__Question 6.16__:*V*. If the collision is elastic, which of the following figure is a possible result after collision?

__Solution__:**Answer:**Case (ii)

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:

= (1/2)mV

^{2}+ (1/2)(2m) X 0

^{2}

= (1/2)mV

^{2}

^{}

**Case**

**(i)**Total kinetic energy of the system after collision:

= (1/2) m X 0 + (1/2) (2m) (V/2)

^{2}

= (1/4)mV

^{2}

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

**Case**

**(ii)**Total kinetic energy of the system after collision:

= (1/2)(2m) X 0 + (1/2)mV

^{2}

= (1/2) mV

^{2}

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

**Case**

**(iii)**Total kinetic energy of the system after collision:

= (1/2)(3m)(V/3)

^{2}

= (1/6)mV

^{2}

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

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