The physics of wakeboarding and water skiing

Recently we told you how many forces act on a surfboard. Today, let’s look at why people don’t fall off when they are wakeboarding and water skiing—well, when they’ve already learned not to fall.

How to overcome gravity

In the physical world, Newton’s third law tells us that every action has an equal and opposite reaction. You push something, and it pushes back.

Objects—like a wakeboard and seawater—repel because their particles, their electrons, are so close that they try to take each other’s place, which is impossible, so a repulsive force occurs.

But let’s start at the beginning. When a wakeboarder is lying in the water waiting to be pulled up by a boat, there are only two main forces at work—gravity, which pulls them underwater, and buoyancy, or Archimedes’ force, which pushes them out of the water.

Gravity (green arrow) pushes down, and buoyancy (blue) pushes up. Source photo: monstertower.com

At this point, the forces are balanced: buoyancy compensates enough for gravity that the person doesn’t sink completely, because the water beneath them is denser than they are with their board. But if they stand on the board, their center of gravity will be above the water, their balance is broken, and gravity will be stronge—they will slowly sink into the water until the forces are balanced again.

The situation changes when the boat starts to move; the rope tightens, and begins to drag the wakeboarder behind it. Here, another struggle unfolds, this time between buoyancy and the friction of the water, which resists the movement of the board or waterskis.

The balance is disturbed—the wakeboard starts pushing the water forward and down, the water pushes it backward and upward, and gravity is so weak that it only can watch.

Water friction (the yellow arrow) pushes the wakeboard back and down, and buoyancy pushes the wakeboard forward and up (the red arrow). Source photo: blog.decathlon

To make the board brake less against the water, the angle should be reduced—for this purpose, the wakeboarder stands on the board to make it almost parallel to the water. On average, it is 10 degrees.

When the speed of the boat becomes constant (and higher than at least 1.3 meters per second, to compensate for gravity), and the board or skis can be stood upon, a new equilibrium occurs—water friction and buoyancy compensate for each other, so that the board glides on the surface of the water.

Somewhere out there, of course, gravity still wants to sink everyone, but no one listens to it anymore.

The power of the rope

In wakeboarding and water skiing, everything always depends on the tension of the rope—because it is the tension inside the rope, the unwillingness of its molecules to break the bonds with each other, that pushes you forward.

The tension depends directly on how much force is acting on the rope; that’s why you should just hold on to it with relaxed hands, and not pull with all your might, because otherwise the rope will pull your hands back in the best tradition of Newton’s law, and you will fall in the water headfirst.

Photo: Jack van Tricht / Unsplash

And when you let go of the rope, the balance is lost—the friction of the water becomes too strong, buoyancy ceases to compensate for gravity, and you find yourself already slowly sinking into the water.

...only to get back on the wakeboard or skis a few minutes later.

Text: Jason Bright, a journalist, and a traveler

Cover photo: Tim Mossholder / Unsplash