## Wednesday, June 13, 2007

### IIT-JEE 2007 Assertion-Reason Type MCQ on Elastic Collisions

The following simple Assertion-Reason Type multiple choice question which appeared in IIT-JEE 2007 question paper is quite simple and is meant for checking whether you have mastered the fundamentals of elastic and inelastic collisions:

STATEMENT-1

In an elastic collision between two bodies, the relative speed of the bodies after collision is equal to the relative speed before the collision

because

Statement-2

In an elastic collision, the linear momentum of the system is conserved.

(a) Statement-1 is True, Statement-2 is True; Statement-2 is a correct

explanation for Statement-1

(b) Statement-1 is True, Statement-2 is True; Statement-2 is NOT a correct

explanation for Statement-1

(c) Statement-1 is True, Statement-2 is False

(d) Statement-1 is False, Statement-2 is True

The important points you have to remember here are:

(i) momentum is conserved in elastic as well as inelastic collisions

(i) in elastic collisions, kinetic energy also is conserved.

[Remember, we are talking of kinetic energy. Note that the total energy is conserved in all collisions in accordance with the law of conservation of energy].

In an elastic collision, the relative speed of the bodies after collision is equal to the relative speed before the collision and this is a consequence of the conservation of kinetic energy.

Therefore, statement-1 is true. Statement-2 also is true, but it is not the reason for statement-1. So, the correct option is (b).

[Note that the ratio of the relative speed of the bodies after collision to the relative speed before collision is called the coefficient of restitution, e. The value of ‘e’ lies between 0 and 1. For perfectly elastic collisions, e =1 and for perfectly inelastic collisions, e = 0]

1. 2. 