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Porter W. Johnson Illinois Institute of Technology
Chicago IL 60616-3793
The student will make careful observations of a ball bouncing off a hard surface
one time or sequentially for several trials, to study how the ball travels as it
bounces across the room. From these observations, he/she will then be able to
describe and explain the motion of the ball.
1. A box containing several small super balls, medium-sized super balls,
hollow rubber balls, solid rubber balls, tennis balls, golf balls,
baseballs, and whatever other types of balls are available.
2. Several meter sticks for measuring the height of the bouncing ball.
3. Several smooth hard flat horizontal surfaces suitable for bouncing
balls---floors, lab tables, sidewalks, and the like.
The initial phase of the activity involves making careful, qualitative
observations of a single bounce. Next, the students in teams should take a ball
and measure quantitatively how high it bounces when dropped from a given height.
Then the various types of balls are categorized as to how well they bounce.
Finally, one should consider the same ball bouncing several times and study the
progressive decrease in the heights to which it goes.
A. Procedures for qualitative study:
Bounce a ball a number of times off the table or desk in front of the class,
catching it on first bounce, and ask students to describe carefully what they
see. Make a list of the qualitative observations---a list of typical
observations is given below:
1. The ball is released from rest and picks up speed until it hits the
surface and bounces off.
2. After bouncing off the surface the ball comes back up to a height which
is less than the height at which it started, before stopping to climb.
3. The higher the distance from which the ball is dropped, the higher it
4. The ball makes very brief contact with the table, seeming to leave it
5. The ball makes a sound when it hits the table, which changes with the
height from which it is dropped.
6. The ball may bounce differently when it hits different points on the
B. Procedures for quantitative study:
1. Each student should be given a ball to drop from a height h of one
meter and measure the distance d to which it bounces upward. The
results should be arranged according to the types of balls and the
group should discuss which balls bounce well, and why.
2. Each student should release the ball from several heights [h = 50 cm,
75 cm, 100 cm, 125 cm, 150 cm, etc] and measure the distance d to which
it bounces. The ratio d/h, which we call the elasticity coefficient,
should be roughly the same for each height.
3. If there is time, a given ball should be allowed to bounce several times
after being released from an initial height h. The sequence of bounce
heights, d1, d2, d3, ..., should be measured. Each time the bounce
height reduces by roughly the same factor, the coefficient of
d1/h = d2/d1 = d3/d2 = ...
The ball begins at rest from height h with potential energy mgh, where m is its
mass and g is the acceleration due to gravity. On first bounce it comes up to a
height d, corresponding to potential energy mgd. The coefficient of
restitution, d/h, is the fraction of mechanical energy remaining after first
Energy is dissipated in the form of heat and acoustical energy. Physicists
believe in the principle of conservation of energy as a consequence of the
symmetry of the basic interaction under translations in the time coordinate.
Bounce a ball across the room to an assistant so that it strikes the floor
several times. Have each student draw the trajectory of the ball in the
notebook, describing its motion as completely as possible.
A ball bounced off a hard surface loses a specific fraction of its mechanical
energy with each bounce.
The bouncing of a ball on a hard surface is essential for an understanding of
baseball, which is played with gusto in Asia, Latin America, the Caribbean
Region, and North America. English Cricket [played throughout the British
Commonwealth Nations] requires similar knowledge of the physics of bouncing
The deep connection between symmetries and conservation laws was first
understood by the Mathematical Physicist Amelia Emmy Noether [1882 - 1935].
Robert K. Adair, The Physics of Baseball, 2nd edition [Harper 1994].