The science of surfing – Part 2
Why do you have to paddle?
Whether you’re on a surfboard or a bodyboard, if a great wave is heading towards you, you have to paddle like mad to be able to catch it. In other words, you have to be traveling with some speed and momentum as the wave hits you to stand any chance of riding along with it.
Why is that?
To travel with a wave, you have to accelerate to the speed it’s traveling. In other words, you have to gain a certain amount of kinetic energy very quickly. If you’ve already got some kinetic energy to start with—if you’re already moving when the wave catches up with you—it’s much easier for the wave to accelerate you a little bit more. Or in simple terms, the faster you paddle, the more likely you are to catch your wave.
Why can small kids ride small waves?
Have you noticed how young kids can ride almost any waves—but older ones can’t?
It’s back to momentum again. To move you forward, a wave has to give you a certain amount of momentum and energy. Both of these depend on your mass (how much “stuff” your body is made from). The bigger you are, the more energy you need to travel at a certain speed. So the older and bigger you are, the bigger the waves you need for surfing—because bigger waves can supply you with more energy. If you’re younger or smaller, you need less energy to move at the same speed, so a smaller wave will do the job.
Why do waves suck you backwards?
At school you learn about two kinds of wave. There are waves like sound, which travel by a sort of push-pull process, making patterns of alternately squeezed up dense air (compressions) and thinned-out, less-dense air (rarefactions). Sounds wave are called compression waves; they’re also called longitudinal waves, because the air molecules that carry sound energy move in the same direction as the wave travels. Then there are waves like light that travel in a familiar, up-and-down pattern. These are known as transverse waves, because they vibrate at right angles to the direction in which the wave travels.
But ocean waves are not like the waves you learn about at school. They move the water surface round in circles as they travel along. Watch a seagull sitting on the ocean as a wave approaches. The gull is sucked backwards up the front of the wave, lifted onto the wave’s crest, pushed forward as the crest passes by, and then lowered down to pretty much the same place it started off in. This happens because ocean wave energy is not traveling purely on the surface of the ocean: it also affects the layers of water underneath. You’ll have noticed this sucking effect if you’ve ever caught waves on a bodyboard. As you lie on the sea surface, you’ll feel yourself being pulled backwards as a wave approaches. This is another reason why you have to be paddling forwards to catch a wave. If you’re not paddling forwards, you’re definitely going to get sucked backwards!
Photo: Layers of water slide past one another at the shore. At the top of the beach, water is sucking back down into the sea. Just offshore, a small wave is breaking inwards and up the beach. In between, another small wave has just broken and is coming to a halt. There are at least three layers of water sliding over one another here. In these calm conditions, the liquid layers are moving in what physicists would call laminar flow.
We’ve reached the end of part 2 of our series about the science of surfing. Now you’ve made it this far, read the last part here.
Already read enough? Come surfing with us while in Phuket. Read more about our surf lessons here.