Everything about surfing, from the ability to stay afloat on the waves to the necessity of duck diving, comes from basic physics. Newton's laws of motion, which describe the movement of matter, contribute heavily to maneuvering and staying afloat. Newton's first law states that objects in motion, like waves, tend to stay in motion, while objects at rest, like a floating surfboard, tend to stay at rest. This is why a surfer has to paddle to catch a wave. According to Newton's third law, every action has an equal and opposite reaction. When a rider pushes down one edge of the board, that edge pushes into the water, which pushes back up against the board. As a result, the board starts to turn. You can see more about this process in How Personal Watercraft Work.
Here's a run-down of other physics principles that affect surfing:
- Buoyancy: The surfboard's buoyancy, or ability to float, comes from its density. The board is less dense than the water underneath it. The board's coating is also waterproof, keeping water from seeping in, soaking the foam inside and pulling the board under.
- Surface tension: The molecules that make up water are attracted to one another, so they create a surprisingly strong film at the water's surface. This film is one reason why a wave holds it shape, and it helps keep the surfboard afloat.
- Gravity: While buoyancy keeps the surfboard afloat, gravity pulls it and its rider toward the water. Gravity's pull helps the rider hold his position on the moving, nearly-vertical face of a wave.
- Mass and shape: The surfboard and its rider both have a center of gravity, which is related to their shape and mass. When riding the waves, the rider can move his center of gravity to shift the board's angle in the water. For example, moving toward the tail of the board will cause the nose to lift up from the water in response.
- Hydrodynamic forces: Hydrodynamic forces are essentially the same as aerodynamic forces. These forces, like lift and drag, can dramatically affect how waves form and how the waves interact with the surfer's board.
Next, we'll examine the specifics of how waves form, especially the dramatic waves found in famous surfing spots.