A box can be moved up an inclined plane with constant velocity by a force of magnitude $F_1$ or down the inclined plane with constant velocity by a force of magnitude $F_2$ . Find the coefficient of kinetic friction $\mu_k$ between the box and the inclined plane. It holds that $F_1=6F_2$ and both forces are parallel with the inclined plane. The angle $\alpha$ between the inclined plane and the horizontal plane is $15^{\circ}$
