Recently I went to an indoor climbing facility downtown, called Rock Ventures. I love rock climbing, there's a thrill in all that built up gravitational potential energy. However, it is completely safe thanks to our friend the pulley. There are multiple types of rock climbing, indoor climbing, free climbing, and solo climbing. Solo climbing has no ropes involved whatsoever, just a person on a cliff. Free climbers use ropes, but only to prevent from injury in segments, not to assist in the climb. Indoor or sport climbing is what I was doing, this type involves a pulley system. The interesting part of this system, not really having to do with pulleys, is the ATC that is used when belaying (yes it stands for Air Traffic Controller, you'll soon see why)
The ATC is used by the belayer, a person on the ground spotting the climber, to control their ascent and descent and how much slack is in the rope. The ATC is a device which the rope goes through to be manipulated by the belayer. There is a lead rope and a slack rope, when they are parallel, the rope is free to slide - in this position the belayer can pull excess rope through as the climber ascends, getting rid of slack makes the climb safer in case the climber would fall. When the slack part of the rope is pulled down towards the ground, there is a frictional force between the ATC and the rope, which under even a large amount of tension, would not allow the rope to slide.
The ATC is a very simple, yet genius device that makes it possible to safely climb for fun, and for that, I thank physics