8. a boy pulls a 20.0-kg box with a 140-n force at 30° above a horizontal surface. if the coefficient of kinetic friction between the box and the horizontal surface is 0.25 and the box is pulled a distance of 28.0 m, what is the net work done on the box?

With the simplified model of the eye, what corrective lens (specified by focal length as measured in air) would be needed to enable a person underwater to focus an infinitely distant object? (be careful-the focal length of a lens underwater is not the same as in air! assume that the corrective lens has a refractive index of 1.62 and that the lens is used in eyeglasses, not goggles, so there is water on both sides of the lens. assume that the eyeglasses are 1.90