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Newton's third law states,For every action exerted by an object A on object B,there will be an equal and opposite reaction on A. In case of rockets,during ejection of hot gases with high velocities,action is gases coming out of end nozzle and reaction is rocket moving upward. What exactly is the action here?I mean if the rocket moving upward is a reaction,what is its action and on what? In other words,What exactly pushes rocket upward?They say,a thrust is produced in opposite direction pushing rocket upward when gases come out with high velocities according to law of conservation of momentum.Okay. But,how is thrust produced?

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The hot molucules of the burning fuel move rapidly to all directions with equal probability inside the burning cavity. This means that if the cavity were closed, for any molecule that impacts on one point in the interior of the cavity there would be (statisticaly) another one impacting anti-diametricaly and with the same rate; this would not cause any thrust since the impact forces on the walls of the cavity would cancel each other. However, if there is a hole in the cavity and some molecules escape through it, then then would be no force to compensate the impacts on the point which is anti-diametrically to the hole. So thrust is achieved...

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  • $\begingroup$ I was thinking of an explanation along the same lines..Thank you. :) $\endgroup$ – Viswanath Reddy Sep 23 '15 at 8:06

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