1. a box of mass 12 kg slides at a speed of 10 m/s acros a smooth level floor, where it enters a rough portion 3.0 m in length. In the rough portion, the box experiences a horizontal friction force of 72N. a) how much work is done by the frictional force? b) what is the velocity of the box when it leaves the rough surface? c) what length of rough surface brings the box coompletely to rest? I dont know how to do it!!