A sled of mass m starts from rest at the top of a snowy hill. A qualitative energy bar chart uses a scale of one square per joule. At the top, the gravitational potential-energy bar is 9 squares tall and the kinetic-energy bar is 0 squares tall. Halfway down, the kinetic-energy bar is 4 squares tall and the gravitational potential-energy bar is 3 squares tall. Based on these bars, has the work done by friction between the two positions been positive, negative, or zero? Briefly justify your answer in one sentence.