Surfing is a sport whose origins may be traced to the ancient Polynesian cultures of the Pacific Ocean. In 1788, Captain James Cook, the English explorer, observed the indigenous people of both Tahiti and the Hawaiian Islands using long wooden boards to move through the waves near shore. American author Mark Twain described his own adventures with Hawaiian surfing in his book Roughing It published in 1871. California, the place most often associated with surfing, first became a hotbed of the sport in the 1920s and 1930s. The California surfing community helped propel the sport into the cultural mainstream in the late 1950s; surfer expressions such as "stoked," "hot dogging," and "wipeout" became a part of modern speech.

Surfing has attained a worldwide appeal due in large part to its utter simplicity. A surfer paddles out into the ocean on a surfboard, and awaits a suitable sized breaking wave on which to ride back to the shore using the wave's energy for propulsion. The more ambitious and talented the surfer, the larger the wave or the greater number of tricks the surfer can execute as the wave travels towards the shore.

Competitive surfing is a subjectively judged and the variability of wave height and speed may impact upon the competitive result.
Although surfing pre-dates the popularity of extreme sports, surfing's inherent physical dangers and its potential for high levels of personal satisfaction, as opposed to achieving a competitive result, warrant its inclusion in the extreme sports category.

The modern surfboard has undergone many changes since the days of the Polynesians. Early surfboards were often as long as 16 ft (5 m), weighing over 100 1b (45 kg), and each was built to support the surfer. Modern surfboards are constructed from synthetic materials such as epoxy, fiberglass, and carbon fiber composites and the boards are usually a shorter length, designed to suit the style of the surfer but also intended to be highly maneuverable in the water. An inexperienced surfer is often directed to a wider and longer board, as the greater the surface area in contact with the water, the greater the stability of the board.

In colder weather or water temperatures below 68°F (20°C), surfers will often wear a wetsuit to protect themselves from the combined effects of cold water and cold air.

Surfing is unique among sports in that the ride on a surfboard is powered by water and the resultant force of gravity alone. The physical object of surfing is to slide down the surface of the wave at the same speed at which the water is moving upwards. If the surfer moves took quickly along the wave surface, the surfer proceed to the bottom of the wave and end the ride as the surfer will no longer be affected by the wave motion. If the surfer moves slower than the wave, the wave will out run the surfer, creating a wipeout. It is for this reason that a surfer can maintain a stable position while riding below the crest of a mammoth ocean wave.

The surfer and the surfboard are balanced on the water surface when the force of gravity acting downwards upon the surfer is precisely equal to the hydrostatic effect, or buoyancy, directed upwards. The center of mass of the board is its balance point; when the surfer moves towards the front or the rear of the board, the board will become oriented accordingly, with the nose or back moved upwards or down to reflect to shift in position by the surfer. A surfer executes a turn on the board by shifting body position to the rear of the board, a movement that creates torque (twisting effect in the motion of the board). The forces of gravity and buoyancy are now directly under the surfer, permitting the turn to be executed. When the surfer pushes downwards on the board, the force of the push is greater than the force of gravity, causing the turn to occur more quickly.

As the surfer moves through the water on the board, the surfer maintains a low, bent leg position, to maximize balance on the board and to respond to any forces directed against the board with subtle changes of position.

A shorter surfboard provides the surfer with greater maneuverability, as the shorter the board, the shorter the axis upon which the surfer is required to turn. A longer surf board possesses an inherently greater moment of inertia, the time period within which an object resists the forces directing it to turn. Conversely, a longer surfboard will tend to travel faster across the surface of the water, due to the relationship between drag and the volume of water displaced by the board. The fins attached to the rear of the surfboard, known as the "skeg" act in a similar fashion to that of a keel on a sail boat, as the fins extending downwards into the water aid in preventing the surfboard from being pushed too far sideways by the force of the moving water.

The International Surfing Association (ISA) is the world governing body of surfing; surfing is also a member of the International Olympic Committee, although surfing is not an Olympic sport. There are also various professional surfing events held in various parts of the world on an annual basis sanctioned by the ISA. Competitive surfing is a subjectively judged and the variability of wave height and speed may impact upon the competitive result.

SEE ALSO Sailing; Sailing physics; Windsurfing.