An iron ball is released from rest from a height of 125 m and falls freely. Taking $g = 10$ m/s$^2$, the time taken to reach the ground is:

1. For free fall use $s = ut + \dfrac{1}{2}gt^2$ with $u = 0$. 2. $125 = 0 + \dfrac{1}{2}\times10\times t^2 = 5t^2$. 3. So $t^2 = \dfrac{125}{5} = 25$. 4. $t = \sqrt{25} = 5$ s; the value 25 s mistakes $t^2$ for $t$ and is the trap. _Source: Balbharati (Maharashtra Board) Class 10 Science & Technology, Ch 1 "Gravitation", p.21_