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This week, we will use the
concepts of momentum and energy

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to examine an application of
great importance-- collisions.

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A collision is an event where
two or more objects exert

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forces on each other over
a short time interval,

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and where the interaction
forces dominate

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over any external forces.

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Common examples are collisions
involving projectiles,

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collisions of motor vehicles,
collisions in sports,

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and collisions between atomic
or subatomic particles.

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It is worth noting that the
colliding objects need not

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actually touch each
other in the collision,

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as long as the interaction force
does not require a contact.

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For example, two
positively charged

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protons shot towards each
other will repel each other due

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to the Coulomb electric force.

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Their interaction can be
treated as a collision

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even if they never
actually touch.

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We will see that momentum is
always conserved in a collision

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as long as external forces
can be entirely neglected.

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However, energy is not
necessarily conserved.

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This leads to the distinction
between elastic collisions,

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where energy is conserved,
and inelastic collisions,

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where energy is not conserved.

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We will also derive a simple,
but powerful principle

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in the case of elastic
collisions in one dimension,

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that the relative velocity
of one particle with respect

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to the other is simply
reversed by the collision.