Why is kinetic energy conserved in collisions?

[Joanofarcs]

talking of KE, i encountered a problem: "when a force of constant magnitude always act perpendicular to the motion of a particle, then KE is constant" - can someone explain this please?

 

[Gerenuk]

The change in kinetic energy expressed with vector quantities is
$$\mathrm{d}E=\vec{F}\cdot\mathrm{d}\vec{s}$$
If the force F and the path ds are perpendicular, then the change in kinetic energy is zero, i.e. the velocity of the particle doesn't change. For example a charge in magnetic field goes in circles, but keeps constant velocity.

 

[Dadface]

Consider a case where I think it is generally accepted that KE is conserved...gas atoms colliding at "moderate" energies.If the total energy of collision is smaller than the minimum excitation energy then,borrowing a phrase from tiny-tim, "kinetic is the only energy in town"and the collision will be perfectly elastic.At high enough energies,these given by quantum theory, some of the energy of collision can be used for excitation and ionisation, the collisions will not be perfectly elastic and the gas will glow.Since elastic collisions can only happen with microscopic objects where quantum effects can be appreciable then I think it is quantum theory that gives the best answer to the question.