Sports Science: The Unpredictable Knuckleball

What do all sports have in common? Activities as diverse as baseball, basketball, cricket, swimming, gymnastics, snooker/billiards, carrom (or carroms) and cycling all involve an application of one or more principles of science. Sports, with its preoccupation with balance, trajectory, speed, timing and spin, is nothing but a living and breathing science laboratory.

What better way of starting a sports segment on the Exploriments blog than with the national US pastime - baseball? Baseball is so fascinating because it is a duel between the batter's skills and the pitcher's skills. While pitchers frequently employ the fastball, it is the knuckleball that is a more interesting Physics study.

The main skill in throwing a knuckleball, other than being able to dig your fingernails in to hold the ball, is to not impart any spin to it. A good knuckleball turns only a few times during its entire flight. The fact that it has no spin leaves it susceptible to wind eddies and the vortices that form over its seam, and this makes it float in unpredictable ways. 

A regular fastball spins fast about its own axis, giving it the angular momentum and rotational inertia which helps it resist changes to its position. This gyroscopic effect helps the ball retain its orientation and course, making it more predictable to follow around and to catch or hit. A knuckleball neither has the spin, nor the gyroscopic effect associated with spin, making the trajectory of the ball difficult to pick out. This video from Reuters TV shows this Physics principle in action: