An 80 kg biker (including the bike) rolls , starting from rest, down a slope from an initial elevation of 20 m. As he does so, rolling friction exerts a constant force of 125 N opposite the direction of motion. (This is unrealistic- but go with it). The path the biker takes has a total length of 40 m. What is the biker's speed at bottom? Show all your work and explain your reasoning