Workshop Plans: Rolling Balls

Have you noticed the theme going?  I'm developing plans for a teacher's workshop at a small Catholic school in about three weeks.  I'm preparing several inquiry-based labs to present and try.  In the past few posts, there have been instructions for a DNA extraction (Biology or Life Science), Alka-Seltzer labs (Chemistry or Physical Science),  and Clouds in a Bottle (Earth Science).  This time it's Physics or Physical Science: Rolling balls.  This is trickier than it appears.  You should change the angle without changing the potential energy.  This lab from Harvard demonstrates just how profound an experiment with rolling balls is, Inclined Planes and Rolling Balls.  Here is an explanation for acceleration as the ball rolls down the ramp.