Porter Johnson - Illinois Institute of Technology
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Porter Johnson Illinois Institute of Technology
Biol Chem Phys Sci Dept
CHICAGO IL 60616-3793
This lesson is suitable for an 8th grade student. To study the motion of
superballs in the air, colliding with and bouncing off smooth surfaces. Basic
features of moving and colliding objects can be demonstrated and studied using
this familiar and fascinating toy.
A supply of superballs of various sizes and colors for use by students in
teams of two. A supply of meter sticks [with some two meter sticks] is needed
for each set-up.
For the initial demonstration of bouncing you will need a supply of various
types of balls, some of which bounce very well, some poorly, and some not at
all. In particular, you should get a happy ball sad ball set [available
at toy stores everywhere].
Begin by bouncing superballs and showing that their bounce is quite "springy" or
"elastic". Show that the smaller superballs bounce just as well as the big
ones. Show that superballs bounce better than tennis balls, ping-pong balls,
and golf balls.
Show the "happy and sad balls", and ask the group to predict how they will
bounce. In particular, have the class discover that the "sad ball" feels a
lot like a superball, so that it might bounce rather well. Unfortunately, it
does not bounce at all, even though the result is not obvious from handling
Practice dropping the ball in front of the class, showing how to release the
ball from rest, measuring the height of the bottom of the ball above the
floor. Also, show how to measure the bounce height, again defined as the
maximum height of the bottom of the ball above the floor on first bounce.
Divide the class into groups of two, and have them each drop a superball from
a height of 100 cm [one meter], and record the bounce heights on the board.
Here is a set of typical bounce heights [in cm], arranged in increasing order:
72 76 78 79 80 80 80 81 82 83 85
Note that there are eleven independent measurements on the list, and that the
median height is 80 cm, with seven of the eleven measurements lying between
77 and 83 cm. Thus, an "eye-ball" estimate of the measured bounce heights is
(80 03) cm.
<-------------- RANGE -------------->
72 76 78 79 80 80 80 81 82 83 85
If your students are sufficiently advanced or "calculator literate", you should
show them how to compute the mean and standard deviation of these numbers;
The elasticity coefficient r, which we define as the ratio of the bounce height
the initial height, is roughly independent of bounce height, as the class can
demonstrate by studying the dropping the ball from initial heights of 50 cm
and 200 cm.
Have the class drop the ball from an initial height of 100 cm and measure
the maximum heights for second bounce. The data [in cm] may look something like
56 58 59 60 63 64 64 66 68 69 72
By "eyeballing" the data, we estimate the measured heights on second bounce to
be about (64 6) cm. Note that, for two
bounces, the ratio of
bounce height to initial height is about r x r = r2. Correspondingly, for three
bounces the ratio would be about r x r x r = r3. One can count 10 - 20
independent bounces for the ball, each of lesser amplitude, before the ball
seems to stop on the floor.
One can summarize by saying that on each bounce, the ball returns to r = 0.80
of its initial height, corresponding to the fact that the fraction 1. - r = 0.20
of the initial energy is dissipated upon collision with the floor.
Impress upon the class that, because the ball loses some mechanical energy
after each bounce, it can never, never bounce higher than the initial release
height, and get them to agree with this basic consequence of energy
conservation. Having thoroughly convinced them of this point, take out a
smaller super ball and put it into a small indentation in the bigger ball.
Hold the bigger ball in your hand, with the smaller one sitting on top of it,
and carefully drop it. Repeat the exercise several times, and observe that,
under optimal conditions, the smaller ball goes several times higher than its
initial drop height. The smaller ball is, in effect, drawing energy from the
larger ball, so that is can go much higher than otherwise. [The Jupiter
slingshot maneuver, in which a satellite can increase its speed by doing a
"hairpin" loop around a major planet, which is used to extend the range of
inter-planetary rockets, operates because of similar principles.
A superball can be made to bounce several times near its initial location,
but it will eventually bounce away because of mis-alignments, imperfections,
etc. Can you make a superball bounce back and forth about a given location.
This is a skill which each class member can acquire, simply by bouncing the
ball on the floor, trying various schemes, and the like. The trick is to
release the ball with an initial horizontal velocity and spin. By picking the
right initial conditions, the ball can be made to bounce back and forth.
The vigorous bouncing of the superball is an useful vehicle for illustrating
and studying the basic concepts of energy conservation, energy transfer, and
dissipation of mechanical energy as heat.
In various types of sponsored gambling, whether legal or illegal, the "house"
gets to keep a certain percentage p of the total amount of the wagers. In
other words, the total amount paid to the winners is the fraction r = 1. - p,
multiplied by the total amount of the wagers. The fraction paid back after
two bets [just like two bounces of the ball] is r2, after three bets it is r3,
etc. Eventually, or course, the "house" ends up with all the money, for the
same reason that the balls stop bouncing. Urge your students to keep these
points in mind when they think of placing bets. If they persist in
gambling, they might wish to learn about the super ball lottery:
A web-based reference on bouncing baseballs is given at the following
This site, developed by the Exploratorium [an excellent interactive science
museum in San Francisco], outlines experiments with baseballs, tennis balls,
and golf balls, and in particular the temperature dependence of the bounce.