What is the optical density of oil and water ?

Optical density of a material determines the ability of the material to reflect light. It is not the same as mass density of the medium, which determines the heaviness of the material. That is why petrol inspire of being less denser in terms of mass than water, it is optically more denser than water.

moving ball hits still ball. mover stops, hit ball moves. momentum transfer.

if they have a finite resultant