To illustrate linear kinematics in a possibly counterintuitive way.
Track assembly with two-ball launcher, as photographed.
Ask the class to vote before doing the experiment.
When the two balls are launched from one end of the track with the same velocities, what will happen: (a) the ball on the straight track arrives at the other end first, (b) the ball on the track with the dip arrives at the other end first, or © the race is a tie - both balls reach the other end at the same time? ◙
(PIRA 1D15.20) See the Demonstration Reference File for several very interesting papers on this apparatus and variants thereof.◙