Question

what is meant by collisions in one dimensions(elastic collisions)?

Answer 1

ellastic non ellastic semi ellastic 3 types.

Answer 2

This is a definition , that you could have easily looked up.

The most simple case of a collision is a one-dimensional, or head-on collision. Because of the conservation of energy and momentum we are able to predict a great deal about these collisions, and to calculate relevant quantities after the collision occurs. For future reference: http://www.sparknotes.com/physics/linearmomentum/collisions/section1.html