The Physics of Surfing

Hardly any surfers think about how much force is exerted on them as they glide down the wave. But we have, and today we’re going to tell you why surfing works at all.

The Yin and Yang of Surfing

Everything we see consists of the tiniest particles: atoms. There are other particles around the atoms: electrons. Objects repel each other when their electrons get so close that they try to take each other’s places; the laws of physics forbid this, so a repulsive force is created.

For example, when a surfer is lying on a board, paddling with his hands, he is repulsed by the water, and he repulses the water. Newton’s third law of motion works here: if you act on something, you get an equal and opposite reaction.

A professional surfer at Teahupoo Beach on the island of Tahiti. Photo: Tim Mckenna/Agence France-Presse — Getty Images. The green arrow marks the effect of gravity (pulling down), the blue shows buoyancy (water pushing the board up).

But why doesn’t the board just sink? The fact is that there are two opposing forces vying for the fate of the surfer at every moment. Not good and evil, of course, but something no less fundamental—gravity and buoyancy, or Archimedes’ force.

Gravity, the attraction to the Earth’s center of gravity, pulls the surfer down. While buoyancy, that is, the ability to float on the water’s surface when your weight is equal to the weight of the volume of liquid you displace, pushes you upward.

These forces are in perfect equilibrium as long as the surfer lies on the board and does not move anywhere—the board, under a person’s weight, displaces exactly enough water around itself to prevent gravity from submerging its owner underwater.

But then, the surfer notices a wave, begins to adjust to it, and the situation quickly becomes more complicated.

Going with the flow

At least once in your life, you’ve probably lowered your palm into a rough current, and felt the water pushing it along the stream. If you turn your palm to the right, for example, at that moment, the water will push it further to the right, and part of the stream will deflect to the left.

A similar thing happens to a surfer’s board when they find themself on the water rising in front of a wave, stand up on the board, and start moving along the wave. There is no more equilibrium: in addition to gravity and buoyancy, the surfer is now subject to hydrodynamic forces, i.e., the forces of water movement within the wave.

The board pushes some water to the side, as seen in the wake.

Australian surfer Blake Johnston off southern Sydney. Photo: Saeed Khan/AFP/Getty Images. The red arrow marks the thrust, which pushes the board in the same direction as the wave moves.

Gravity keeps pushing down, Archimedes’ force keeps pushing up, and hydrodynamic forces keep pushing forward, thanks to the movement of the wave. As a result, the surfer begins to glide forward and down through the wave, gaining speed.

If you manage to control gravity (this requires standing on the board at the right moment, right before the place where the wave breaks) and stay on the wave, the speed will be enough to compensate for the force of gravity. As a result, the uncontrolled sliding downwards stops, and the surfer will continue to move along the wave with increasing speed, choosing their direction: up or down on the wave.

If they want to, surfers can glide faster than the wave if they learn to use the up and down motion on the wave, in the same way skateboarders accelerate on a ramp, crouching in the middle and rising on the rise. This technique is called pumping.

Dance to the rhythm of hydrodynamic forces

Catching a wave is only half the success, because staying on it is not so easy.

Waves «break», creating water tunnels for surfers, because of the approach to the shore: the lower part of the wave slows down due to friction and irregularities of the bottom, while the upper part begins to move faster than the bottom. That’s how you get the swirling motion of the water.

Friction also affects the surfer’s board— the faster they move forward in the wave, the more the friction of the water pushes them back. Frontal resistance, that is, the friction of the board against the water, is the fourth force that acts while you are surfing. To weaken it, the bottom of the board should be as smooth as possible, because any irregularities will slow you down.

Hawaiian American athlete Carissa Moore at the 2020 Tokyo Olympics, where surfing made its debut as an Olympic discipline. Photo: Francisco Seco/AP. The pink arrow shows the drag, which acts in the opposite direction of the surfer’s movement on the wave.

And to avoid falling over too soon, a surfer needs to control their center of gravity: bending their knees so their center is closer to the board and it’s harder for the water to drop them.

Valerya Milovanova / Windy.app

Since the effects of forces change depending on the position of the board and the surfer, they have to turn sideways to stay on the wave. If the center of gravity is in the middle of the board, it floats parallel to the water surface because its weight and buoyancy are balanced. But if you need to turn, you should stand on the back of the board, so the back edge will go deeper and the front edge will rise above the water, allowing through the slope to give the board a new direction.

You achieve a kind of dance with the elements, with dozens of tons of fast-moving water at the most dangerous «breaking» part of the wave, where the surfer slides down, then climbs up to balance the four forces of nature at once.

Text: Jason Bright, a journalist, and a traveler

Cover photo: Nele Viaene / Unsplash