Stone Skipping the Scientific Way

Quirk writes "National Geographic has a bit on the scientific analysis of stone skipping. Using a machine launching aluminum disks Lyderic Bocquet, a physics professor at the University of Lyon, and his colleagues discovered the 'magic angle' of 20 degrees as that required to maximize skipping. 'Jerdone Coleman McGhee of Wimberley, Texas, holds the current Guinness Book of World Records title for a 1992 toss that yielded an impressive 38 bounces across the Blanco River in central Texas'"

Comments

Just wondering . . .

[millisa]

I am horrible at skipping stones, but the best I ever got was on lake oneida up in NY right before it froze over (I think it was like 10 skips; yeah, I suck). I wonder how much the other type of degrees (temperature) effects things . . . physics/chem geek want to wax eloquent?

Well that's all fine and dandy...

[PoitNarf]

that a human can skip one 38 times, but there is no mention on how many times the machine they built was able to do it. Just watch, this is gonna lead to some wacky robotics competition where teams try to construct different robotic launchers to see which can skip more times or longer distance.

Re:Well that's all fine and dandy...

[Cat_Byte]

I live near a river (100 yards away) and grew up here. Personally the 38 skip record sounds weird to me since you can skip a small one many more times than that with the near flat angle of entry & make the skips about 1" apart. I've never counted but I would estimate over 50 from the continuous stream of splashes as it skims the water.

Now here is the kicker. If it's on a river the water isn't perfectly flat. I wonder if their "magic angle" took wave size into account? You really have to get a higher angle to keep it from diving into waves if necessary. No I didn't RTFA. It was /.ed.

Re:Well that's all fine and dandy...

[XNormal]

Now here is the kicker. If it's on a river the water isn't perfectly flat.

It depends on the amplitude of the waves. On each skip a random angle added or subtracted from the ideal 20 degrees. If this minimum of energy loss per skip around 20 degrees is relatively symmetrical you should still get the optimum at 20 degrees. For really high wave amplitudes you might hit the water at an angle that is too sharp and not skip at all so in those cases a shallower angle may be preferred.

Personally the 38 skip record sounds weird to me... ... I've never counted...

Consider the possibilty that your estimate is incorrect. Even 25 skips looks like "a lot".

All about salt water

[Anonymous Coward]

You centrally located people haven't experienced stone skipping until you have been to the ocean. Easy to get 15-20 skips in a calm inlet. Dense salt water makes it that much easier.

I'm sure at the dead sea you could really make 'em go.

Re:mass versus skip number

[inode_buddha]

Actually, I *have* skipped bricks on the Niagara river (living a few minutes from the Falls). To answer the previous poster about what the variables might be, just IMHO:

1. Angle of attack
2. area of rock surface
3. rate of spin
4. velocity
5. flatness of surface

Somehow these all interract; for example, its difficult for me to skip a stone below a certain weight/area.

Re:mass versus skip number

[lgbarker]

Some serious skipping was done during World War II. From http://www.kensmen.com/combatlessons4.html ".... In dropping bombs, the bombardier should allow for at least a 60 ft.. bounce and skip ..."

A well researched problem already?

[wrmrxxx]

The article suggests that this is the first time this type of problem has been scientifically studied. As far as I know this kind of problem has been very thoroughly studied for aerospace purposes: a planet's atmosphere is the pond, and a spacecraft is the stone. A google search for 'skip trajectory' shows up lots of serious research.

skipping massive objects

[Anonymous Coward]

I don't remember the details, but the History channel had a show on about a British aviation (WWII) engineer who was in charge of developing a plane and payload to destroy damns. The plane, flying low, would drop the payload 100's of feet before the damn. Skipping a dozen or more times, bounce off the target, roll back towards the base, due to inertia , and explode.

Very interesting, he had to find the best shape, weight, and attack angle.

Re:skipping massive objects

[rilister]

the 'aviation engineer' in question is Barnes Wallace. The 'Bouncing Bomb' was used in the successful attacks to flood the industrial Ruhr Valley in Germany, by destroying the reservoir dams. He chose 220mph, rather than 25mph. I guess the stalling speed of a Lancaster bomber is a little high for that.

I've skipped stones (at a precise 20degrees) at Ladybower reservoir in Derbyshire, UK, where the practise flights were made - like many WW2 bombing operations, there was no attention to 'collateral damage':

Guardian Unlimited
Two dams out of the three targeted - the Mohne and Eder - were breached. Thirteen hundred civilians were killed, including nearly 500 slave labourers from the Ukraine - mostly women. Local towns were flooded, trains washed off their tracks and six electricity stations put out of action.

You mean like dambusters?

[adept256]

The massive spinning bombs that were designed to bounce along the water before sinking and exploding in front of a dam? That technology was developed and used successfully in world war II by the english.

Dam Busting Bombs

[_aa_]

I recall watching a documentary about Barnes Wallis, a British scientist who during WWII invented and "perfected" a dam busting bomb. A rather large (multi ton) spinning cylinder full of explosives that would be dropped from a plane at remarkably low altitude over water directly at a dam at high speed, resulting in the bomb's skipping, like a stone, until it would collide with the dam. The bomb would then sink, but it's spinning motion would keep it tight to the dam until it exploded.

