Introductory textbooks have routinely stated Newton's Second Law as: a =
Sigma(F)/m, or Sigma(F) = ma. (There are some exceptions; for example,
Eugene Hecht's text). This is a special case and is true only when the
mass is constant. There should also be an equal emphasis on the fact
that it is possible to generate a force at a constant velocity when the
mass changes as well, such as in rocket motion or in water jetting out
of a hose. Even Newton does not state his Second Law the same way,
instead indicating that force is proportional to the rate of change in
momentum. I think that should be the more general and the correct way to
state the second law. Please give me some feedback. Thank you.