Problem $2$

A retaining wall against a mud slide is constructed by placing $1\mathrm{.}2-m-$high, $1-$

m long and $0\mathrm{.}25-m-$wide rectangular concrete blocks $(=2700k\frac{g}{m^3})$ side

by side. The friction coefficient between the ground and concrete blocks is

$0\mathrm{.}4,$ and the density of the mud is $1400k\frac{g}{m^3}.$ What is the mud height for

which the blocks will overcome friction.