You tie a rope to a box of mass m = 8.0 kg and pull on it with a constant force T = 50 N with this force oriented at an angle 0 = 40 above the horizontal. The block starts at rest, and there is friction between the block and the ground, but the coefficient of friction is unknown. After pulling it a distance d = 6.0 m, you observe its speed to be v = 5.8 m/s. What is the magnitude of the frictional force exerted on the block over this distance?