(d) As is clear from figure
[image:4_image125.emf]
$T\sin\theta = \frac{mv^{2}}{r},\ T\cos\theta = mg$
Dividing, we get
$\tan\theta = \frac{v^{2}}{rg} = \frac{r}{g}\left( \frac{2\pi}{T} \right)^{2}$
$\frac{2\pi}{T} = \sqrt{\frac{g\tan\theta}{r}} = \sqrt{\frac{g\tan\theta}{l\sin\theta}} = \sqrt{\frac{g}{l\cos\theta}}$
or $T = 2\pi\sqrt{\frac{l\cos\theta}{g}}$