From @Stevens R. Miller : Challenge to my friends who are science nerds (yes, if you are even reading this, that includes you): My son is learning physics. At the moment, he is studying Newton's laws of motion. In dealing with force and acceleration, I have been impressing upon him that, if an object maintains a constant velocity, that means it is not experiencing any acceleration. From the equation F=ma, that also means that the net force applied to that object is zero. For a bit, he had a tough time understanding how an object might continue to move if no force acted on it. That got us into a discussion about momentum, defined as mv. At which point, he stopped me cold with this one: "Dad, if an object is moving at constant velocity, and that means its acceleration is zero, which also means the force acting on the object is zero, doesn't that mean its mass is undefined?" He defended his question thus:  F=ma.  Constant velocity implies a=0.  a=0 implies that ma=0, and therefore, by , F=0,  F=ma implies that m=F/a.  By , , and , constant velocity implies that m=0/0.  0/0 is undefined,  By all of the foregoing, the mass of an object moving at constant velocity is undefined. At which point he demanded that I explain how an object can have a defined momentum when moving at a constant velocity, if momentum is mv and, therefore, (0/0)v. I could use a little help here.