$(a)$ Define kinetic energy.
$(b)$ The masses of a scooter and a bike are in the ratio of $2:3$,but they are moving with the same speed of $108 \, km/h$. Compute the ratio of their kinetic energy.

(2:3) Kinetic energy is the energy possessed by an object due to its motion.
$(b)$ Given the ratio of masses $\frac{m_1}{m_2} = \frac{2}{3}$ and the velocity $v_1 = v_2 = 108 \, km/h$.
The formula for kinetic energy is $KE = \frac{1}{2}mv^2$.
Since the velocity is the same for both,the ratio of kinetic energies is $\frac{KE_1}{KE_2} = \frac{\frac{1}{2}m_1v^2}{\frac{1}{2}m_2v^2} = \frac{m_1}{m_2}$.
Therefore,the ratio of their kinetic energy is $2:3$.