Explanation

(b) When a body is revolving in circular motion it is acted upon by a centripetal force directed towards the center. Water will not fall if weight is balanced by centripetal force. Therefore
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$mg = \frac{{mv}^{2}}{r}$
$\Longrightarrow \ v^{2} = rg\ \ \ldots(i)$
Circumference of a circle is $2\pi r.$
$Time\ of\ revoluation = \frac{2\pi r}{v}$
Putting the value of $v$ from Eq. (i), we get
$T = \frac{2\pi r}{\sqrt{gr}} = 2\pi\sqrt{\frac{r}{g}}$
$Given,\ r = 4\ m,\ g = 9.8\ \frac{m}{s}^{2}$
$\therefore T = 2\pi\sqrt{\frac{4}{9.8}}$
$\Longrightarrow T = \frac{4\pi}{\sqrt{9.8}} = 4s$