Smashing physics – Physical Science: Forces And Motion – physics of tennis

Nicole Dyer

What makes Venus Williams a mighty force in tennis? Hard-hitting strokes, a searing 127-mph serve, and physics!

At 21, the tennis powerhouse has nabbed 17 championship titles–Sports Illustrated crowned her 2000 Sportswoman of the Year. But to nail success, Venus had to battle crippling wrist injuries and critics who claimed she’d never make a comeback. “I didn’t let it get to me,” Venus says. “I kept working hard and believing.” Read on for more about the driving forces that have propelled Venus Williams to the top.


The larger the ball, the slower it travels through air,” says physicist and tennis expert Howard Brody at the University of Pennsylvania. Why? Think friction, a force that opposes motion when two objects rub together. For example, after Venus whacks the ball and sends it hurling, the ball collides with millions of invisible air molecules. The molecules rub against the ball’s surface and slow it down.

Actually, with so many power servers like Venus in the game today, tennis officials, spectators, and even players themselves complain that pro tennis has become too fast, making it that much tougher to return a serve. “We’re concerned that as players continue to get stronger and faster, the game will become far more serve dominated and less entertaining,” Brody says.

One possible solution: use a larger tennis ball. The International Tennis Federation, the sport’s official governing body, is investigating the possibility of increasing a standard-size ball’s diameter–which ranges from 65 to 69 millimeters–by 6 percent. “That would increase the ball’s total surface area [area of the surface surrounding a solid shape] by 12 percent, creating more friction between the ball and surrounding air molecules,” says Brody. “When the game slows down, you spend less time bending over to pick up the ball and more time hitting it.”


Like all people, a tennis player relies on his or her body’s estimated 640 muscles (tissue made up of fibers) for movement. Skeletal muscles move the body when they contract, or shorten, and pull on the bones. But a powerful racquet stroke relies particularly on two large, fin-shaped chest muscles–one on the left side of the body and the other on the right–called the pectoralis major. “Your chest muscles are the driving force behind a strong serve and forehand stroke,” says Duane Knudson, a biomechanist (scientist who studies the physics of human movement) at California State University at Chico.

Due to its large size, the chest muscle is designed to exert large amounts of force. The “pec” starts at the top of the rib cage and stretches horizontally to the collarbone where it attaches to the deltoid (shoulder) muscle (see diagram). When you swing a racquet forward, nerves in the muscle fire tiny bolts of electricity that cause it to tighten and exert a force that pulls the arm in toward the body. Pecs also help rotate the shoulders. The larger the muscle, the stronger the force.


While beefy chest muscles may add power to your stroke, Knudson says it’s important not to overtrain them. “That could throw off your posture,” he explains. “It’s best to strive for balance and develop all of your major muscles.”


You’ve probably noticed that most athletic balls–baseballs, footballs, racquetballs–aren’t fuzzy. The exception, of course, is the tennis ball. The reason is twofold, says ball engineer Bill Bishop at Wilson Sporting Goods in Elk Grove Village, Ill. First, the felt coating helps slow the ball down. “The fuzz increases air friction and keeps the ball from speeding over the net like a bullet,” he says. “The fluffier the ball, the more friction it experiences and the slower it travels.” During a hard-hitting match, the fuzz wears down and the ball travels faster. That’s why pro players like Venus are required to use new balls every nine games–to keep Se match in control.

The felt also helps a player control the ball’s spin (rotation), and alter its trajectory, or distance it travels, over the net. For example, to give the ball topspin, an effect that causes the ball to rotate forward and travel a shorter distance, a player must swing the racquet from low to high. As the racquet strikes the ball, the fuzzy coating lodges inbetween the strings, allowing the ball to be tugged upward with the player’s stroke. Try that with a baseball!


A “sweet spot” is an area on a racquet where a player can slam the ball with maximum force (physical movement) and feel minimum vibration in the arm. “Basically, it’s where it feels good to hit the ball,” says Brody.

In general, every racquet has two sweet spots: one in the middle of the racquet and the other about 4 to 5 cm away from the racquet’s throat. Thanks to the principle of conservation of momentum–which dictates that energy is always transferred between two colliding objects–a racquet absorbs some of an incoming ball’s momentum, the mass of a moving object multiplied by its velocity (momentum: mass x velocity). The transferred momentum causes invisible molecules in the racquet and strings to vibrate. If you nail one of the “sweet spots,” the racquet returns the vibration energy to the ball–and not your arm. If you miss the sweet spot, the vibration energy travels through the racquet and you feel like you’re holding a jackhammer!


When a tennis ball smashes into Venus’s racquet, the strings absorb the ball’s force, stretch backward, and gain potential energy, or stored energy. As the strings bounce back to their original shape, the potential energy converts into kinetic energy, or moving energy, and the ball flies through the air. “A racquet’s strings work like little trampolines,” says Brody. “They’re designed to give back 95 percent of the energy they store.” If you loosen strings and make them more elastic, they’ll rebound more energy to the ball and add power to your stroke. The downside: You lose accuracy. The more the strings deform backward, the sharper the angle at which the ball rebounds. Conversely, tighter strings give you more ball control, but less power. Ready to play?

