PURPOSE: To illustrate linear kinematics in a possibly counterintuitive way.
DESCRIPTION: 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 (c) the race is a tie - both balls reach the other end at the same time?
SUGGESTIONS: Ask the class to vote before doing the experiment.
Note that there are only two conditions required for this result:
(1) no friction (very nearly the case), and
(2) the slope of the track cannot exceed that of a rolling brachistochrone (another way of saying that the ball must remain in contact with the track at all times and cannot have any loop-the-loops).
REFERENCES: (PIRA 1D15.20) See the Demonstration Reference File for several very interesting papers on this apparatus and variants thereof.
EQUIPMENT: Track assembly with two-ball launcher, as photographed.
SETUP TIME: None.
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