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# If you want to pile up sand onto a circular area of radius R, the greatest height of the pile to be erected without spilling sand on to the surrounding area is ($\mu$ - coefficient of friction between sand particles)

$\begin{array}{1 1} R \\ \mu R \\ \mu^2 R \\ \frac{R}{\mu}\end{array}$

When the inclination of slant side of the heap reaches the angle of friction sand will begin to slide down.
For maximum height $\mu=\tan \alpha$
$\mu=\large\frac{h}{R}$
$h=\mu R$
Hence b is the correct answer.

edited Jan 26, 2014 by meena.p