[YTNUKLR]

F(t)=dP/dt=(d/dt)mv=v(dm/dt) + m(dv/dt)

a=dv/dt

Therefore,

a=(F(t)-v(dm/dt))/m(t) , where v is the relative velocity of mass to the inertial frame (vehicle, projectile, etc.)

This is where we get a=F/m ; however, this eqn. doesn't generally take into account the fact that mass may be lost at some relative velocity from the vehicle (i.e. a rocket).

An interesting expansion on F=ma, I think.