A box of weight w is accelerated up a ramp by a rope that exerts a tension T. The ramp makes an angle a with the horizontal, and the rope makes an angle 6 above the ramp. The coefficient of kinetic friction between the box and the ramp is μk' Show that no matter what the value of a, the acceleration is maximum if θ = arctan μk (as long as the box remains in contact with the ramp).