A block of mass m rests on a horizontal surface (figure). The block is pulled by a massless rope with a force at an angle θ. The coefficient of static friction is 0.6. The minimum value of the force needed to move the block depends on the angle θ. 

(a) Discuss qualitatively how you would expect this force to depend on θ. 

(b) Compute the force for the angles θ = 0°, 10°, 20°, 30°, 40°, 50°, and 60°, and make a plot of F versus θ for mg = 400 N. From your plot, at what angle is it most efficient to apply the force to move the block?