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Soccer FOCUS Question How do different forces affect the movement of a soccer ball?

Physics Gets a World Cup Crowds are cheering and waving flags from around the world. What sport are they watching? Soccer! Every four years, soccer teams face off to compete in the World Cup. It is an exciting game and a thrilling tournament! Soccer is a game of skill and endurance. It is also a game of physics. Physics is the science of matter and energy. Physics explains the speed and direction of the ball. Physics also explains. why a ball can curve around players. Maybe the World Cup of soccer should be called the World Cup of physics!

Playing the Game Soccer is a team sport. During a soccer match, the eleven players of one team try to move a ball down the field and into the goal of the other team. The other team tries to do the same thing in the opposite direction. Newton's first law of motion explains how a soccer ball moves. According to this law, a moving object keeps moving until a force acts on it. Also, an object that is not moving stays at rest until a force acts on it. When a soccer player kicks a soccer ball, she applies force to the ball. Will a moving soccer ball keep going forever if no one touches it? Of course not! Forces act on the soccer field. These forces affect the motion of the ball.

Soccer Balls That Go Up Must Come Down Sometimes, a soccer ball moves on the ground. Other times the ball flies through the air. What keeps a kicked soccer ball from flying straight out into space? Gravity is the force that pulls objects toward Earth's surface. It pulls the soccer ball back to the ground. Air resistance is the force of air pushing against a moving object. It pushes against the flying ball, slowing it down. A soccer ball that is kicked up will eventually come back down to the ground.

Acceleration Not every kick makes the ball go fast and far down the field. Some kicks move the ball just a little bit. The ball may move from side to side. When players dribble the ball, they make small, controlled kicks. This keeps the ball between their own feet and helps them move it around the field. Acceleration is the change in an object's speed or direction of motion. Newton's second law of motion says that acceleration is related to how much force is used. A small kick leads to small acceleration. The ball moves just a little. Dribbling uses small forces. During a big kick, a large force is applied. The acceleration will be large. This moves the ball farther than small forces do. Think About It How can you tell which player on this page is applying a larger force?

Kicks and Curves One of the most famous soccer plays of all time was a free kick by Brazilian soccer player Roberto Carlos. A free kick starts the game again after it is stopped for a foul or another reason. What made the free kick by Carlos so famous? The ball curved around a whole group of players and flew right into the goal! How did the ball curve around the players? The force of the kick made the ball move forward toward the sideline. But because the ball was spinning, its motion changed due to the Magnus effect. This caused the ball to curve.