A box of mass 6 kg lies on a rough plane inclined at an angle of 30 to the horizontal. The box is held in equilibrium by means of a horizontal force of magnitude P newtons, as shown in Fig. 2. The line of action of the force is in the same vertical plane as a line of greatest slope of the plane. The coefficient of friction between the box and the plane is 0.4. The box is modelled as a particle.
Given that the box is in limiting equilibrium and on the point of moving up the plane,
find,
(a) the normal reaction exerted on the box by the plane,
(b) the value of P.
The horizontal force is removed.
(c) Show that the box will now start to move down the plane.
Christ sake.
I had trouble with this, the mark scheme just doesnt make sense.