IIT JEE , AIEEE Forum - Physics , Mechanics

bvsatyaram

The basic definition of Elastic collision is that the energy is conserved. If u review the definition of Coefficient of restitution, u will find that it has nothing to do with energy.

But mathematically, it happens that coefficient of restitution is 1 for perfectly elastic collisions.
So, both the definitions are true for perfectly elastic collisions.
Now coming to the problem, I would like to stress on the exact definition of Coefficient of restitution.

e = (rel. vel. of seperation)/(rel. vel. of approach)
In this definition we talk about two relative velocities. Note that both the relative velocities MUST be taken in the direction of common normal. So, inthis particular proble, when u use the definition of Coefficient of restitution, u need to take the relative velocities along the common normal. ie. normal to the sphere at the point of contact. If u go by this method, u will get the same answer as u get by conservation of energy method.

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