Bouncing Back
Evan Zabawski | TLT From the Editor February 2013
A famous children’s toy explains two curiosities.
This may explain why so many people are frustrated at the way a Super Ball bounces.
THOSE OF YOU WHO TOOK PHYSICS IN HIGH SCHOOL might recall learning about Newton’s Third Law of Motion, which states that for every action there is an equal and opposite reaction. To demonstrate this effect, teachers often use collisions involving balls. A ball dropped without spin will bounce straight up, whereas a ball dropped at an angle will bounce off the floor at an equal and opposite angle. But in either case, a ball dropped with spin will depart in a direction dependent on the direction of spin and one other factor.
I distinctly remember my physics teacher asking the class which way we thought a basketball would go if he dropped it with topspin. The consensus was that the ball would bounce away from him, and the demonstration proved us right. Next he asked what would happen if he pushed the ball away from him with topspin instead of allowing it to fall straight down. The consensus was that the ball would depart on an even steeper angle away from him, and again the demonstration proved us right.
He explained that what we were witnessing was the coefficient of restitution and the conservation of momentum and energy. He added we were going to learn how to calculate how many successive bounces a given ball would have by observing the bounce height of the first rebound. He conjectured we should also be able to calculate the distance the ball would travel (before it started rolling) by measuring the angle of the first bounce.
He admitted he would not complicate it by accounting for the spin on the ball and demonstrated that throwing the basketball with backspin greatly affected the first bounce and the ball’s forward momentum. At this point, he smiled and grabbed another ball, one we immediately recognized as a large Super Ball. He asked what would happen if he threw it with backspin. Without waiting for an answer, he threw the ball and to our surprise the ball did not continue across the class at a greater angle, similar to the basketball, but rather it returned almost perfectly to his hands.
Sadly, whether due to lack of time or exclusion from the required curriculum, he did not delve into a detailed explanation for the bizarre behavior of the Super Ball—its coefficient of friction. Most balls, like a basketball, grip the floor enough to transfer some of their energy. Therefore some energy is consumed, reversing the spin of the ball as the ball continues in its original direction at a lower speed. A Super Ball, however, is able to grip so strongly that it reverses direction (and spin) so that it returns at nearly its original speed. This may explain why so many people are frustrated at the way a Super Ball bounces.
Unbeknownst to the average user, both the coefficient of restitution and the coefficient of friction of sports equipment and playing fields are carefully regulated to ensure fair and consistent game play. So even if your favorite football team succumbs to a “bad bounce,” understand that you can really only blame the ball’s irregular shape.
For you football fans, there is one more influential connection. The name Lamar Hunt should ring a bell. He founded the American Football League in 1959 when the National Football League would not grant him a license for an expansion team in Dallas. The AFL competed with the NFL until 1966 when both organizations merged and agreed to a season-ending title match on Jan. 15, 1967.
Trying to come up with a catchy name for this match, Hunt was influenced by the college “bowl” games played on New Year’s Day and his children’s latest toy—the Super Ball. He suggested the name Super Bowl, which lasted for two seasons before the NFL commissioner requested a contest to come up with a new name. None of the submissions won over the judges, and 47 years later we still watch the Super Bowl.
Evan Zabawski, CLS, is a reliability specialist in Calgary, Alberta, Canada. You can reach him at evan.zabawski@gmail.com.