In an elastic collision of two billiard balls,which of the following quantities remain conserved during the short time of collision of the balls? (i.e.,when they are in contact)
$(a)$ Kinetic energy.
$(b)$ Total linear momentum.
Give reason for your answer in each case.

(B) During the short time of collision,the billiard balls undergo deformation,which leads to the storage of potential energy $(PE)$.
Since some kinetic energy $(KE)$ is converted into potential energy during the deformation phase,the total kinetic energy is not conserved during the contact time.
However,for the system of two balls,the resultant external force acting on the system is zero.
According to the law of conservation of linear momentum,if the net external force is zero,the total linear momentum of the system remains conserved throughout the collision process,including the time of contact.