A heavy crate of weight 15 N lies on a rough slope inclined at an angle of 30∘ 30^\circ\,30∘ to the horizontal. An external force of 18 newtons acts on the crate parallel to and up the slope, such that the crate is in limiting equilibrium and on the point of moving up the slope.
Find the coefficient of friction between the crate and the slope.
The 18 N force is removed. Determine whether or not the crate moves.