To demonstrate conservation of energy in a simple system.
Galileo's pendulum, as photographed.
The pendulum is hung from the upper peg with the lower peg interrupting its swing to the right. When started from the left at a given height, the pendulum rises to that same height on the right, after being stopped by the lower peg.
Herbert T. Wood, The Interrupted Pendulum, TPT 32, 422-423 (1994).