Forces!

When you surf, there are several forces at work. The forces of gravity and buoyancy are working against each other. Gravity is pulling the surfboard down. Buoyancy is pushing it up. Which force is greater? It depends on the density of the surfboard’s materials. As you can guess, the materials used in surfboards today are much lighter and less dense than the wooden materials used in the past. Surfboards are also less dense overall than water. This is what allows them to float in the first place!

Of course, the body of the surfer adds weight. Weight is equal to mass times the force of gravity (9.8 m/s2). If a surfer sat perfectly still on a surfboard in the water, that surfboard would likely begin to sink. Keeping a low center of gravity is one of the most important techniques to be a good surfer. To remain balanced and stable, surfers crouch, keeping their center of gravity low. Surfers can also change their position on the board. If a surfer stands in the middle of the board where buoyancy and weight are balanced, the board will be flat. If the surfer moves back, the nose of the board will go up, making it easier to steer.

Meanwhile, drag from the water creates friction in the opposite direction a surfer is moving in. This helps the surfer control the surfboard’s movement.

So how do you use physics to catch a wave? When you are lying on a surfboard, waiting to catch a wave, you are at rest. But when you see a wave approaching, you begin to paddle your arms so that you can move towards it. You accelerate until your velocity is the same as the velocity of the wave. At this point, gravity, buoyancy, thrust and drag are in perfect balance.