A zip line is a very simple mechanism for getting quickly from Point A to Point B. It's a practical tool for the military, and it's a fun experience for kids. A significant slope is the first necessary ingredient for a working zip line. That's because it provides the automatic gravitational pull on the weight of the person using the zip line, thus causing them to slide downward toward the other end (Point B). A slope could begin on the roof of a building and end at an adjacent building, several floors down. It could begin on a tree at the top of a hill and end on a tree at the bottom of a hill. The slope does not have to be extremely steep, but the steeper it is the faster the zip line will work. This makes it quicker and a lot more fun.
Rope is the actual "line" portion of the zip line, and it is attached to a point at the upper end of the slope, stretched tight and attached to a point at the lower end of the slope. If the slope provides the power (gravitational force), then the rope provides the means. The person using the zip line literally zips down the line, or rope, until he or she reaches the other end.
A slope is the power. A rope is the means. A pulley is the mechanism. A pulley used in a zip line is usually covered with a heavy-duty plastic case and attached to a set of handles. You have to have something to hold to grab. The pulley is placed on the rope before it is installed on the two points of the slope. The pulley itself is on top of the rope. The handles are beneath the rope. When the person at the top of the slope grabs the handles and launches him or herself off the ground, the gravitational force acts on the weight of the body, pulling it downward on the slope. This causes the pulley to turn on the rope. The pulley turns, moving the handles down the rope. The handles are holding the person.