Q: A smooth sphere A is moving on a frictionless horizontal plane with angular velocity ω and centre of mass velocity v. It collides elastically and head on with an identical sphere B at rest. Neglect friction everywhere. After the collision their angular speeds are ω_{A} and ω_{B} respectively. Then,

(a)ω_{A} < ω_{B}

(b)ω_{A} = ω_{B}

(c) ω_{A} = ω

(d)ω_{B} = ω

Ans: (c)

Sol: Since , there is head on elastic collision between two identical spheres , hence they will exchange their velocities i.e. A comes to rest and B starts moving with linear velocity v . As there is no friction anywhere , torque on both the sphere about their centre of mass is zero & their angular velocities remains constant .

ω_{A} = ω & ω_{B} = 0