A ball falls off a table and reaches the ground in 1 s. Taking $g = 10$ m/s$^2$, its speed on reaching the ground and the height of the table are:

1. Starting from rest, final speed $v = u + gt = 0 + 10 \times 1 = 10$ m/s. 2. Height fallen $s = ut + \dfrac{1}{2}gt^2 = 0 + \dfrac{1}{2}\times10\times(1)^2$. 3. $s = \dfrac{1}{2}\times10\times1 = 5$ m. 4. So speed is 10 m/s and the table height is 5 m; using 10 m for height forgets the factor $\tfrac12$ and is the trap. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 1 "Gravitation", p.24_