Why Is Total Kinetic Energy Conserved in Some Collisions but Not in Others?

[jack1234]

For this question
http://tinyurl.com/2btmot
the solution for question 6(c) is assume conservation of energy(total kinetic energy before collision = total kinetic energy after collision)

I am very confused here.

May I know why sometimes when two body collides, the total kinetic energy before collision is not equal to the total kinetic energy after the collision(although their momentum is equal)? Where are the kinetic energy goes?

For this question, what make us think that this is an elastic collision?

 

[muppet]

Energy is always conserved, as you know. However, it doesn't have to stay in the same form; it's sometimes dissipated from a system. Replace your airtrack with sandpaperand you'd quickly conclude that friction would be massive, so loads of energy would be wasted overcoming that. If the spring wasn't there, the gliders would crack together, and energy would be lost as sound; my old school used magnets instead to achieve the same effect.
In general, think about where energy can be lost from your system. If the question includes features that minimise those losses (the classic keyword is "smooth surface") then you're being encouraged to assume an elastic collision.