# How do stones skip?

I know how to do it, I'm just wondering what's happening.  It's spinning and .....????

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BIGHAIRYDUDE7 years ago
MAGIC!
mole1 (author) 7 years ago
Can I split 'Best Answer' between kelseymh and RedneckEngineer? You've both been great at clearly explaining all this to someone with zero background.
If either of you said you were professional teachers I'd believe it. You're really good! Thank you!!
7 years ago
Well I'm honored to even hear this. I'm not a teacher nor am I as well versed in physics as the local physicist, kelseymh. So thanks for the compliment. Just wish it would have been something harder than this. :)
RedneckEngineer7 years ago
The stone is hitting the surface of the water at such an angle and speed that the forward momentmen overcomes the pull of gravity and bounces off the water. Each time the stone hit the water it is slowed down till gravity wins and pulls it in to the water.
mole1 (author)  RedneckEngineer7 years ago
OK, I'm with you so far, but why does it bounce up instead of just skid across the water?
7 years ago
If you throw a stone at the ground does it bounce back up or skip? It can't flow through the ground so the forces moving it have to be redirected in another direction till the energy is gone. Same line of thinking in the water skipping. Once the stone sinks the energy used is dispersed into the water to make waves. It's all really simple examples of Newtons laws.
mole1 (author)  RedneckEngineer7 years ago
Oh. I never thought about it, but tossed rocks usually DO bounce. So the water is just another solid as far as the stone is concerned - until it slows down?

I really appreciate your explanations. I've never studied physics. It sounds like I missed a lot of good stuff.

So how does the spin of the stone fit into all this?
7 years ago
The spin is just a result of being thrown. It's not really needed per say, but it can help. Just as a bullet spins, when it hits off a wall and bounces the spinning has nothing to do with it, just the angles involved. I've not studied physics but I have read a lot of books and I feel on something like this it's pretty simple.
7 years ago
Physics is all the good stuff! It's how the world works, where we came from and where we're going. Everything is physics, either right on the surface (planes, cars, skipping stones) or way deep down (nerve impulses, nuclear reactions).

By the way, I'm a physicist :-)
7 years ago

To your first question, "So the water is just another solid as far as the stone is concerned - until it slows down?" Yes, indeed. At high speed (high Reynolds number) the fact that water is incompressible means that it can't "get out of the way" and behaves like a solid. Think about doing a belly flop from a high-diving board; same thing.

To your second question, "So how does the spin of the stone fit into all this?" The spinning isn't relevant to the stone bouncing off the water. It is relevant to the stone keeping a fixed orientation in flight before it hits the water. The spin (just like the spin of a Frisbee) gives the stone gyroscopic stability -- a spinning disk has a high resistance to being tilted -- so it will hit the water flat, just the way you threw it.

If you throw a sphere (like a small golf ball, or a "skipping bomb") you don't need any spin. The object is symmetric, so no matter how it rolls or tumbles in flight it'll hit the water the same way.
mole1 (author)  kelseymh7 years ago
Thanks! I did check out the link earlier, but I missed seeing the point of the spin.

So a marble could skip on water if it were going fast enough and struck the water at about 20 degrees?

One more bit of beach-lore to check out with you guys... I've been told that the cross section of a skip-able stone has to be more or less a flattened oval. The edges have to be rounded and the bottom has to be at least slightly convex. True?

7 years ago
Yes, a marble could skip on water. It would be interesting to do the calculations. You might check out the two "see also" links at the end of that article, on different types of "skipping bombs", which are spherical or cylindrical.

I don't know whether the shape is really critical to the skipping process itself, but I can make some guesses. First, that shape is going to be easy to hold in the hand, and to throw flat with spin (so it'll keep a fixed angle of attack). The convex bottom means that the impact angle can be more variable, since the rock will still hit the water with an appropriate shallow tangent.

You should probably look at some of the citations in the Wikipedia article, and read the primary literature on the subject, for more definitive answers.
mole1 (author)  kelseymh7 years ago
Thanks again. I don't know about calculations, but trying a sling shot with a marble at a beach sometime might be fun.
Skip bombs certainly answer the question about shape... never would have guessed they could skip.
The article in Nature looks like it would be good. I'll try to get access to it.

7 years ago
Nature articles are generally behind a pay-wall :-( One thing to try is to use Google Scholar to find the article, then click on the "Cached" link to get the version Google stashed on their own server. Also, if an article is physics-related, you can look for a preprint on arXiv (search by title or by author).
lemonie7 years ago

Water is a funny-thing, it can behave in weird ways. Stones "bounce" off a flexible surface.

L
kelseymh7 years ago
http://en.wikipedia.org/wiki/Stone_skipping