A sled with mass 8.00 kilogram moves in a straight line on a frictionless horizontal surface. At one point in its path, its speed is 4.00 m/s; after it has traveled 2.5 meter beyond this point, its speed is 6.00 m/s. Use the work-energy theorem to determine the force acting on the sled, assuming that this force is constant and that it acts in the direction of the sled's motion.