# newtons 3rd law.... how does it work in space ?...

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#### tomorows_scientist

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&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ok so acording to newtons third law for every action there is an equal and oposite reaction right. ok so in space you for example shoot off a rocket and there is fire shooting out the back pushing the rocket along. well acording to newton there has to be an equal and oposite reaction but in space there really isnt anything for the force to act apon is there??? for example... so your flame shooting out the back of your rocket. the flame is creating a force on the rocket well then acording to his law there has to be a force beeing pushed out at well space, but well there isnt anything there in space like air or some other object for the force to act on is there?? so what is the forces (the flame) oposite reaction pushing on (acting on)?... <div class="Discussion_UserSignature"> </div>

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#### eburacum45

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<p>The stuff that comes out of the back of the rocket, the propellant, has mass, and therefore momentum. So when the rocket fires its&nbsp;motor, it is essentially'pushing' against the mass of the propellant. If you add up the momentum of the rocket moving in one direction and the propellant in the other, they will be equal to each other, which satisfies Newton's third law.</p> <div class="Discussion_UserSignature"> <p>---------------------------------------------------------------</p><p>http://orionsarm.com  http://thestarlark.blogspot.com/</p> </div>

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#### derekmcd

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ok so acording to newtons third law for every action there is an equal and oposite reaction right. ok so in space you for example shoot off a rocket and there is fire shooting out the back pushing the rocket along. well acording to newton there has to be an equal and oposite reaction but in space there really isnt anything for the force to act apon is there??? for example... so your flame shooting out the back of your rocket. the flame is creating a force on the rocket well then acording to his law there has to be a force beeing pushed out at well space, but well there isnt anything there in space like air or some other object for the force to act on is there?? so what is the forces (the flame) oposite reaction pushing on (acting on)?... <br /> Posted by tomorows_scientist</DIV></p><p>The action inside a rocket engine happens within the combustion chamber.&nbsp; As the gas or plasms expands, it pushes against the combustion chamber propelling the rocket forward... The equal and opposite reaction is the speed of the exhuast exiting the craft.&nbsp;</p> <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>

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#### CalliArcale

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<p>This is actually the reason why rocket engines (and jet engines) are called "reaction motors" -- they operate solely on the principle of Newton's Third Law rather than by the more complex means of propellers (which use Newton's Third Law as well, acting as a fan, but also a few other principles to sort of claw their way through the air -- this is a gross oversimplification, of course).</p><p>You don't need to be in a vacuum to test this.&nbsp;&nbsp; Get in a small boat such as a canoe on a very calm day (or on an indoor pool) with a supply of baseballs.&nbsp; Paddle out a ways, and then try to cancel your motion as best you can.&nbsp; Once everything has settled down, start throwing baseballs, all in the same direction.&nbsp; You will experience a slight thrust!&nbsp; Yet you haven't been pushing against the water, or against the shore.&nbsp; What, then, have you been pushing against?</p><p>Against the baseballs, of course.&nbsp; ;-)</p><p>One field where this is particularly important is artillery.&nbsp; Newton's Third Law doesn't just make rockets go -- it also makes guns recoil, and a really big artillery piece mounted on a destroyer can really mess up your navigation (and your aim) when it fires.&nbsp; You can see one solution to this problem in any pirate movie -- the cannons are mounted on rails so that they can roll backwards when they fire, absorbing the recoil (which is itself the reaction to the action of the same explosion that propelled the cannonball out of the cannon).&nbsp; Infantry cannons were mounted on caissons, which were allowed to roll freely when the cannons fired, acheiving the same objective.&nbsp; The reaction has to go *somewhere*.</p> <div class="Discussion_UserSignature"> <p> </p><p><font color="#666699"><em>"People assume that time is a strict progression of cause to effect, but actually from a non-linear, non-subjective viewpoint it's more like a big ball of wibbly wobbly . . . timey wimey . . . stuff."</em>  -- The Tenth Doctor, "Blink"</font></p> </div>

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#### DrRocket

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<p><BR/>Replying to:<BR/><DIV CLASS='Discussion_PostQuote'>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; ok so acording to newtons third law for every action there is an equal and oposite reaction right. ok so in space you for example shoot off a rocket and there is fire shooting out the back pushing the rocket along. well acording to newton there has to be an equal and oposite reaction but in space there really isnt anything for the force to act apon is there??? for example... so your flame shooting out the back of your rocket. the flame is creating a force on the rocket well then acording to his law there has to be a force beeing pushed out at well space, but well there isnt anything there in space like air or some other object for the force to act on is there?? so what is the forces (the flame) oposite reaction pushing on (acting on)?... <br />Posted by tomorows_scientist</DIV></p><p>There are two ways to look at rocket thrust, and they are equivalent.</p><p>One is via conservation of momentum.&nbsp; Gas a high velocity is exhausted out of the nozzle.&nbsp; So you have mass of gas exiting at a velocity from the back of the rocket.&nbsp; That mass with the associated velocity has a momentum.&nbsp; The rocket must increase velocity in the opposite direction, picking up a momentum ( mass x velocity ) in the forward equal to the momentum of the exhaust gasses in the opposite direction.</p><p>The other way is to look at pressure within the rocket and out to the exit plane of the nozzle.&nbsp; If you integrate the pressure over the surface to which it is applied you get a net force vector acting on the rockete.&nbsp; That net force vector is the thrust and it pushes the rocket forward.&nbsp; Essentially the force is the chamber pressure times the area of the nozzle throat (acutally the projection of the throat onto the forward dome) plus a smaller contribution from the rapidly dropping pressure inside the nozzle exit cone applied to the cone itself.&nbsp; Except for&nbsp;the projection of the throat onto the forward dome the other pressure X area elements inside the chamber cancel out.</p><p>Neither of these calculations require that there be anything "to push on", and rockets work just fine in a vacuum.&nbsp; In fact they work best in a vacuum because 1) there is no air resistance and more importantly because 2) the exhaust gasses can be expanded out to zero pressure and therefore to maximum velocity available within the laws of thermodynamics.<br /></p> <div class="Discussion_UserSignature"> </div>

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