Waterskidding

Now that’s a neat little trick. Once again that characteristically Japanese sense of humor gives us an opportunity to glimpse some interesting physics in an entertaining venue. Of course we aren’t too surprised to see a water skier plane over the water, or to see a rock skipping esthetically across a placid pond. However, a water slide entry propelling someone into an “unaided” skid across a swimming pool seems a more rare and special event — even though the physical principles are the same.

Although describing the specifics of the motion of an object across the surface of a fluid can be rather complex, we can distinguish between whether an object will sink or slide with a simple application of Newton’s second law: In order to stay on the surface, the upward force exerted by the fluid must be equal to the downward force of gravity (the weight force) acting on the object.

Let’s look at a water skier. If he simply stands on his ski in the water, he sinks part-way below the surface. In this case the upward force exerted by the water surface is not enough to support his weight. As he begins to sink, the buoyant force of the water will increase until it balances his weight. At this point he floats, but he’s partially submerged. Once the boat starts pulling him, however, Newton’s third law is going to help him plane along the surface. It works like this: As its velocity increases, the ski pushes increasingly harder into the water. The third law — the principle of action and reaction — tells us that the water will push back on the ski equally hard. With sufficient speed, this force will balance the weight force, and the ski and skier can merrily plane along the surface of the water indefinitely, or until the skier gets tired.

In the video, it’s the same idea. Each of these intrepid water-sliding daredevils needs to be moving fast enough to stay on top of the water. However, in the preliminary unsuccessful attempts, the fellas simply apply too much drag when they hit the pool. Once they slow down, the game is over. However, notice the beautiful aerodynamic form of our champion. As he comes off of the slide he holds his streamlined legs just above the surface, there is very little drag, he maintains speed and heading, and he successfully planes across the pool.

Next time you’re at the water park give it a try. You might like it!

Adam Weiner is the author of Don’t Try This at Home! The Physics of Hollywood Movies.