Tim, thanks for that explanation. I had never considered it quite that way, that mass is a measure of inertia, not matter. Wow, that makes some sense.

David Simmons, yes, Newton did in fact use the time derivative of momentum. It only simplifies to F=ma in most cases. However, one place it definitely does not is in the rocket equation. Basically any time you're burning fuel for acceleration, mass is not constant.

Mr. X, "quantity of movement" - thinking about that phrase, I would come up with displacement, or distance. Quantity means amount, and movement indicates changing location. The first derivative of displacement is velocity, second derivative is acceleration.

SEG9585, another way to consider the relativistic mass problem is to think of it as increasing kinetic energy, not increasing mass.