7. A 20.0 kg box slides down a 4.0 m high frictionless hill, starting from rest. At the bottom of the hill it moves across a long horizontal surface. The horizontal surface is rough, with a coefficient of kinetic friction of 0.53. Eventually the box comes to rest. a. What is the kinetic energy of the box when it reaches the bottom of the hill? b. What is it's velocity at this point? c. How much work is done by friction on the box as it slides across the rough surface? d. How far does the box slide on the rough surface