Why don’t certain stones, and pieces of shale, etc. sink when hitting the water? How it is possible for the stones to skim?
How do you mean? Some volcanic rock floats because of gas bubbles trapped inside. But I think you’re talking about throwing stones at tthe water so that they skip over the surface. That is due to the shape to the stone and how it is thrown, not its composition.
I’m not sure of the exact physics, but I’d WAG it’s similar to the physics behind the re-entry angle window a space capsule must be within to avoid “skipping” off the atmosphere and off into outer space. Fluids (liquid or gas) often exhibit similar behavior, so it shouldn’t be too surprising.
The physics must depend on the geometry of the rock, the smoothness (or choppiness) of the water, velocity, etc.
- Jinx
When the rock hits the water, it has a slant so that its front edge is higher than its trailing edge. As it sinks into the water, its forward speed is causing the water in its path to be deflected downwards, which pushes the rock up. It’s effectively a bounce, its downward momentum causing it to sink farther than it would under its own weight, resulting in a greater lifting force, so it gets launched again.
In other situations, this would cause the rock to tumble, since the force is mostly on its trailing edge. But since the rock is spinning, it resists tumbling and keeps spinning in mostly the same axis until it bounces again. The force on the rear edge does cause precession, though, and the rock starts tilting to one side somewhat.
I wouldn’t, since it’s only a WAG, but this is one of my all-time favorite pastimes, so here goes:
The shape of the stone is important for the best skipping (sorry, that’s what I’ve always called it). The ideal stones in my experience have large, flat, but most importantly smooth bottoms. A relatively flat top is best, but of slightly lesser importance.
I have the best luck with stones which are, believe it or not, relatively dense. Force equals, if memory serves, mass times acceleration, so as a rule, you’ll get more for your money with a denser stone.
The water should be reasonably calm, but don’t think this means swimming-pool-first-thing-in-the-morning-or-bust. I’ve had great success skipping stones into the surf when I was able to get a few square yards of smooth water ahead of the breakers. Recently, I managed to get a stone to skip right over the waves that were crashing into the beach and into the relatively calm water behind them. Way cool.
Now, to your actual question. My WAG is that it’s the combination of forces at work. Forgive my weak physics as I try to explain this. A correctly thrown stone will have be moving almost parallel to the surface of the water. Gravity acts on it, but up to a certain point, the stone will continue moving across the water. When the stone strikes the surface of the water, the density of the water and (maybe) surface tension combine to make bouncing back up into the air the physical path of least resistance, though it loses some velocity in the act. This process continues until the velocity of the stone can no longer overcome gravity and it finally sinks.
It can be done with objects that are not flat at all, as Mr Barnes-Wallis demonstrated.