1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Definitions of mass.

  1. Jul 27, 2010 #1


    User Avatar
    Gold Member

    In physics, we use the concept of mass, and we use a unit of mass.
    It would appear that we must first define the concept of mass, to get to a position where we can establish a unit of mass.

    Mass not rigorously defined

    To my knowledge the concept of mass is not rigorously defined in Physics. We can opt to make equivalence of inertial mass and gravitational mass part of the very definition of mass, or we can choose to define mass independently. That is not codified.

    (Incidentally, it's remarkable that enormous effort has gone in deciding on a definition of 'planet' that would make Pluto either a planet or not a planet, but the concept of mass is pretty much left free.)

    If we want to know whether two objects have the same mass then we put them on a balance, a procedure that compares their respective gravitational masses.
    Assumption of equivalence of gravitational mass and inertial mass is everywhere in our procedures. Therefore in effect equivalence of gravitational and inertial mass is part of the very definition of mass, even though there's no formal statement to that effect.

    As I said, in physics defining the concept of mass is left free. Then how do physics student learn what mass is? Well, they absorb the concept; they learn by osmosis. In any discipline the most pervasive concepts tends to remain undefined, because those concepts are everywhere anyway.

    In a recent thread on this forum I saw someone insisting that to define the concept of mass only the concepts momentum and velocity ought to be used. Implicitly (but nowhere explicitly) that person was arguing that the concept and unit of mass should work with the concept of inertial mass only, disregarding gravitational mass.
    Last edited: Jul 27, 2010
  2. jcsd
  3. Jul 27, 2010 #2
    There is already an ongoing discussion of these very ideas here in this thread; basically the concept of inertial mass and how it relates to gravitation.

    You may want to read first and put your input there.
    Also note our discussion on Dennis Sciama's formulation which provides the underlying relationship for the equivalence of inertial and gravitational mass.
    Last edited: Jul 27, 2010
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook