What happens when a perfectly elastic ball collides with a completely inelastic ball? It is shown that the outcome depends on the stiffness of each ball. A standard textbook problem in mechanics is to calculate the outcome of a head-on collision between two balls using conservation of momentum and kinetic energy. It is easily shown that the outcome depends on the relative mass of the balls and their initial speeds. In practice, there is always a net loss of kinetic energy during a collision, which can be expressed in terms of the coefficient of restitution (COR) for the collision. The COR is defined as the relative speed of the two balls after the collision divided by the relative speed before the collision. In a perfectly elastic collision the COR is unity. In a completely inelastic collision, the COR is zero.

1.
K.
Sullivan
, “
What’s in a name: Why do we call a bouncy ball bouncy?
Phys. Teach.
57
,
229
231
(
April
2019
).
2.
E.
Etkina
,
D.
Gitomer
,
C.
Iaconangelo
,
G.
Phelps
,
L.
Seeley
, and
S.
Vokos
, “
Design of an assessment to probe teachers’ content knowledge for teaching: An example from energy in high school physics
,”
Phys. Rev. Phys. Educ. Res.
14
,
010127
(
2018
).
3.
Available at www.arborsci.com or as larger Choositz Decision Balls from Educational Innovations, USA.
4.
R.
Cross
, “
The coefficient of restitution for collisions of happy balls, unhappy balls, and tennis balls
,”
Am. J. Phys.
68
,
1025
1031
(
Nov.
2000
).
5.
Readers may view the video at TPT Online, http://dx.doi.org/10.1119/10.0009101 under the Supplemental tab.
6.
R.
Cross
, “
The bounce of a ball
,”
Am. J. Phys.
67
,
222
227
(
March
1999
).
7.
A.
Nathan
Characterizing the performance of baseball bats
,”
Am. J. Phys.
71
,
134
143
(
March
2003
).