Happy Sad Ball Background Reading

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©2025 Arbor Scientific All Rights Reserved
Happy Unhappy Balls
P6-1000
INSTRUCTIONAL GUIDE
• Happy ball
• Sad ball
• Instructional Guide
Recommended for activities:
• Inclined Plane (P3-3541)
Happy and Unhappy Balls sometimes known as Happy Sad Balls, are a pair of black spheres which
appear to be similar. The “unhappy” ball is formed from a proprietary rubber compound developed and
manufactured under the trade name “Norsorex,” while the “happy” ball is made of conventional
neoprene rubber. Although the two balls appear to be quite similar, their physical properties are
markedly different.
Low v. High Hysteresis: Hysteresis is a measure of the rate of restitution of the natural tendency of
rubber to return to its original shape after deformation. This rate of restitution is caused by internal
frictional forces resulting from the molecular structure of the rubber. The dead or “unhappy” ball exhibits
the greatest hysteresis.
Rate of Restitution: The ball with low hysteresis (the “happy” ball) exhibits a more rapid return to its
original shape, resulting in its greater bounce (it has a high coefficient of restitution).
Paradoxically, as the
balls are cooled below
room temperature, the
bounce of the “happy”
ball is diminished
somewhat while that of
the dead ball increases.
Figure 1 shows the
changes in the energy
absorption rate of
Norsorex with changes in
temperature.
Coefficient of Friction: The molecular structure of the two types of rubber is also responsible for
discrepant qualities in the surface friction of the balls. The “happy” ball exhibits lower surface friction
and rolls more rapidly than the dead ball.
Background
Contents

(800) 367-6695 | www.arborsci.com
©2025 Arbor Scientific All Rights Reserved
Challenge your students to perform the following experiments and come up with logical explanations for
the observed events.
1. A Rolling Ball Gathers Momentum: Set up an inclined plane at least one meter in length
and roll both balls simultaneously from the same starting point. Note which ball reaches the
end of the incline first. Does it do so consistently? What clues does this provide about the
friction of the balls relative to the surface and in comparison, with each other? The
“unhappy” ball, of course, because of its higher coefficient of friction, rolls more slowly. It is
this friction which makes rubber of this type very desirable for racing tires where road
adhesion is required at high speed. Too much friction, however, will cause heat buildup and
excessive tire wear, so this property must be balanced by blending high-friction rubber with
a more firmly vulcanized rubber.
2. Sphere of Flying: Drop both balls onto a hard surface from a fixed height and determine
the height of the first bounce (if any). Place both balls in a container of ice or a freezer for
about twenty minutes and repeat your measurements. What differences are observed? Why
does the dead ball bounce? Would you think that the dead property of the rubber is
temperature dependent? Why?
3. The Chiller Instinct: Determine the rate at which each ball returns to shape after being
compressed by a vise or heavy tongs. Chill the balls first as before, then repeat. Are there
differences in the rate at which the balls return to shape?
Restitution is the “desire” of a substance to return to its original shape, almost a molecular memory of
its original form. The dead ball has a very low coefficient of restitution, the other ball a high coefficient;
each property can have practical benefits. In running shoes, for example, the superior ability of a “dead”
rubber to absorb shocks helps to alleviate the tremendous pounding to the foot, leg, and ankle. A rubber
with a high coefficient of restitution – that is, one with a lot of bounce – would be ideal for handballs or
other applications. Ask your students to think for practical uses for both types of rubber.
Chemical Formulation: Figure 2
outlines the primary steps in the
formation of Norsorex. First, ethylene
cyclopentadiene is converted to the
monomer norbornene via a Diels-Alder
reaction, then the monomer is
converted polymerized by a process
which opens the norbornene ring,
creating alternating bonds between the
five-member ring and the newly-exposed double bonds. This polymerization process means that
vulcanization can be done utilizing the double bond.
Astro Blaster (P1-5000) The Astro Blaster lets you dramatically demonstrate the Law of Conservation
of Momentum to almost any age level. Drop it on a hard surface, then stand back as the top ball
bounces to heights up to 5 times the original drop!
Dropper Popper (P6-6075) The ultimate "Super Ball" is really only half a ball! Turn this popper inside
out and drop it.
Experiments
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