Question

Two identical ball bearings in contact with each other and resting on a frictionless table are hit head-on by another ball bearing of the same mass moving initially with a speed V. If the collision is elastic, which of the following figure is a possible result after collision?

Answer 1

Case (ii)

It can be observed that the total momentum before and after collision in each case is constant.

For an elastic collision, the total kinetic energy of a system remains conserved before and after collision. 

For mass of each ball bearing m, we can write: 

Total kinetic energy of the system before collision:

Case (i)

Total kinetic energy of the system after collision:

Hence, the kinetic energy of the system is not conserved in case (i).

Case (ii)

Total kinetic energy of the system after collision:

Hence, the kinetic energy of the system is conserved in case (ii).

Case (iii) 

Total kinetic energy of the system after collision: 

Hence, the kinetic energy of the system is not conserved in case (iii).