Welcome to the latest Demo Highlight of the week! This week, we’re taking a look at a popular demonstration used to introduce the concept of energy: C8-04, the Hill Track. Dave Buehrle introduces it in the video below.

The ball starts out with a certain amount of gravitational potential energy based on its height above the base. As it rolls down the track, it converts this potential energy into kinetic energy. This includes both the kinetic energy related to its linear motion along the track, and the rotational kinetic energy of the ball spinning as it moves. So if the ball is released on the high end of the track from a height exactly equal to the height of the hill in the middle, it doesn’t quite make it over the hill, it doesn’t quite reach that same height. Some energy has been lost to friction, but importantly, some energy is still in the form of kinetic energy as the ball is still rotating. Thus, for the ball to get over the hill, it has to start out slightly higher than the hill to compensate for this.

Hill Track, viewed end-on

The PhET Collection at the University of Colorado has a simulation related to this demonstration. The Energy Skate Park simulator ( is a simpler (non-rotating) system that lets you experiment with a simulated skateboarder on a variety of tracks, including one with a hill in the middle. Try it out at home, and see how the initial position affects how your skater moves!

This demonstration, though simple, is also used in advanced classes. You can see it cross-listed as demonstration P2-41, and it is often used in the teaching of quantum mechanics to illustrate the concept of potential wells.