# Newton's 4th law

Gold Member
Why do we not refer to Newton's universal law of gravitation as Newton's 4th law. Why are his first three packaged together: inertial mass, f=ma, action/reaction, while his 4th, f=Gmm/r^2 stands alone?

Because these three laws are the fundamental basis for describing motion in Newtonian mechanics. They are collectively referred to as "Newton's three laws of motion". Obviously his law of universal gravitation doesn't constitute a "4th law of motion", and so there you have it.

AlephZero