Wallis' research involved countless stone skipping tests, that inevitably resulted in the discovery of the perfect angle.

The bombs themselves enjoyed marginal success, succesfully destroying 1 of 3 objectives, if I'm not mistaken.

http://simscience.org/cracks/dambusters.html - Interesting videos and more information.

Re:I could see it. . .

[Bastian]

I mean, the trailing edge is the edge that will hit the water first if you're talking a 20 degree angle, and if the stone skipped, the center of force would have to be behind the stone's center of gravity (otherwise it would sink).

If it skips soon enough, it could be far enough behind the center of gravity to cause the stone to flip. But I doubt it happens all the time, because I can't see getting it to flip the same speed every time. If it doesn't flip by about 180%, the stone would soon hit at a bad angle and sink. The chances of even getting three or four skips in a row would probably be ridiculously small, but I can get at least that many skips fairly consistently.

skipping cannonballs

[tamarik]

We recently sailed out to Ft Jefferson in the Dry Tortugas. We read where the soldiers would heat cannonballs to red hot and shoot them at enemy ships. They even made an oven with 4 magazines in it for the job. They used layers of wet and dry padding between the poweder and ball in the cannons. My pics don't say what size balls but they were one of 12, 18, or 32 lbs.

These balls would skip along the saltwater and bury themselves in the ships at waterline level where the seamen couldn't get to them. The balls would then burn through the boats hull, hopefully starting a fire.

See, there were even geeks back then with a lot of time and resources on their hands. This must have taken a lot of practice.

Also visited Fort Pulaski outside of Savannah GA. These 2 forts were designed to be very similiar in so many aspects. But there is no mention here of this kind of ball skipping. Where Ft Jefferson is surrounded by water, though, Ft Pulaski only has it near in a 45degree arc, and that's more than a 1/4 mile away. The ships channel is out of cannonball range these days; maybe it wasn't back then.

Re:Don't the stones flip?

[dlakelan]

The stone is usually spinning around an axis that is more or less vertical. The angular momentum of the stone makes it much more difficult to flip over via forces exerted by the water interface. In order to have it flip over, the axis of the spin now has to become horizontal requiring a tremendous torque.

A stone fired at a lake with no initial spin might easily tumble in the manner you're describing, but probably wouldn't skip nearly as well.

missing something

[CAIMLAS]

It seems to me that they're missing several somethings that are fairly important (other than the physics of the stone): the velocity and spin of the stone (to say nothing of the water surface's dynamics).

That said, my personal record was achieved when I was 12 at a cub/boy scount camp. It was in a little river/creek (maybe 10 feet across, no deeper than 1' in most parts, with lots of smallish smooth disc-shaped stones perfect for skipping). My group was out hiking, and we had a competition. Everyone else was picking more roughly-shaped stones off the shore, and not venturing into the water.

Having grown up watching my uncles skip stones on their lake since I was very young, I probably knew a thing or two about stone skipping that the others didn't, simply by example. At any rate, I took a step or two out into the water, and grabbed the smoothest stone I could find.

This was all after the scout master said the person with the most skips gets a candy bar. IIRC, I was the last to have my turn at winning the candy bar. Everyone started bitching about how I was cheating because I didn't take the rock from the shoreline. (bah!) I got into the water, and got as close as I could to the water, and threw the stone upstream like a frisby.

The end result: 23 skips, at least half an hour of people trying to come close to half as many skips, and a candy bar for me back at camp. And a dozen pissed off cub scouts for 4 more days. :P

Re:mass versus skip number

[dunee]

Of course, once the government got hold of this technology, they would put it to use bombing Iraq.

Actually, that's been already done. Not in this war, and surprisingly, it did not involve Iraq (though bonus points will be awarded for proving there is a link after all).

The bombing method the Dambusters used during WW2 employed a similar principle of skipping stones.

Re:mass versus skip number

[Dylan2000]

Wish I had Mod points, cause that was funny. For all mods who didn't understand or who are reading at +2 please click and read about the British scientist who created a bomb which skiped along the water surface, impacted the wall of a dam, rolled down to the wall's base and then exploded, destroying an important German manufacturing area in WWII.

Great film, but also some awesome science.

Re:mass versus skip number

[xmedar]

It was Barnes Wallace who invented the Bouncing Bomb used against the German dams, though as the film makes clear he borrowed it from Horatio Nelson who used to bounce cannonballs off the water to get a more devestating impact against a target, as is made clear in the film, Barnes was a complete ubergeek, which is best summed up by an exchange in which a military man sat behind a desk asks him how he intends to get hold of a Wellington bomber to test his theory to which he replies "I'll tell them I designed it".

Are they sure it isn't 19 degrees, 28 minutes?

[Anonymous Coward]

That's the Kelvin limit angle. The angle the wake waves make from a boat.