(i)[2]
Find an inequality that gives the coefficient of friction between the box and the plane.
(ii)[5]
The coefficient of friction between the box and the plane is in fact $0.19$. A girl applies a force of $50\,\text{N}$ to the box, acting down the line of greatest slope of the plane, for $5\,\text{m}$. She then stops pushing. Use an energy method to determine the speed of the box after it has moved another $5\,\text{m}$.
(iii)[2]
The box then moves onto a plane inclined at $20^\circ$ below the horizontal. It travels down a line of greatest slope of this plane. The coefficient of friction remains $0.19$, and the girl is no longer pushing the box. Find the acceleration of the box.