Category Archives: Physics

A Man's Vacation Home is His Sandcastle

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How to construct the perfect sandcastle

Just a bit of water enables one to turn a pile of dry sand into a spectacular sandcastle. Too much water however will destabilize the material, as is seen in landslides. Here we investigated the stability of wet sand columns to account for the maximum height of sandcastles. We find that the columns become unstable to elastic buckling under their own weight. This allows to account for the maximum height of the sand column; it is found to increase as the 2/3 power of the base radius of the column. Measuring the elastic modulus of the wet sand, we find that the optimum strength is achieved at a very low liquid volume fraction of about 1%. Knowing the modulus we can quantitatively account for the measured sandcastle heights.

Snell's Window on the Olympics

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Olympic Swimmers

I did a bit on Snell’s Window last summer, and you can see that effect quite clearly in photos 1 and 8. If you look carefully at photo #6, you can see the edge of the window at the top of the shot. Below that there is the total internal reflection, which is quite obvious with the first two swimmers. Now imagine you’re a predatory fish, and your target is swimming away from you and approaches the surface — he would see another fish come into view — the reflection, and if the fish then breached the water it would disappear completely, except for any glimpses possible from the disruption of the surface.

Toys in the Office: DIY Edition

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I have an airzooka air cannon as part of my office armament, and it does an admirable job of shooting projectiles of air. However, one of my colleagues had expressed an interest in upping the ante, so we took it upon ourselves to make one using a 5-gallon bucket.

We followed the general path of the steps outlined in an instructable, though we substituted light-duty bungee cords for the elastic.

Cutting out the end of the bucket is a tad messy with all the plastic shavings. In cleaning up my clothes I employed a version of a trick we used in the navy when getting ready for an inspection — a few windings of tape on your hand, sticky-side out, does a good job at grabbing lint. Or plastic shavings.


Another deviation from the basic instructions was that since we used bungee cord, we drilled holes in the bucket for the anchor end, and zip-tied the other ends together, having removed the hooks. That also allowed us to use 3 lines.

The handle is temporarily an optics post. The bucket is just strong enough for it, so we need to add a second layer of something to shore that up and put both a proper handle on the body and a proper grip on the elastic. No metal, though, since a failure of some sort could be very bad news if this became a slingshot. We should be ready for our picnic next week.

That Shifty Hydrogen Atom

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Landmarks—Lamb Shift Verifies New Quantum Concept

In the second quarter of the 20th century, quantum theory faced some serious challenges, including unexplained details of atomic spectra and difficulties calculating basic properties of charged particles. In 1947 Willis Lamb and Robert Retherford of Columbia University discovered an unexpected detail in the hydrogen spectrum, later called the Lamb shift, that became an essential clue in solving both problems. The measurement agreed with new calculations and was the first indication that the theoretical approach called renormalization could resolve the mathematical infinities that had threatened to derail the progress of quantum mechanics.

Pew Pew Pew

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BELLA Laser Achieves World Record Power at One Pulse Per Second

On the night of July 20, 2012, the laser system of the Berkeley Lab Laser Accelerator (BELLA), which is nearing completion at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), delivered a petawatt of power in a pulse just 40 femtoseconds long at a pulse rate of one hertz – one pulse every second. A petawatt is 10^15 watts, a quadrillion watts, and a femtosecond is 10^-15 second, a quadrillionth of a second. No other laser system has achieved this peak power at this rapid pulse rate.

Impressive. Multiplying the two, we see that this is ~40 Joules of energy per pulse, which isn’t a lot of energy, but getting the pulses short and repeating them is the hard part. Lots of interesting things happen when you get a large energy density — there are nonlinear processes that depend on some exponent of field strength.

Can't Wait for "Dawn of the Flick II"

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Dawn of the Flick: The Doctors, Physicists, and Mathematicians Who Made the Movies

Today, we know that persistence of vision does not work like the progenitors of film thought it did (the retina does not retain images like Dr. Paris and others theorized), and many movies are filmed in 3-D and displayed on screens that are sometimes five stories tall. But as the technology of moving images has progressed and the group experience of attending a film has gotten increasingly sophisticated, a parallel trend has emerged, namely, to permit anyone to watch anything, anywhere, on a tiny cell phone. Curiously, this medium that was once limited to the single viewer appears to be circling back to that one-at-a-time format. The irony is not lost on Balzer. “It’s a very interesting loop,” he says, pun intended.

So Do You Feel Lucky, Mr. Fly?

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Seeing as this is a bug-a-salt, the most powerful salt gun in the world, you have to ask yourself one question … do I feel lucky.

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Bug-A-Salt

A little BotE calculation: flies are of order 1 cm, and salt grains around 100 microns. Scale this up to human size and it’s like being hot by a chunk of salt 15-20 mm across, or around 75 caliber. At a pretty decent clip, too. So, ouch! And several chunks, really, so (n * ouch), which is why it can do in the fly. I may have to break down and get one of these and use my slow-motion camera to measure the muzzle speed. All in the name of science, of course.

h/t to Moontanman