In physics, mass is a property of a physical body which determines resistance to being accelerated by a force and the strength of its mutual gravitational attraction with other bodies. The SI unit of mass is the kilogram (kg).

Mass is not the same thing as weight, even though we commonly calculate an object's mass by measuring its weight. A man standing on the Moon would weigh less than he would on Earth because of the lower gravity, but he would have the same mass.

For everyday objects and energies well-described by Newtonian physics, mass describes the amount of matter in an object. However, at very high speeds or for subatomic particles, special relativity shows that energy is an additional source of mass. Thus, any stationary body having mass has an equivalent amount of energy, and all forms of energy resist acceleration by a force and have gravitational attraction.

There are several distinct phenomena which can be used to measure mass. Current experiments have found no difference among any of the ways used to measure mass:

  • Inertial mass measures an object's resistance to being accelerated by a force (represented by the relationship F=ma).

  • Active gravitational mass measures the gravitational force exerted by an object.

  • Passive gravitational mass measures the gravitational force experienced by an object in a known gravitational field.

  • Mass-Energy measures the total amount of energy contained within a body, using E=mc²