Consider elastic collision of a particle of mass m moving with a velocity u with another particle of the same mass at rest. After the collision the projectile and the stuck particle move in directions making angles and respectively with the initial direction of motion. The sum of the angles
a)		b)		c)		d)

(b)

Let particle with mass , move with velocity u,and and be velocity after collision. Since , elastic collision is one in which the momentum is conserved , we have

               …..(i)

In perpendicular direction

             …..(ii)

Also elastic collision occurs only if there is no conversion of kinetic energy into other from, Hence

                             ….(iii)

Squaring Esq.(i)and (ii)and adding we get

Using Eq.(iii),we get

Hence    

Or              

When two identical particles collide elastically and obliquely,

One being at rest, then they fly off in mutually perpendicular directions.