You don’t have to be Venus Williams to ace our “What’s Your Tennis IQ?” quiz. All you need is physics know-how.

1. Does the size of a tennis

ball affect how fast it travels?

a. Yes

b. No

2. What muscle does a tennis

player rely on most?

a. Biceps muscle

b. Pectoral muscle

3. Why is a tennis ball

covered with yellow fuzz?

a. To speed the ball up

b. To give more ball control

4. What’s a


“sweet spot”?

a. An area on the

strings that produces

very little vibration when


b. The handle where you

hold the racquet

5. How does a racquet’s string

tension affect how far the ball


a. More flexible strings

allow for more powerful


b. Tighter strings allow for

more powerful rebounds

For answers, read on!

Venus Williams, 21

Birthplace: Lynwood, Calif.

Height: 6’1″ Weight: 169 lb

Championship Titles: 17, (including two Olympic gold medals)

Weapon: Explosive serve

Quote: “I’m looking to be the best.”


1. Bend knees and twist hips to gain potential energy (PE), or stored energy.

2. Spring upward to convert body’s PE into kinetic energy (KE), or moving energy.

3. Fully extend racquet arm toward ball.

4. Transfer racquet’s force to ball.

5. Use turning force (torque) to swing racquet across body.

Stroke of Power

Newton’s third law of motion states that for every action there is an equal and opposite reaction. At left, Venus applies a torque, or turning force, to her hips. An equal and opposite force applied to her trunk powers her swing.


Tennis Heats Up!

The inner core of a tennis ball is stuffed with air–or tightly packaged gas molecules–which expands or compresses depending on the air’s temperature. How do hot and cold temperatures affect a tennis ball’s bounce? Try this experiment to find out.

YOU NEED: 3 cans balls (moat standard cans contain 3 balls) * thermometer * tape measure * shoebox * masking tape * bucket of ice (or refrigerator) * chair * zip-top plastic bags * paper * pen * a partner


1. Remove three new tennis balls from a can. Place balls in a shoebox and close the lid. Allow the box to bake in the sun for two hours.

2 Remove three tennis balls from the second can. Place balls in bag(s) and place in icebox. Allow the balls to chill for two hours.

3. Remove three new tennis balls from the third can. Let them sit at room temperature for two hours.

4. Remove the lid on the shoebox.

5. Use the thermometer to measure the air immediately surrounding the tennis balls. Take three readings for each ball. Record your results.

6. Repeat Step 5 for the chilled and room-temperature tennis balls.

7. Choose a hard surface that you can drop the balls on–such as concrete, linoleum, or a tennis court.

8. Stand on a sturdy chair (have your partner spot you) and hold the tape measure 2.5 meters (98.4 inches) from the ground. The zero mark on the tape measure should touch the floor.

9. Hold a heated ball at the 2.5-meter mark. Allow the ball to drop straight to the ground. Record the height of its first rebound. Repeat the procedure five times with each of the three balls.

10. Repeat Step 9 for the chilled and room-temperature tennis balls.

11. Average your height data for the heated balls. (Add up the 15 different measurements and divide the total by 15.) Record the average rebound height.

12. Repeat Step 11 for the chilled and room-temperature tennis balls.


Which set of balls rebounded highest? Why? How do hot which cold temperature affect the amount of force (pressure) exerted on the ball’s outer shell?


Cross-Curricular Connection

History: Research the history of the sport of tennis. Which country takes credit for its invention? Which professional player has had the most consecutive victories?

Did You Know?

* Tennis was included in the first modern Olympic Games in Athens in 1896. It was withdrawn from the Olympic Games in 1924, then reinstated in 1988.

* A skilled player can control the trajectory and rotation of a tennis ball by using different racquet strokes. The backspin stroke, for example, causes the ball to rotate counterclockwise and travel a greater distance.

* There are four general tennis court surfaces: grass, clay, cement, and synthetic. Each one has its advantages and disadvantages. Clay, for example, causes the ball to bounce very slowly, and increases the volley time between players.

National Science Education Standards

Grades 5-8: change, constancy, and measurement * form and function * motions and forces * transfer of energy * structure and function in living systems

Grades 9-12: change, constancy, and measurement * form and function * motions and forces * interactions of energy and matter * matter, energy, and organization in living systems * understandings about science and technology


Sports: The Complete Visual Reference by Francois Fortin, Firefly Books, 2000

“The Tennis Sport Science” Web site by Cislunar Aerospace:

“Simply Super” by S. L. Price, Sports Illustrated for Women, Nov/Dec 2001


Smashing Physics

Directions: Write a paragraph using the vocabulary words provided.

1. Describe Venus William’s powerful serve. (Kinetic energy, pectoralis major, racquet)

2. Report on a hard-hitting match at the U.S. Open. (Friction, topspin, fuzz)


Answers should include these points:

1. Pectoralis major is a large chest muscle. The force it can exert powers a racquet stroke. 2. Kinetic energy is moving energy. Tennis ball fuzz increases air friction, which slows the ball. Topspin causes a tennis ball to rotate forward and travel a shorter distance.

COPYRIGHT 2001 Scholastic, Inc.

COPYRIGHT 2002 Gale Group

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