If a car accelerates from $20\text{ m/s}$ to $40\text{ m/s}$, its speed doubles, but the destructive energy it carries upon impact quadruples.

The kinetic energy of a body of mass $m$ moving with velocity $v$ is given by the formula $KE = \frac{1}{2}mv^2$. If the velocity is doubled ($v' = 2v$), the new kinetic energy becomes $KE' = \frac{1}{2}m(2v)^2 = 4 \times (\frac{1}{2}mv^2) = 4 \times KE$.

Always distinguish between momentum ($p \propto v$) which scales linearly, and kinetic energy ($KE \propto v^2$) which scales quadratically with velocity in competitive exams.