Q.A stone is tied to a string and whirled such that the stone performs horizontal circular motion. It can be seen that the string is NEVER horizontal.

A conical pendulum is a mass (the bob) suspended by a string from a fixed support point, made to revolve so that the bob traces a horizontal circle at constant speed while the string itself sweeps out the surface of a cone of constant half-angle θ measured from the vertical. Only two forces act on the bob: its weight $mg$ (vertically downward) and the string tension $T$ (along the string, toward the support). Resolving the tension into vertical and horizontal components: the vertical component $T\cos\theta$ must balance gravity exactly ($T\cos\theta=mg$, since the bob stays at constant height), and the horizontal component $T\sin\theta$ supplies the centripetal force needed for the circular motion ($T\sin\theta=\dfrac{mv^2}{r}$).

Combining these two conditions with the geometric relation $r=l\sin\theta$ (from a string of length l) and $v=\dfrac{2\pi r}{T_{period}}$ gives the pendulum's period of revolution, $T_{period}=2\pi\sqrt{\dfrac{l\cos\theta}{g}}$ — equivalently $2\pi\sqrt{h/g}$, where $h=l\cos\theta$ is the vertical drop from the support point to the plane of the circle, the same functional form as a simple pendulum's period with length h. Unlike a simple pendulum, which swings back and forth in a single vertical plane, the conical pendulum's steady, unidirectional horizontal-circle motion makes it a clean physical example of uniform circular motion driven by a resolved component of tension.