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.

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There may be increase in $PE$ while balls are in contact, this decreases $KE$. Hence, $KE $ is not conserved. Here, resultant external force is zero, hence the total linear momentum is conserved,

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