# Physics of Sports

By Brianne McCurley

Watching a baseball soar over the Green Monster at Fenway Park, seeing a tennis ball hit at more than 100 miles per hour at Wimbledon, witnessing Tiger Woods drive a golf ball onto the green in Amen Corner or Ben Roethlisberger throwing a 40-yard touchdown pass are commonly observed sports performances in which physics play a key role.

Magnus (effect) force is a reaction that the Encyclopedia Britannica defines as "the generation of a sidewise force on a spinning cylindrical or spherical solid immersed in a fluid (liquid or gas) when there is relative motion between the spinning body and fluid." The Magnus force reaction occurs in many sports with the curve of a baseball, soccer ball, golf ball and tennis ball.

The force and torque exerted on the handle of a baseball bat, as well as the spot where the ball meets the bat, all determine how far and where a baseball will travel. To hit a baseball the maximum possible distance, the trajectory off the bat should have a 35-degree angle. A batter exerts between 6,000 to 8,000 pounds of force on the ball. This force is required to change a 5 1/8 ounce ball from a speed of 90 mph to a speed of 110 mph. This distorts the baseball to half of its original diameter and the bat is compressed one fifth of its size.

When a soccer ball travels through the air, wind flow, air speed and pressure can all affect its trajectory. As a playerâ€™s leg comes in contact with the ball, the drag and force strongly influences its trajectory. A Magnus force reaction occurs when a player strikes the ball, attempting to induce a shot that bends.

Golf balls are dimpled to increase the Magnus effect. As a golf club comes in contact with a ball, the presence of the Magnus force reaction occurs because the ball has backspin. That same Magnus effect can cause a ball to hook or slice if it has sideways spin. The windup, the toss and the strike of a tennis ball are all steps in completing a serve. Physics is a factor in each of the steps. As a player tosses the ball up, he or she presses their feet against the court, using ground reaction forces to create elastic potential energy. The rotations of their legs, hips, trunk and shoulders produce the maximum angular motion in order to hit the ball at exactly the right time with the right amount of force. In biomechanics, this is known as "the kinetic chain principle."

Next time you are watching a sporting event or casually playing a game on your own, think about the physics involved each time you touch the ball.

### Classroom Discussion

• Other than in sports, where does physics play an important role in everyday life?
• Corking a baseball bat is illegal. Why would a cork-lined bat give a batter an advantage?