Editing Collision (section)

== Types of collisions ==
[[File:Deflection.png|right|thumb|250px|[[Deflection (physics)|Deflection]] happens when an object hits a plane surface. If the kinetic energy after impact is the same as before impact, it is an elastic collision. If kinetic energy is lost, it is an inelastic collision. The diagram does not show whether the illustrated collision was elastic or inelastic, because no velocities are provided. The most one can say is that the collision was not perfectly inelastic, because in that case the ball would have stuck to the wall.]]
Collision is short-duration interaction between two bodies or more than two bodies simultaneously causing change in motion of bodies involved due to internal forces acted between them during this. Collisions involve forces (there is a change in [[velocity]]). The magnitude of the velocity difference just before impact is called the '''closing speed'''. All collisions conserve [[momentum]].  What distinguishes different types of collisions is whether they also conserve [[kinetic energy]] of the system before and after the collision. Collisions are of two types:
#'''[[Elastic collision]]''' If all of the total kinetic energy is conserved (i.e. no energy is released as sound, heat, etc.), the collision is said to be ''perfectly elastic''. Such a system is an [[Idealization (science philosophy)|idealization]] and cannot occur in reality, due to the [[second law of thermodynamics]].
#'''[[Inelastic collision]]'''. If most or all of the total kinetic energy is lost ([[Dissipation|dissipated]] as heat, sound, etc. or absorbed by the objects themselves), the collision is said to be [[Inelastic collision|''inelastic'']]; such collisions involve objects coming to a full stop.  An example of this is a baseball bat hitting a baseball - the kinetic energy of the bat is transferred to the ball, greatly increasing the ball's velocity. The sound of the bat hitting the ball represents the loss of energy.  A "perfectly inelastic" collision (also called a "perfectly plastic" collision) is a [[limiting case (mathematics)|limiting case]] of inelastic collision in which the two bodies [[Coalescence (physics)|coalesce]] after impact. An example of such a collision is a car crash, as cars crumple inward when crashing, rather than bouncing off of each other. This [[Crashworthiness|is by design]], for the [[Automotive safety|safety of the occupants]] and bystanders should a crash occur - the frame of the car absorbs the energy of the crash instead. 

The degree to which a collision is elastic or inelastic is quantified by the [[coefficient of restitution]], a value that generally ranges between zero and one. A perfectly elastic collision has a coefficient of restitution of one; a perfectly inelastic collision has a coefficient of restitution of zero. The line of impact is the line that is collinear to the common normal of the surfaces that are closest or in contact during impact.  This is the line along which internal force of collision acts during impact, and Newton's [[coefficient of restitution]] is defined only along this line.  

Collisions in [[ideal gases]] approach perfectly elastic collisions, as do scattering interactions of [[sub-atomic particles]] which are deflected by the [[electromagnetic force]]. Some large-scale interactions like the slingshot type gravitational interactions between satellites and planets are almost perfectly elastic.