Something i have always wanted to do is go on a zip-line in a forest or down a hill. However, what i did not think about are the many applications of physics in zip lining. The most obvious the use of gravity to propel you down the line. Gravity will act down on you at the center of you mass and accelerate you in a wild ride of fun. The zip-line actually works by putting a contraption on the line that has multiple wheels in it. The line is notched in the wheels and a low amount of friction allows the wheels to spin very fast while traveling down the wire (pulled by the force of gravity). At he end of the zip-line the most common way to stop is using a spring. The spring will be coiled around the wire so that when you hit it, it cushions your impact. The more you compress the spring the harder it will push back on you which insures that you will not hit the wall/tree/post it is attached to.