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?