The physics of zip-lining are very important because it is an activity that can potentially be very dangerous. When zip-lining, the line must be somewhat angled down so that the person can actually zip down the line. There is friction when pulley glides along the wire. The rider accelerates when the force of gravity pushes them down the line. The extra friction that occurs when the rider pulls down on the line to break results in a decreasing velocity and a negative acceleration. The next time you ride a zip line be sure to think of all the physics involved.