Consider a small bucket full of water tied to a string whirled around in vertical circle of radius r without water falling down. At the topmost position when the speed of the inverted bucket is v,
(A) $\frac{{{v^2}}}{r} = g$ necessarily
(B) $\frac{{{v^2}}}{r} < g$ necessarily
(C) $\frac{{{v^2}}}{r} > g$ necessarily
(D) None of the options given
Solution
Consider water as the system. Let, m be the mass of water. The weight mg acts downwards and the normal reaction N also acts downwards.
$mg + N = \frac{{m{v^2}}}{r}$
$ \Rightarrow \frac{{m{v^2}}}{r} \ge mg$
$ \Rightarrow \frac{{{v^2}}}{r} \ge g$
Hence, Option (D).
