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==Origin==
[http://nordan.daynal.org/wiki/index.php?title=English#ca._1100-1500_.09THE_MIDDLE_ENGLISH_PERIOD Middle English], from [[Latin]] collision-, collisio, from collidere
*[http://en.wikipedia.org/wiki/15th_century 15th Century]
==Definitions==
*1: an [[act]] or instance of colliding : clash
*2: an encounter between [[particles]] (as [[atoms]] or [[molecules]]) resulting in exchange or [[transformation]] of [[energy]]
==Description==
A '''collision''' is an isolated [[event]] in which two or more moving bodies (colliding bodies) exert [[relatively]] strong [[forces]] on each other for a relatively short time.
==Dynamics==
Collisions involve [[forces]] (there is a [[change]] in [[velocity]]). Collisions can be elastic, meaning they [[conserve]] [[energy]] and [[momentum]]; inelastic, meaning they conserve [[momentum]] but not energy; or totally inelastic (or plastic), meaning they [[conserve]] [[momentum]] and the two objects stick [[together]].
The [[magnitude]] of the [[velocity]] [[difference]] at impact is called the closing speed.
The field of [http://en.wikipedia.org/wiki/Dynamics_(mechanics) dynamics] is concerned with moving and colliding objects.
==Elastic and Inelastic Collisions==
A perfectly [http://en.wikipedia.org/wiki/Elastic_collision elastic collision] is defined as one in which there is no loss of [http://en.wikipedia.org/wiki/Kinetic_energy kinetic energy] in the collision. An inelastic collision is one in which part of the kinetic energy is changed to some other [[form]] of [[energy]] in the collision. Any macroscopic collision between objects will [[convert]] some of the kinetic energy into [http://en.wikipedia.org/wiki/Internal_energy internal energy] and other forms of energy, so no large scale impacts are perfectly elastic. [[Momentum]] is conserved in inelastic collisions, but one cannot track the kinetic energy through the collision since some of it is converted to other forms of energy. Collisions in [http://en.wikipedia.org/wiki/Ideal_gases ideal gases] approach perfectly elastic collisions, as do scattering [[interactions]] of [http://en.wikipedia.org/wiki/Sub-atomic_particles sub-atomic particles] which are deflected by the [[electromagnetic]] force. Some large-scale interactions like the slingshot type [[gravitation]]al interactions between [[satellites]] and [[planets]] are perfectly elastic. Collisions between hard [[spheres]] may be nearly elastic, so it is useful to [[calculate]] the limiting case of an elastic collision. The [[assumption]] of [[conservation]] of [[momentum]] as well as the conservation of kinetic energy makes possible the [[calculation]] of the final velocities in two-body collisions.
[edit] Mathematical description
Let the [[linear]], [[angular]] and internal [[momenta]] of a [[molecule]] be given by the set of r variables { pi }. The state of a molecule may then be described by the range δwi = δp1δp2δp3 ... δpr. There are many such ranges [[corresponding]] to different states; a specific state may be denoted by the index i. Two [[molecules]] undergoing a collision can thus be denoted by (i, j) (Such an ordered pair is sometimes known as a [[constellation]].)
It is convenient to suppose that two molecules exert a negligible ''effect'' on each other unless their centre of [[gravities]] approach within a critical distance b. A collision therefore begins when the respective [[centres]] of [[gravity]] arrive at this critical distance, and is completed when they again reach this critical distance on their way apart. Under this [[model]], a collision is completely described by the matrix , which refers to the constellation (i, j) before the collision, and the (in general different) constellation (k, l) after the collision.
This notation is convenient in proving Boltzmann's [http://en.wikipedia.org/wiki/H-theorem H-theorem] of [http://en.wikipedia.org/wiki/Statistical_mechanics statistical mechanics].[http://en.wikipedia.org/wiki/Collision]
[[Category: Physics]]
==Origin==
[http://nordan.daynal.org/wiki/index.php?title=English#ca._1100-1500_.09THE_MIDDLE_ENGLISH_PERIOD Middle English], from [[Latin]] collision-, collisio, from collidere
*[http://en.wikipedia.org/wiki/15th_century 15th Century]
==Definitions==
*1: an [[act]] or instance of colliding : clash
*2: an encounter between [[particles]] (as [[atoms]] or [[molecules]]) resulting in exchange or [[transformation]] of [[energy]]
==Description==
A '''collision''' is an isolated [[event]] in which two or more moving bodies (colliding bodies) exert [[relatively]] strong [[forces]] on each other for a relatively short time.
==Dynamics==
Collisions involve [[forces]] (there is a [[change]] in [[velocity]]). Collisions can be elastic, meaning they [[conserve]] [[energy]] and [[momentum]]; inelastic, meaning they conserve [[momentum]] but not energy; or totally inelastic (or plastic), meaning they [[conserve]] [[momentum]] and the two objects stick [[together]].
The [[magnitude]] of the [[velocity]] [[difference]] at impact is called the closing speed.
The field of [http://en.wikipedia.org/wiki/Dynamics_(mechanics) dynamics] is concerned with moving and colliding objects.
==Elastic and Inelastic Collisions==
A perfectly [http://en.wikipedia.org/wiki/Elastic_collision elastic collision] is defined as one in which there is no loss of [http://en.wikipedia.org/wiki/Kinetic_energy kinetic energy] in the collision. An inelastic collision is one in which part of the kinetic energy is changed to some other [[form]] of [[energy]] in the collision. Any macroscopic collision between objects will [[convert]] some of the kinetic energy into [http://en.wikipedia.org/wiki/Internal_energy internal energy] and other forms of energy, so no large scale impacts are perfectly elastic. [[Momentum]] is conserved in inelastic collisions, but one cannot track the kinetic energy through the collision since some of it is converted to other forms of energy. Collisions in [http://en.wikipedia.org/wiki/Ideal_gases ideal gases] approach perfectly elastic collisions, as do scattering [[interactions]] of [http://en.wikipedia.org/wiki/Sub-atomic_particles sub-atomic particles] which are deflected by the [[electromagnetic]] force. Some large-scale interactions like the slingshot type [[gravitation]]al interactions between [[satellites]] and [[planets]] are perfectly elastic. Collisions between hard [[spheres]] may be nearly elastic, so it is useful to [[calculate]] the limiting case of an elastic collision. The [[assumption]] of [[conservation]] of [[momentum]] as well as the conservation of kinetic energy makes possible the [[calculation]] of the final velocities in two-body collisions.
[edit] Mathematical description
Let the [[linear]], [[angular]] and internal [[momenta]] of a [[molecule]] be given by the set of r variables { pi }. The state of a molecule may then be described by the range δwi = δp1δp2δp3 ... δpr. There are many such ranges [[corresponding]] to different states; a specific state may be denoted by the index i. Two [[molecules]] undergoing a collision can thus be denoted by (i, j) (Such an ordered pair is sometimes known as a [[constellation]].)
It is convenient to suppose that two molecules exert a negligible ''effect'' on each other unless their centre of [[gravities]] approach within a critical distance b. A collision therefore begins when the respective [[centres]] of [[gravity]] arrive at this critical distance, and is completed when they again reach this critical distance on their way apart. Under this [[model]], a collision is completely described by the matrix , which refers to the constellation (i, j) before the collision, and the (in general different) constellation (k, l) after the collision.
This notation is convenient in proving Boltzmann's [http://en.wikipedia.org/wiki/H-theorem H-theorem] of [http://en.wikipedia.org/wiki/Statistical_mechanics statistical mechanics].[http://en.wikipedia.org/wiki/Collision]
[[Category: Physics]]