest
a reply to: Sremmos80

In the parable of the rolling chair, the thrusting arms are the engine. Within the atmosphere, I always figured the engine used the resistance of the atmosphere for thrust.

No, it doesn't. This is high school level Newtonian physics. There are LOTS of good middle school/high school sophomore level books that will walk you through basic Newtonian mechanics.

Yeah...pffffft...it's not like this is rocket science or anything! Wait...

posted on Oct, 25 2015 @ 01:29 AM
common mistake is to think that space is empty.

Newtons laws of motion still apply
think of it like this if our atmosphere is a thick soup then space is a patchy thin fog of particles.
the thrust is a force if you apply sufficient force movement will occur in the opposite direction regardless
of the density of the medium it sitting in
The main difference is that in an atmosphere the medium is dense enough to slow the craft due to friction
(not counting for the effect of gravity)
where in space the medium is so thin as to have almost no drag at all so as long as the the thrust is applied
the craft will accelerate to a very high speed and keep that speed once the thrust is stopped
until it either meets a dense gas cloud or an atmosphere where drag will apply once more.

also another effect is solar wind (a stream of particles) which can also push a craft in open space
just like water on a boat. but you need to be outside of a planets gravity to notice.

posted on Oct, 25 2015 @ 01:32 AM

originally posted by: Sremmos80
a reply to: BELIEVERpriest

That link explains it pushing off the gravitation pull, or at least that is what I understood it as saying.

Or is your question about what happens when the craft in question is away from all planets/stars?

Oh, my, no. What part of that link did you think was telling you that rocket thrust was created by pushing off of gravity?

Acceleration caused by rocket thrust is independent of any of that. It's basic Newtonian mechanics, the entire 'equal and opposite reaction' part.

eta:

I should add, in a vacuum, you get even MORE thrust out of a conventional rocket thruster, because the equation that tells you how much force the rocket generates has a couple of main components. One is related to Newtonian reaction, that is, the mass and velocity of the crap you're pitching out the rear. The other is related to pressure differential between the nozzle opening and the inside of the combustion chamber. If the pressure outside the nozzle is effectively zero, you get a lot more contribution from the pressure differential term.
edit on 25-10-2015 by Bedlam because: (no reason given)

posted on Oct, 25 2015 @ 01:37 AM

originally posted by: Sremmos80
a reply to: BELIEVERpriest

I am no expert but always understood it that space isn't completely void of things to push off of. That link explains it pushing off the gravitation pull, or at least that is what I understood it as saying.

Or is your question about what happens when the craft in question is away from all planets/stars?

The link says that remaining stationary and coasting are "free" as in no extra force is needed. This would be due to the 'vacuum' factor if space. So if there is nothing (dense enough) to slow a craft down, then surely there is nothing (dense enough) to push off of.

So far no answers in this thread. I hear parroting, but no explanation as to why.
edit on 25-10-2015 by BELIEVERpriest because: typo

posted on Oct, 25 2015 @ 01:38 AM

originally posted by: BELIEVERpriest
So far no answers in this thread. I hear parroting, but no explanation as to why.

The parrots are giving you your answer, but you are unable to parse it. Try again.

You do NOT need anything "to push off against". Your mental image of what's going on is wrong. That's your answer.

posted on Oct, 25 2015 @ 01:38 AM
a reply to: ShayneJUK

It has nothing to do with whether there are any particles in space or not. It has everything to do with conservation of momentum - if your rocket is expelling particles out one end, there can be no net change in total momentum of this system, and thus the rocket will go the other way. This principle would still work in a perfect vacuum.

posted on Oct, 25 2015 @ 01:41 AM

originally posted by: Bedlam

originally posted by: BELIEVERpriest
So far no answers in this thread. I hear parroting, but no explanation as to why.

You do NOT need anything "to push off against". Your mental image of what's going on is wrong. That's your answer.

Very well, then why?

posted on Oct, 25 2015 @ 01:45 AM
a reply to: BELIEVERpriest

Because physics.

Not really sure how to explain it any clearer than above.

posted on Oct, 25 2015 @ 01:47 AM

originally posted by: BELIEVERpriest

originally posted by: Bedlam

originally posted by: BELIEVERpriest
So far no answers in this thread. I hear parroting, but no explanation as to why.

You do NOT need anything "to push off against". Your mental image of what's going on is wrong. That's your answer.

Very well, then why?

Why is your mental image wrong? I don't know, but I'm going to assume it's one of the following...

1) You are still a freshman or sophomore in high school
2) your high school was worse even than Stephens County High (places hand over heart), which, to be honest, seems to have been at least as good if not better than other small high schools in the southeast for biology and physics. Especially for the sort of high schools where animal husbandry was mandatory.
3) you didn't pay attention to the physics of motion classes, but this takes up the majority of high school level science, so I don't know how you managed it

posted on Oct, 25 2015 @ 01:51 AM
a reply to: mc_squared

i am aware.

i was trying (poorly) to simplify what space actually IS and that the thruster doesnt care about the medium
beyond the affect of drag

posted on Oct, 25 2015 @ 01:52 AM
a reply to: Bedlam

I'll admit, I'm ignorant to the principle, but thats not what I meant.

Why does that physical law work? All of the other laws make sense to me, but never this one.

You can continue to make assumptions about my intellectual aptitude, or you can help me understand.

posted on Oct, 25 2015 @ 01:55 AM
a reply to: Bedlam

Thought I read that, musta missed something there.

Now I feel rather silly about saying it, not sure why it didn't hit me when I originally typed it

edit on thSun, 25 Oct 2015 02:00:38 -0500America/Chicago1020153880 by Sremmos80 because: (no reason given)

posted on Oct, 25 2015 @ 01:57 AM
a reply to: BELIEVERpriest

You're still thinking that you need something to push off of in order to give the spaceship momentum, which isn't necessarily true. When you look at propulsion systems, like a plane's propeller, it uses air particles to push through the engine and produce thrust. This, I believe, is the key issue you're having with the concept. Because there are no air particles to use and generate thrust with out in space, explosions do the trick.

When a rocket's fuel is lit, it produces a controlled explosion, and that energy is focused out a specific direction. It generates thrust, not by pushing off anything, but simply with it's own energy, so the ship that rocket is attached to all of a sudden has this energy being jetted out one end, and thus is propelled to go the opposite direction of that controlled explosion.

The same thing goes for using other forms of generating energy to produce movement, such as using compressed gasses (that don't need to ignite, necessarily). When their energy is expelled, and focused in one direction, that energy also triggers the ship (or device) it's attached to to move in the opposite direction.

Because there is no resistance in space (other than gravity and actual solid objects), even the smallest release of energy can push something in a way that it will continue in that direction forever (or until it is snagged by gravity or hits a solid object).

posted on Oct, 25 2015 @ 02:01 AM
a reply to: BELIEVERpriest

In the vacuum of space, rocket engines and thrusters (basically small rocket engines) have a bell shape. One end open and the other end closed. In your scenario of pushing against the wall, you are actually playing the role of the expanding gases pushing on the closed end of the bell. You are not the rocket ship.

In space, your wall wouldn't be attached to the mass of our entire planet. As you pushed against the wall (the closed end of the bell), the wall would have moved away from you and you would move away from the wall at the same time. As the role of the expanding gases you would keep moving (out of the open end of the bell) because there is no friction from your chair's wheels to slow you down.

The rocket ship will keep dumping more of you into the bell as long as it needs more force.

Hope this helps.

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