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Difficulty in understanding a Rocket travelling in space.

  1. Nov 21, 2007 #1


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    Hi all,

    I'm afraid this may sound rather trivial but a friend of mine brought up this topic and we're quite confused about it: how does a rocket change its velocity in space?

    My understanding is that the rocket releases gas, and this gas will cause the rocket to accelerate. If you take Newton's second law, the net force of the rocket must be constant, but it is losing mass equilavent to the gas it releases. So in order for the net force to remain constant the rocket must accelerate. At least that can explain how the rocket changes it's velocity mathematically. But understanding using pure physics is where my problem occurs.

    How exactly does the gas "push" the rocket forward in space? Space is a vacuum is it not, so the gas musn't be colliding with any particles, if the gas isn't colliding with anything then how does it push the rocket forward?
    If I were to try my understanding of newtons laws, the the rocket should stay the travelling at a constant velocity until an unbalanced external force acts on it. This unbalanced external force will be provided by the equal and opposite force been exerted by the gas released. I guess why question really is how does the gas provide this force?

    Basically what I'm asking is action and reaction are opposite and equal but can some expalin why this so? Or have I merely delved into a matter that cannot be understood only we just know it to be the case and it apply it appropiately to situations like the rocket?

    Or am I missing something in relation to how the rocket accelerates?
  2. jcsd
  3. Nov 21, 2007 #2

    Doc Al

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    The rocket accelerates by expelling gas. The rocket pushes on the gas, and--from Newton's 3rd law--the gases push back on the rocket creating a net force on the rocket. Thus the rocket accelerates. The gas is also accelerating as it is pushed out into space. (It doesn't need anything to push against, other than the rocket.)

    A more down to earth example might help. Imagine you are stuck on a perfectly frictionless frozen pond and you want to start moving (accelerate) in order to reach the shore. Luckily, you happen to have a rock in your hand! You toss the rock: You push on it and it pushes back on you. Thus the force of the rock pushing back on you gives you the velocity needed to start moving towards shore. This is essentially the same idea as the rocket and its expelled gases.
  4. Nov 21, 2007 #3


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    A simple way of looking at it is like this:

    First imagine that the combustion chamber of the rocket is completely enclosed. The fuel is ignited and the resulting hot gasses attempt to expand. As they do so, they exert outward pressure (force) on the walls of the combustion chamber. in this situation all the forces balance out and there is no net force acting on the chamber in any one direction. The gasses push in all directions with equal force.

    Now open on end of the chamber. The gases still expand and still exert force against the inside of the chamber, but the gases at the open end pass through without exerting any force against the chamber. The gasses on the opposite side of the chamber still do exert a force on to the wall of the chamber. And since this force is not balanced out by a force on the open end of the chamber, we end up with a net force acting on the chamber in this direction and the chamber (and the rocket it is attached to) accelerates in this direction.
  5. Nov 21, 2007 #4


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    Thanks very much Janus, I get it now:D

    Oh and Doc Al thanks aswell.
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