The Physics Behind Surfing

When you are out in the ocean sitting on your board, you may not think about the Physics involved in catching a wave.

Enjoying Physics

When your are on the water, you don't whip out a calculator to figure out how to surf, you just surf. But there is a ton of physics involved in surfing. But as a surfer, you become a master of physics and hydrodynamics without even knowing it.

Where do you experience physics as a surfer?

Surfers are constantly in search for perfect waves and, like other waves in physics, ocean waves are the result of a transfer of energy. (We'll go over this in a future blog post)

The science of surfing begins as soon as you and your surfboard hit the water. A surfer waits on the board looking for that perfect wave to catch. It is here that they experience buoyancy. The board's size and construction help it displace a lot of water. At the same time, a buoyant force equal to the weight of the displaced water pushes up on the board. This counteracts the surfer's weight and allows them to float while they wait to paddle for a wave. That is buoyancy!

As this surfer paddles out, she is floating almost level with the surface of the water. Note the gentle wave passing the middle of the surfboard. (photo: exploratorium.edu)

At rest, the gravitational force and the buoyant force are equal and opposite. The net force on the surfer plus the board is zero. (photo: exploratorium.edu)

As the surfer catches the wave, they experience acceleration. As the wave gets near, the surfer paddles like mad to match the wave's speed. The board forms an angle with the water, and which creates pressure on the bottom of it. As a surfers keeps their balance and pops up, the pressure forces you and your board out of the water, to skim along the surface. The increased forward momentum makes the surfer more stable as they stand up and surf along the wave.

Move back on a surfboard, behind its center of mass, and the nose of the board tilts up until the buoyant force aligns with gravity once again. As this surfer moves to the right, the tail of the board pushes hard on the water it’s moving through, bringing him to a stop. (photo: exploratorium.edu)

When the surfer moves back on the board, gravity and buoyancy move out of alignment and create a torque—a twisting force—on the surfboard. The board rotates until the forces are realigned. (photo: exploratorium.edu)

The surfboard rotates until the buoyancy force through the center of mass of the displaced water is aligned with the gravity force on the surfer. As the board rotates, the center of buoyancy—the center of mass of the displaced water—moves toward the back of the board. When buoyancy and gravity are again in alignment, there’s no longer any torque. (photo: exploratorium.edu)

The fins on the surfboard allow a surfer to alter their speed and direction as they reposition their weight. And parts like the rails, channels, shape, etc. also affect the way forces are distributed to make the surfer's rider even more enjoyable.

As the surfer catches the wave, the water pushes the surfboard forward. At last, the net force isn’t zero! The net force accelerates the surfer to the speed of the wave and beyond. (photo: exploratorium.edu)

Notice how Lori reaches from a deep crouch to touch the moving water of the wave. She’s riding a longboard, which floats high in the water. (photo: exploratorium.edu)

Meanwhile, the water particles of the wave crest are also accelerating. This forces the crest to move a bit faster than the bottom of the wave and topple over itself because of a force called gravity. When the wind speed, the ocean bottom and the coastline work perfectly together, this "break" can form hollow tunnels that surfers call barrels!

Eric, on a shortboard, is taking a ride down the face of a wave. He’s ahead of the break, in balance, with knees bent. A surfer moving along the face of a wave can travel at a speed faster than the wave itself. (photo: exploratorium.edu)

Once the surfer is up to speed on the wave, all of the hydrodynamic forces of the moving water on the surfboard add up to oppose gravity, resulting in a zero net force. (photo: exploratorium.edu)

Check out this awesome Ted Ed video by Nick Pizzo, postdoctoral researcher at Scripps Institution of Oceanography, that share more about the physics of the waves.

Surfing Physics Vocabulary

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.

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