The Physics of Surfing

“While I get to work a top Mauna Kea, Jorge gets a surfing lesson from Assaf Azouri, who studies the physics of ocean waves.

“We are PhD comics and we want to know why…”

“Good, without letting go, get the back foot flat. Stand up and turn your body this way and drop your butt down. There you go- that’s your stance. Yeah, you got it.”

“Well I’m working in oceanography, in particular I study physical oceanography, which is basically trying to understand physics of the ocean, the movement of currents around the ocean, and waves. I’m interested in what we call seiche, oscillations of the sea level on what we call the normals. If you force these oscillations at certain frequencies we can have oscillations that are very amplified within the harbor. As a result, these can cause damage to boats that are slamming against or against the docks. We just try to understand what is the mechanism that is involved in generating these very energetic seiches.”

“Where do waves come from?”

“First of all in order to have what we call ‘surfable’ waves, you need to have a large storm the ocean with very strong winds that blows over the ocean surface for a long period of time; over a very large area, if possible, and over long distances. So with a combination all these three you will have large waves that will reach the coast. Now, once you have these conditions, you will have a wide array of waves that have different wavelengths and heights that are inside the storm region. Once they leave the storm area, they start to be more organized because these waves have a property, that’s called dispersion, that the longer wave travels faster than the shorter wave.”

“Faster?”

“Faster.”

“Why is it that longer wavelengths travel faster?”

“That’s how the physics of the system works. Mathematically, that’s what you get from solving the Navier Stokes equation.”

“Really?”

“Yeah, you have to apply boundary conditions on the upper surface of the ocean and the bottom. And then you get a solution that gives you the dispersion.”

“So there’s a storm over here - it’s blowing, it’s kicking up water like crazy, but the longer waves kind of break out because they travel faster.”

“They travel faster than the shorter wavelengths.”

“So that’s why at a beach like Waikiki, you have these nice beautiful breaks, because they all come from very far away from whatever conditions they were in…”

“Yes especially in the south to waves are more separate from each other, but this is changing once you reach the shallow region. When this is happening then waves are traveling at the same speed no matter what is the weather.”

“What are the physics of surfing? Do you ever think about physics when you’re surfing?”

“I try mostly not, because I will get distracted and won’t catch too many waves. But I think the most important forces are basically gravity, the force that pulls you downward, buoyancy force that’s basically pulling you upwards, and you also have the you have also the drag force which is the friction, it goes against the direction that you’re moving. It’s a balance between forces, just those three forces.”

“So how do I catch a wave?”

“Ok, so basically this is the balance that you always have when you’re on a surfboard. Even if you’re sitting just moving a little bit, you have this same balance. But when you want to catch a wave, the first thing you want to do is to obtain a speed that is equivalent to the speed of the wave, so you have to accelerate from rest. And once you are starting to move look at the crest of the wave front of it you want to use its acceleration downwards starting down the face of the way and at certain point you reach a point that you can move the same speed of the wave and that’s when all of these forces are equal. you don’t have any more acceleration, so the gravitational force balances exactly the drag force and the buoyancy force or lift force.

When I’ve done enough work, I can take a short break for a couple of hours and just go surf. That’s the nice thing about living in Hawaii you know.”

“It’s hard to beat this.”

“Very hard.”

“You said your professor also surfs, right?”

“Yeah, he does surf, but I never surfed with him yet.”

“But you never asked him?”

“No, we just talked about surfing, we just never surfed together.”

“Would that be weird?”

“Maybe, but really fun also.”

“Alright, Assaf, let’s go surfing!”

“Okay, sounds good.”

“How did you get interested in waves?”

“Surfing is pretty popular in Israel, it’s where I grew up.”

“Really, Israel?”

“Yeah, the waves are not that great, but they are much better than one would think for this part of the world.”

“My undergrad patting my board. Everyone can tell the beginner with this board”

“Yes, we can with the board like that.”

“How did you get into the study of waves?

“I started my PhD in the physics department but at some point I realized I want to do something slightly different. So I found out about the physical oceanography division. So I thought it would be nice is going to be the application of physics to something that I connect very much- the ocean.”

“What do you mean you connect to the ocean?”

“I have never lived more than like 5-10 minutes drive from the ocean always felt the need to be close to the ocean. You don’t think about many things when out there, just when’s the next wave coming and what I’m going to do a wave. Have great time away from anything else that happened before or, you know, a great distraction from reality.

“How did you pick where to go for graduate school?”

“Well it was very hard for me to pick because I was looking for a place that combined a few elements that are very important to me. And the first thing I wanted to find a place that has great waves because that’s the thing I like most.”

“Did you find yourself we’re not being able to go surf because you’re so you’re simulating wave on the computer?”

“Not simulating but yeah. I just basically analyzing data that I’ve converted to wave heights and periods and things like that so I’m analyzing data that is related to waves in the ocean. So I’m doing something that is related and interests me more, you know. So when I go surf I feel like I want to know even more to understand what I am doing.”

“So when you’re surfing it’s actually research?”

“You could think of it like that but I try to keep that on the desk, you know. When I go out in the water I maybe try to think ‘oh, how this is working’ and try to understand better so I am thinking more. But in the water, I just don’t think too much. I just surf.”

“Just surf. Yeah, I’m jealous. I wish I had done my PhD here.”

“You could get another one.”

“Another one? I will not be doing that – even for Hawaii.”