Re: Can you tell me about the physics involved in soccer?

Soccer, like most sports has countless examples of really cool physics. I will limit my discussion to the physics involved in kicking a ball along the ground or into the air and straight or curved.

The position of the foot relative to the center of the ball and the direction of the applied force of the foot determine the flight direction of the ball.

A soccer ball is a sphere with it center of mass located at its geometric center. A Force applied along a line that does not pass through the center of mass will produce a torque and will induce a rotation. Think of the kick as an applied force along a direction (sounds like a torque to me).

The distortion of the ball due to the force of the kick is typically large only for a very brief time and will not in general change the resulting direction of the kick too much. This distortion does affect the energy transferred to the ball, but I will defer on speaking about that.

Let's look at airborn versus ground limited flight of the ball.

If the foot at or above the center of the ball, the ball will travel level to the ground. A "topped" ball will roll across grass or carpet while a "true" struck ball will skid on carpet until the frictional force has done enough work to create a condition of rolling without slipping. It is practically impossible to kick a ball on the ground in a manner to generate both distance and curvature. However, the same sort of interaction between friction and the ball occurs when it is airborn.

A ball kicked into the air can curve significantly. This used be known as a banana kick and is the soccer equivalent of a Baseball curve ball or slider. The kicker strikes the ball off-center with the instep of his foot. This will cause the ball to come off the foot with spin. A right footed kick will drift to right-to-left relative to the facing of the kicker. Seriously good players can perform the same trick with the outside of the foot producing a left-to-right drift in the flight of the ball.

A soccer ball can on rare occassion, be kicked with no spin on it. In this case the ball can "dance around" like a knuckle ball or simply float with slight bobbing like a frisbee.

If the ball is kicked to make it spin around an axis that is close to perpendicular to the ground. One side of the ball will be spinning into the direction of flight and the over side will be spinning away from the direction of flight. This produces an imbalance in the lateral forces on the ball -- This is usually called the Magnus Effect, although this is not quite correct. When the ball travels through a fluid (like air), the fluid is pushed out of the way like a when a boat travels through water. The fluid pushes on the object moving through it. If the object spins while in motion the fluid moves around the object in an asymmetric pattern.

It is the asymmetry of this pattern which causes circulation of the fluid and pushes the object off its non-spinning course. This is the fluid mechanics view of how airplane wings (airfoils) work.

It the object is spinning about an axis parallel to the direction of flight, stability is created because small surface irregularities are in a sense "averaged out."

Previous answers on the Magnus Effect exist on our site. Also look into the flight of a curve ball in baseball for more information. The same sort of push/fade or hook/slice effects as soccer kick happen in a football kick, a golf ball struck by a club, or baseball hit by a bat.

Sincerely,

Tom "Big Toe" Cull

P.S.

The Magnus was first empirically noticed from cannon balls in flight during battles. Cannon balls were pretty close to spherical and were shot with only accidental spin. The lack of spin created an instability to lateral forces like wind or an irregular surface which caused the cannon ball to deviate from its predicted path sometimes. It was noticed that a spinnning cannon ball would often fly in a more steady path, the problem was putting a predictable spin on the cannon ball. The Magnus effect also affected the flight of bullets fired from guns until the barrel of the gun was designed to spin the bullet along its long axis -- this is known as rifling and was discovered during the Civil War.