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The Physics of Baseball

Illustration of baseball showing axis of spin, direction of travel, direction of spin and flight path of ball.

Written by Jake Stump
Photographed by Raymond Thompson Jr. Illustrated by Lindsey Estep

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If Hank Aaron were friends with a physicist, perhaps Hammerin’ Hank would’ve knocked a few more out of the park to cement his status to this day as all-time home run hitter. There’s a science to baseball, and with the 2020 season heating up, two faculty members in the Department of Physics and Astronomy broke down the physics behind America’s favorite pastime. Grab your peanuts and Cracker Jacks for a quick science lesson.


HOW DO YOU HIT A HOME RUN? 

Explained by D.J. Pisano, interim chair and associate professor of physics and astronomy, White Sox fan 


p = mv 

p = momentum 

m = mass 

v = velocity


Baseball Hall of Famer “Wee Willie” Keeler had a motto to “hit ‘em where they ain’t.” Presuming that you are able to make contact with the ball at the sweet spot of the bat and hit it at an appropriate angle (i.e., not into the ground), how do you get the ball to travel the greatest distance? 


When the bat collides with a ball, you are transferring the momentum (the product of mass times velocity) from the bat to the ball. 


To maximize the momentum of the bat, you can either increase the mass of the bat or swing the bat faster. The speed which you can swing a bat, however, depends not on the actual mass of the bat but on its “swing weight,” or moment of inertia. 


If all the weight of the bat is at the tip, then the moment of inertia is higher and you will swing the bat slower. On the other hand, a higher swing weight bat will transfer momentum more efficiently. 


So what’s the answer? Quite simply, you want to swing the heaviest bat you can as fast as you can but, in reality, you want to swing the bat you are most comfortable swinging. That is why Babe Ruth used a 40-ounce bat to hit 60 home runs in 1927 while Roger Maris hit 61 home runs with a 33-ounce bat in 1961. 

Sketch of baseball player holding bat.

ALUMINUM VS. WOOD BATS 

Explained by D.J. Pisano 


The average major league baseball player will swing a bat at about 

60 mph 


Professional baseball players hit with wooden bats while college players and other amateurs use aluminum (or composite) bats. 


How do different bat types influence how a ball travels? Most obviously, it is possible to make a strong aluminum bat that weighs less than a wooden one, allowing for a bigger bat to be swung faster. Even if a wooden bat and aluminum bat are the same size and weight, a ball will have a larger bounce off of an aluminum bat due to the “trampoline effect.” 


When a ball hits a bat, both the ball and bat are compressed during the collision before springing back into shape. The average major league baseball player will swing a bat at about 60 mph. With a solid wood bat there is relatively little compression, and a ball pitched at 90 mph will bounce off with an exit velocity of 90 mph. 


An aluminum bat, however, is hollow, and can exhibit a larger trampoline effect, sending the same ball out with an exit velocity closer to 100 mph. Because of this effect, there are strict rules in college baseball limiting the amount of bounce produced by an aluminum bat and thus assuring that fans will hear the satisfying crack of the bat in the major leagues for years to come. 


Sketch of hand holding baseball.

HOW DOES A CURVEBALL CURVE? 

Explained by Sean McWilliams, assistant professor of physics and astronomy, Phillies fan 


Newton's 3rd Law: 

Every action has an equal and opposite reaction


Just after the Civil War, a teenager named “Candy” Cummings discovered that by throwing a ball with a rapid spin, he could make its path curve. Cummings rode the success of his invention all the way to the Baseball Hall of Fame. 


The explanation behind the success of the pitch lies in the study of aerodynamics. When a ball spins as it flies through the air, air passes more rapidly over one side of the ball than the other. 

A fundamental rule of physics, Bernoulli’s principle, tells us that the faster that air passes over a surface, the lower the pressure will be on that surface. The lower pressure on one side will cause the air to deflect in that direction and, so by Newton’s third law (that every action has an equal and opposite reaction), the ball will deflect the opposite way. This effect is known as the Magnus force. 

If a ball is thrown with a forward spin instead of the usually backward spin, then the deflection is downward and produces a vertical drop different from what we would expect from gravity alone; this is known as a curveball. If the spin is partially sideways, then the deflection is both down and sideways, and the pitch is called a slider. Both pitches operate due to the same principle, which is also the principle that produces lift as air passes around a wing and allows airplanes to fly.