What is The joule?

Example 66.3 (First computations)

A $0.45\,\mathrm{kg}$ football at $20\,\mathrm{m}/\mathrm{s}$: $E_k = 0.5 \times 0.45 \times 20^2 = 90\,\mathrm{J}$. A $60\,\mathrm{kg}$ sprinter at $10\,\mathrm{m}/\mathrm{s}$: $0.5 \times 60 \times 100 = 3000\,\mathrm{J}$. A $1000\,\mathrm{kg}$ car at $50\,\mathrm{km}/\mathrm{h}$ — convert first: $50 \div 3.6 \approx 13.9\,\mathrm{m}/\mathrm{s}$ — $E_k = 0.5 \times 1000 \times 13.9^2 \approx 9.7 \times 10^{4}\,\mathrm{J}$: nearly a hundred thousand joules. The same car at $100\,\mathrm{km}/\mathrm{h}$: four times as much — $3.9 \times 10^{5}\,\mathrm{J}$. The formula’s warning, in numbers.