Mass

Also called Inertial mass, Gravitational mass

Mass is the scalar measure of how much matter a system contains, in kilograms. It plays two separate roles in physics: resistance to acceleration, and the source of gravitational attraction.

Mass is a scalar. It has no direction and it does not change when you move an object somewhere else, which is the first thing that separates it from weight.

Two different jobs hide behind the one word.

  • Inertial mass is the mm in the second law: how hard a system is to accelerate. Double it and the same net force produces half the acceleration.
  • Gravitational mass is the mm in the law of universal gravitation: how strongly a system attracts and is attracted by other masses.

Nothing in the logic requires those two numbers to be the same, and the CED notes that their equivalence has been verified experimentally. One line shows why it matters. For an object in free fall the only force is gravity, so

minertial a=GM mgravr2m_{\text{inertial}}\, a = G\frac{M\, m_{\text{grav}}}{r^2}

and if the two masses are equal they cancel. The acceleration left over does not depend on the falling object's mass at all, which is why a feather and a hammer fall together in a vacuum.

Kilograms, always. Not newtons: those measure force, and an object described as weighing "70 kg" is being given its mass.

Compare it directly with weight, which is a force in newtons and does change when the object moves to another planet.

Back to the physics glossary