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Energy is always conserved but in a collision some, or all of the kinetic energy may be converted into other forms of energy such as sound and heat.

There are two extreme types of collision:
(a) elastic - a perfect collision, no kinetic energy is lost on impact
(b) inelastic some of the kinetic energy is lost on impact
In real life most collisions are somewhere between these two extremes.

Momentum is also conserved in all collisions, and the law of conservation of momentum is:

Momentum before collision equals momentum after collision as long as no external forces act on the objects.
© Keith Gibbs 2009