I was looking through this forum, and I noticed one of those problems where a rocket ejects mass to get into orbit. F=dp/dt Now the arguments I saw went something like this: p=mv F=d(mv)/dt=v(dm/dt)+m(dv/dt) But that equation is for a point particle I believe. All you can say for a rocket is: F=m*a where a is the acceleration of the center of mass. For one thing, in the equation F=d(mv)/dt=v(dm/dt)+m(dv/dt) what is v? It's the relative velocity between the ejected mass and the rocket right? How does F=dp/dt "know" information about how the mass is going to be ejected? You could chuck the fuel with a small velocity or high velocity. I think when you write F=d(mv)/dt you do that because relativity says m is a function of v (which is a function of t). But we are still assuming a point particle, and I don't think this expression can honestly be used for a rocket problem. However, I see everyone doing that, so I'm not too sure of myself on this one.