Free fall: the motion of a bouncing ball

Measurements

Introduction

The motion of a bouncing ball can be studied in detail with an ultrasound motion detector. Before these little marvels were available, no one - absolutely no one - had see the graphs that follow.

Procedure

A ball was held under a Vernire motion detector (BAT) as shown. The ball was pumped up very tightly so that it bounced many times and spent very little time on the floor between bounces. The data rate was set at 40 points per second and the averaging for veolcity calculation was reduced to 3 points. The motion detector was held as still as possible. The ball was released suddenly, taking care to take both hands out of the data zone as quickly as possible.

Several attempts were required to get a good set of distance-time data points because the hard ball tended to bounce out of the data zone.

Data

The position-time plot (with upwards defined as positive).

Each section of the curve is a parabola.

The velocity time graph is a series of straight lines with the same slope. The acceleration is constant when the ball is clear of the floor. Fitting straight lines to the segments of the velocity graph gives the acceleration due to gravity as....

Table 1

The acceleration, when the ball is clear of the floor, is constant and close to -10 m/s/s when the ball is clear of the floor.

From Table 1....

Note: the measured value of g is a little less than the well known - 9.80 m/s/s due to air resistance, when using a large inflated rubber ball. Dropping a large steel ball-bearing puts dents in the floor, but returns a value of g equal to -9.80 m/s/s, within errors.

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Index