Paper detail

Frictionless Racquetball?

When a ball hits a surface, does the angle of reflection always equal the angle of incidence? Not at all! Depending on the interplay of the ball's spin and speed and the force of friction, the ball's behavior after the bounce may differ dramatically. Surely some experienced racquetball, ping-pong, pool, tennis and players alike take advantage of this fact. A physics teacher, in turn, can take advantage of the fact that by observing the bounce of a ball her students can determine the coefficient of friction between the ball and the floor. All it takes is a typical video (smartphone) camera and some standard software However, this seemingly straightforward method may lead to surprises. If one wants to double check the value of the coefficient of friction by observing the second, third etc. bounces, one may find the the force of friction "does not turn on". We investigate why this happens, state a hypothesis and run some testing experiments. Teachers who would like to include an illustration of the scientific cycle at work in their classes may find this paper useful.