Ocean waves, generated by the interplay of lunar and earth gravity, travel very long distances across open water. When a wave reaches land, having no more water in which to move forward, it breaks upon the shore. How many millennia have humans been observing this? A long time.
Standing in shallow water very near the shoreline, we would see the hapless bodysurfer waiting to experience the thrill of catching a wave and riding it the short distance until he and the wave are tossed onto the sandy beach. If you happened to be at Ka'anapali beach, Maui, Hawaii, yesterday, that adventurous bodysurfer would have been, perhaps, me, or one of the other hundreds of free-floating small-time adventurers.
The body surfer is not a real surfer, you know. He's just a clueless vacation visitor, not really serious about investigating the larger potentials of the great swells on the north shores of these far-flung islands. He doesn't have the board. He just has his own body, which he has trained to float. In my case, I learned to float at YMCA summer day camp in Jackson, Mississippi, long about 1956 or so.
Consider one wave coming in--the one that I'm going to jump into in just a few seconds here. How does this little dance between the wave and the bodysurfer work?
When the wave is still out in deep water where there is nothing to alter its somewhat ideal sine, or cosine, or bell-jar shape, it is a force of energy moving through the water, rearranging the shape of the water surface as it moves forward. It is moving the water somewhat, mostly up and down, at any chosen point of the ocean surface. Within the force of the wave there is kinetic energy moving those millions of molecules of H²0. But the wave is not really moving as much water, or anything else, as it actually has the physical power to do. So within the wave there is, along with the kinetic energy that is in constant use, potential energy.
So every wave that travels across the Pacific, approaching the beach, is a combination of both kinetic energy and potential energy. When the kinetic force of it hits the beach, the potential energy is suddenly converted to strong kinetic action and the wave totally expends itself on the sand. All that physical force erupts upon the shore and upon whatever happens to be there, be it a surprised snoozing sunbather, a sand castle, or a bodysurfer like me. Over long periods of time, this wave action churns rocks down into fine sand, and this is how we get our beaches. It also steals sunglasses and plastic cups and rubber rafts and other stuff that we consumers drag to the strand with us.
What the bodysurfer seeks to do is partake of that thrilling moment within the wave when its potential energy is instantaneously being converted to kinetic force, and thereby producing for him/her a few seconds of very fine sporting excitement. This occurs when the wave's natural shape is being violently altered by its contentious encounter with the sandy bottom. Entering shallow water, that potential energy has nowhere to put all those water molecules that were formerly being moved up and down in such a gentle, rolling manner. Suddenly there is no more "down" available to the wave, because where there was deep water before there is now something solid that does not yield to the wave's force. In this case, the "something solid" is the edge of the island, i.e. the shore.
So the rambunctious wave tosses all those water molecules up into the air, in a kind of tantrum. Like a spoiled child that has grown accustomed to having its way all the time, the wave shouts with much sound and fury that signifieth nothing, if I can't have my wavy way here, I'm going to throw all this liquid in the air! Waugh! Now it's splash and crash time.
So suddenly the formerly tame motion becomes an eruption of spray through the air, and foam upon the sand. That noble wave that hath traveled afar all the way from Japan or wherever--it just gives up the ghost and dies right there on the coast of Hawaii.
But not before this thrill-seeking tourist can get in on a little super-planetary wave action, ha! I love it.