Definition. A particle, also called mass point, is a model of a real object. A particle is point-like and thus has all of its mass concentrated at one point.

Comment 1. Being point-like, a particle has no internal structure and does not rotate. A particle's motion is entirely translational. A particle can have momentum, = m , and kinetic energy, KE = m/2 v², where m and are the particle's mass and velocity, respectively, and v = || the particle's speed. A particle also can have angular momentum, relative to a point separate from the particle.

Comment 2. Depending on the circumstances, a real object can either be represented by a particle or must be must be represented as an extended object. Circumstances are common in which even very large objects, e.g., the earth or the sun, can usefully be represented by a particle.