I am a member of the club baseball team at Lehigh and last week we played a classic wiffle ball game for practice. Playing club baseball doesn’t require the time commitment and pressure of a varsity sport, but provides the opportunity to compete, get exercise and travel to some local schools. To take a break from our normal practice routine, we played wiffle ball last week in Grace Hall. Obviously, a wiffle ball is completely different than a regular baseball. Hard baseballs are heavier, made of hide and thread, and have seams on the surface. Wiffle balls are much lighter made of plastic, and have holes in them that allow air to flow into and out of the middle of the ball. Last year, in Fluid Mechanics, a group did a presentation on throwing a baseball and how air currents affected the movement of the ball. They looked at several pitches and tried to explain why they moved the way they do. I noticed that the movement of a wiffle ball seemed unpredictable and changed much more rapidly than a traditional baseball, when we were playing this week. So, I searched popsci and found an article that had been recently posted about the science behind a wiffle ball.
Coincidentally, a mechanical engineer at Lehigh’s biggest rival, Lafayette, recently performed research on wiffle balls. She placed the ball in a wind tunnel, measured the airflow, and concluded that “the shifting balance of forces inside and outside the ball is what makes it so devilishly hard to hit.” I can attest to the fact that a wiffle ball is much more difficult to hit than a baseball. But when you do make contact, the path of the ball is unpredictable which makes it difficult for fielders to catch and control. If you think about it, a swing and contact with a bat will supply much more force (torque) on the ball than throwing it, so the affects of airflow are personified when the ball is hit.
Here is what the researcher concluded from her study:
The strengths of the internal and external forces shift constantly while the ball is in flight. The net of the forces is what dictates the ball’s path.
The holes are on just one side. They disrupt airflow, increasing turbulence over that half of the ball.
More turbulence means less drag on that side, resulting in an upward “lift” force.
Air rushing into the holes creates vortices that whirl inside. The ball’s orientation, spin, and velocity all affect how those vortices develop.
Vortices create a force that can change the ball’s direction.On faster pitches, the interior force typically overpowers the external force.
She also explained how to give oneself an advantage in making the ball move more when thrown, thus making it more difficult to hit. One could block one or some of the holes on the ball or make the holes larger and smoother to increase airflow. In the major leagues, it is illegal to scuff or scratch a ball purposely, but scoring a wiffle ball on one side would increase the friction with air and cause rotation to increase or decrease. The last part of the article talked about throwing a knuckleball. The idea is really interesting, you throw the ball with all the holes perpendicular to your body, facing the target. The internal and external air forces are in constant equilibrium, and when the ball rotates slightly, dominant airflow will shift and cause a dramatic change in direction.
It’s really interesting to learn about the science behind simple games and tasks. Science dictates the world around us, and the physics of a game like wiffle ball can drastically change how we perceive it. I was able to make many connections with this article to my Fluid Mechanics class from last year. Air can be treated as a liquid and one can examine the forces that air applies to the ball.