Editing Elastic collision (section)

{{Short description|Collision in which kinetic energy is conserved}}
{{More citations needed|date=September 2020}} 
[[Image:Translational motion.gif|thumb|right|300px|As long as [[black-body radiation]] (not shown) doesn't escape a system, atoms in thermal agitation undergo essentially elastic collisions. On average, two atoms rebound from each other with the same kinetic energy as before a collision. Five atoms are colored red so their paths of motion are easier to see.]]

In [[physics]], an '''elastic collision''' occurs between two [[physical object]]s in which the total [[kinetic energy]] of the two bodies remains the same. In an ideal, perfectly elastic collision, there is no net [[Energy transformation|loss]] of kinetic energy into other forms such as [[heat]], noise, or [[potential energy]].

During the collision of small objects, kinetic energy is first converted to potential energy associated with a [[Coulomb's law|repulsive or attractive force]] between the particles (when the particles move against this force, i.e. the angle between the force and the [[relative velocity]] is obtuse), then this potential energy is converted back to kinetic energy (when the particles move with this force, i.e. the angle between the force and the relative velocity is acute).

Collisions of [[atom]]s are elastic, for example [[Rutherford backscattering]].

A useful special case of elastic collision is when the two bodies have equal mass, in which case they will simply exchange their [[Momentum|momenta]].

The ''[[molecule]]s''—as distinct from [[atom]]s—of a [[gas]] or [[liquid]] rarely experience perfectly elastic collisions because kinetic energy is exchanged between the molecules’ translational motion and their internal [[Degrees of freedom (physics and chemistry)|degrees of freedom]] with each collision. At any instant, half the collisions are, to a varying extent, ''[[inelastic collision]]s'' (the pair possesses less kinetic energy in their translational motions after the collision than before), and half could be described as “super-elastic” (possessing ''more'' kinetic energy after the collision than before). Averaged across the entire sample, molecular collisions can be regarded as essentially elastic as long as [[Planck's law]] forbids energy from being carried away by black-body photons.

In the case of macroscopic bodies, perfectly elastic collisions are an ideal never fully realized, but approximated by the interactions of objects such as [[billiard balls]].

When considering energies, possible [[rotational energy]] before and/or after a collision may also play a role.