Equal And Opposite Reactions

[dunr]

when in space, what do the thrusters push against to change direction of a vehicle

[VHG]

The vehicle.

[bert_h]

They don't "push against" anything. It's a common mistake to think that that's the only way they could work. Instead, it's a thing called "conservation of momentum" - when you're stationary in space, and you push something away from yourself, you recoil in the opposite direction, just enough to keep the total momentum at zero. So the thrusters only work because they're puffing out stuff that has MASS.

You could try it on an ice rink sometime, where it's so smooth that you're certainly not "pushing against" anything; walk or skate on with a heavy bag, and hurl the bag along the ice away from you. Whoops! There you go in the opposite direction.

[dunr]

Yes VHG but when I lift the hatch on my car I push on the hatch & the floor.

[dunr]

Your analogy bert-h, on the ice ,proves the rule "every action has an equal and opposite reaction" - so where is the reaction in space?
I know that it works but i would like to understand how.

[chakka35]

I don't understand, dunr, why you think that being in space makes any difference. Wold you explain, please.

[Fitzer]

As per bert's answer, the thrusters 'throw' exhaust gases out, pushing the vehicle in the opposite direction.

[heathfield]

Imagine an empty box in space with one side missing. Throw in a stick of dynamite. The dynamite explodes, creating a force that expands in all directions. Some of the force is wasted by passing out through the empty side, but some of it presses against the closed side opposite the empty side. This moves the box in that direction. Consider the thrusters as a continually exploding stick of dynamite

[bert_h]

Hi there Dunr, you ask "where's the reaction?" but you've accepted the ice analogy. Where is the reaction in that? You apply an action to the bag, and you then experience a reaction from the bag. The thrusters apply an action to the gases they expel, and they (plus the vehicle they're attached to) then experience a reaction from the exhaust gases. The question "where?" doesn't apply very well in either case, but if you can answer it on the ice, then it's the same answer in space.

[Teddio]

Forces always act in pairs: no exceptions. Consider the spacecraft's thruster rocket in the vacuum of space. The nozzle pushes against the exhaust gas molecules, and the exhaust gas molecules push against the nozzle. Both forces are equal and opposite at all times. The force on the nozzle accelerates the spacecraft in the direction of the force. (Newton's 2nd and 3rd laws)

[Old_Geezer]

The thrusters chuck stuff out in one direction. The reaction is the main craft changing velocity.

[dunr]

Thank you all. I understand it now. Didn't think about gasses pushing against the nozzle, I could only think the gasses should be pushing against an atmosphere - which isn't there.

[mibn2cweus]

There are a couple of ways of looking at this. A rocket is self-propelled by (in any given brief period of time) accelerating a relatively small portion of its mass (the pressurised gas it is burning) to very high velocities in a direction opposite to its own acceleration. This gas, which is highly pressured by burning in a combustion chamber is expelled through a nozzle which provides a containment in which the gas can be further accelerated by virtue of its own pressure, to be expelled in a specific direction producing the counterforce that propels the rocket in the opposite direction.

For an example of this, compare the force given by a water hose with no attachments to one with a nozzle which restrains the flow allowing the water to develop pressure which is then expelled in a directional stream. Even though the overall flow is reduced by the constriction of the nozzle, the accelerated flow in a given direction produces a noticeable increase in the counterforce one feels holding the nozzle.

[Teddio]

Thank the Lord for Copy & Paste !

[otmu]

Dont forget folks! the vacuum of outer space is not as empty as we once thought, in fact, outer space is dense with many weird and wonderful particles, photons neutrinos to name a few, and these particles have mass however small or negligible they still have mass, and in theory could be rolled between your fingers, or, even pushed against.. *Clearly not an answer*, just food for thought..