A tennis ball thrown vertically upward reaches a maximum height of 4.05 m. Taking $g = 10$ m/s$^2$, its initial velocity was:

1. At the highest point the final velocity $v = 0$ and $a = -g = -10$ m/s$^2$. 2. Use $v^2 = u^2 + 2as$: $0 = u^2 + 2(-10)(4.05)$. 3. So $u^2 = 2 \times 10 \times 4.05 = 81$. 4. $u = \sqrt{81} = 9$ m/s; 81 m/s mistakes $u^2$ for $u$ and is the trap. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 1 "Gravitation", p.21_