# Glossary

**collision**- A brief interaction between two or more particles or bodies in close proximity.
**completely inelastic collision**- A
*collision*in which the colliding bodies stick together, resulting in the maximum loss of*kinetic energy*consistent with*conservation of momentum*. **conservation of kinetic energy**- The principle that the total kinetic energy of any isolated system is constant.
**conservation of linear momentum**- The principle that the total
*linear momentum*of any isolated system is constant. **elastic collision**- A collision in which
*kinetic energy*is conserved. **inelastic collision**- A collision in which
*kinetic energy*is not conserved. **internal force**- In the context of a given system, an internal force is a
*force*that acts within the system and which has a*reaction*that also acts within the system. **isolated system**- A
*system*which cannot exchange*matter*or energy with its*environment*. In the context of*mechanics*, an isolated system is one that is subject only to*internal forces*. **kinetic energy**- The energy that a body possesses by virtue of its motion.
**law of conservation of linear momentum**- See
*conservation of linear momentum*. **linear momentum**- The
*momentum*associated with the*translational motion*of a body. For a*particle*of*mass*travelling with*velocity*, the linear momentum is . **mass energy**- The
*energy*that a body possesses by virtue of its*mass*, as given by , where is the speed of light in a vacuum. The existence of mass energy is one of the many implications of the*special theory of relativity*. The mass energy of a free particle is the difference between its (total)*relativistic energy*and its*relativistic translational kinetic energy*. Mass energy is also known as*rest energy*. **momentum**- A
*vector*quantity, useful in various situations as a measure of a body's tendency to continue in its existing state of*rotational*or*translational motion*. **principle of conservation of linear momentum**- See
*conservation of linear momentum*. **relativistic collision**- A collision involving sufficiently high speeds that its analysis requires the use of the relativistic relations for momentum and energy rather than their Newtonian counterparts. Relativistic collisions are often
*inelastic*and are characterised by the creation of new particles and an associated increase in*mass energy*(at the expense of*kinetic energy*). **relativistic energy**- According to the theory of special relativity the total energy (including the
*mass energy*) of a particle of mass travelling with speed is **relativistic kinetic energy**- According to the theory of special relativity, the translational kinetic energy of a particle of mass and speed is equal to its total relativistic energy minus its mass energy
**relativistic momentum**- The
*momentum*of a body according to the*special theory of relativity*. For a particle of (rest) mass , travelling with velocity , the relativistic momentum is At speeds which are small compared with the speed of light, , this reduces to the Newtonian expression . **superelastic collision**- A collision in which the
*kinetic energy*increases, typically as a result of the release of*potential energy*. **system**- That part of the Universe which is the subject of an investigation.