Question #dc233

From a classical physics point of view, mass is in some sense defined by Newton's second law.
#F=ma#
Which means that when we apply a force to an object, the amount by which it accelerates depends on some property of the object, which we call mass. Now we can compare the mass of objects by seeing how they accelerate when we apply the same force to them.

Newton's theory of gravitation gives a law for how strong the force of gravity is by:
#F_g=G(m_1m_2)/r^2#
where #G# is some constant, #m_1# and #m_2# are the masses of the objects attracting each other and #r# is the distance between them. It seems odd that this gravity has anything to do with this number that determines how easy it is for an object to be accelerated, but it has to be, because we know that any two objects take the same amount of time to fall from a certain height (when we ignore air resistance). This means that on heavier objects, gravity needs to pull harder to get it to fall at the same rate.

This allows us to compare masses using scales, which is quite an easy way to weigh objects, but it does let us lose sight of the original definition of mass.

For a more complete discussion on the definitions of mass and forces, you could look in a university text book on classical mechanics. To check if the answer I was about to write, I've used "Classical Mechanics" by John R. Taylor (not because I think it's the best book around, but because it was the book we used when I took the courses on classical mechanics at my university, other books will probably do just as well). By the way, the answer I was about to write, checked out with what was in the book, hence this answer.