Zip wire
As one of my CAS activities (at UWC's Teachers do them too) I supervise a group of students who instruct groups of young children in rock climbing. When the weather is really bad we can't climb up rocks so we slide down them instead. To be more precise we slide down a rope like the one in the picture but with rather more attention to safety. Yesterday was one such rainy day and one of the instructors was a HL physics student so the conversation naturally turned to physics. Why is the slide so much faster when its wet? When does the kid reach maximum velocity? If the rope snapped would the kid be catapulted into the fjord? With so many questions this seemed like a good topic for a design exercise. All you need is a rope slide with adjustable tension.length and height and an assortment of at least 5 different sized children.