A particle of mass m rests on a rough inclined surface at an angle �Pi/6

Question

to the horizontal. When a force of magnitude P is applied parallel to the
surface pointing up the slope, the particle is on the point of slipping down the
slope, whereas when a force of magnitude 2P is applied parallel to the surface
pointing up the slope, the particle is on the point of slipping up the slope.
Show that the coe�cient of friction is
sqrt(3/9)?