Question

Which of the following statement is true for a system of two balls moving with equal speeds in opposite directions ?

• A. Both momentum and kinetic energy are zero.
• B. Momentum is not zero, kinetic energy is zero.
• C. Momentum is zero, kinetic energy is not zero.
• D. Both momentum and kinetic energy are not zero.

Answer 1

The correct option is C Momentum is zero, kinetic energy is not zero.

Assume velocity of first ball is $v$ then velocity of second ball is $-v$, as both are moving in opposite directions.

Momentum of the system is given as $P=mv+(-mv)=0$
And total Kinetic energy is given as $=2\times \frac{1}{2}m{v}^2=m{v}^2$