suppose you're firing a cannon in space: of course, assuming a stationary cannon in the begining, the momentum of the ball must be equal and opposite that of the cannon itself. Now let's say I double the mass of the cannon, of course that is going to affect the velocity of the cannon, but an undergrad just asked me if the velocity of the cannon ball would also change. he claims that if you increase the mass of the cannon, the velocity of the projectile must also increase. I'm not so convinced that his reasoning is correct. I can see that it could possibly be true, but I don't see why it would necessarily be so. my hunch is that the velocity of the projectile would be (ignoring gravitational interactions) constant and that the velocity of the cannon must change to conserve momentum. If I am in error here, please light the way.