An unexpected result of Tiger practicing his putting on someone else’s green.
Tiger Woods drives sales of physics book sky-high
A photograph showing a copy of Get A Grip On Physics by John Gribbin on the floor of Tiger Woods’s wrecked SUV has seen the book rocket up Amazon’s bestseller chart
We recently got a delivery of some electronics (OK, it was a bunch of lasers. You can’t build a death ray without lasers. Oh, wait, did I say death ray? Forget that. I meant clock), and the packaging for each had both a shock sensor and a freeze sensor.
Naturally, I had to take the freeze sensor apart. It’s a bimetal disc that’s either concave or convex, depending on how you look at it. The two metals have different coefficients of thermal expansion, and if you form them in a strip, this will cause them to bend like a springboard as one expands or contracts faster than the other. This is useful for thermostats and indicators.
In this case, the original shape is concave (let’s pick a convention) at room temperature, and that’s the minimum energy configuration. To change it to a concave shape by cooling, the metal on the outside has to contract faster, but it won’t do this continually, as happens with curve changes of flat strips — the other metal is at a low energy, and will have internal forces trying to keep it there. It’s only after the second metal can overcome those forces that the disc changes to convex. By that point, though, there’s a bit of energy stored up in the system, and as the disc pops into its convex shape, it pushes a red indicator button out to show that it has fired. You can calibrate this type of device, and by choosing the right manufacturing conditions, have different indicators fire at different temperatures — many are set to be triggered when the temperature drops below freezing, or thereabouts.
Here’s what it looks like once you’ve taken it apart.
The disc is covered by the white holder with the red plunger, and is covered by the paper mask and plastic container. The plunger pops out when the disc is thermally triggered.
Here’s what it looks like firing in slow motion. I discarded the container and paper mask, and put the disc directly on a cooler freeze-pak, with the holder and plunger resting on it.
Dot Physics has been assimilated by the Borg, but (unsurprisingly) Rhett, through his blood, sweat and tears, continues to post cool stuff. This time, it’s an angular momentum demo and explanation. I’ll post the video here as a teaser; go to the post for the explanation
I agree with a couple of commenters there — I also like the demonstration where you spin the wheel and sit on a rotatable stool, and then reorient the wheel.
What’s a Lagrangian? and something you can do with it: Harmonic Oscillator #2 – Lagrangian Formulation, at Built on Facts.
So why all that extra trouble? In this case, just to see how things work. But in other cases, this is our only option. There are plenty of times (for instance, a roller coaster on tracks) where the forces involved (such as the tracks on the car) are so blisteringly complicated as to be practically impossible to solve. However, it’s easy to write the relation between the track height and the potential energy, and the Lagrangian formulation can automatically give us a much simpler differential equation to solve.
Why triangular snowflakes grow
While most snowflakes are hexagonal, triangular forms have long been observed. When Libbrecht and Arnold grew snow crystals in the lab they found far more triangular forms than would be expected from mere random growth perturbations.
Air moving past a falling crystal will increase its growth, they note. A tiny factor – perhaps a piece of dust on the crystal – that causes a small change in growth of a perfect hexagon will also tilt the crystal, changing the way air moves past the crystal and – in their model – increasing the way the crystal grows at certain points and inducing a more triangular appearance.
Also: Unusual Snow Crystals. Be sure to check out the photo galleries.
Update: and The Unbelievable World of Snowflakes
Letters of Note: “He is a second Dirac, only this time human.”
Robert Oppenheimer’s letter of recommendation on behalf of Richard Feynman.
The reason for telling you about him now is that his excellence is so well known, both at Princeton where he worked before he came here, and to a not inconsiderable number of “big shots” on this project, that he has already been offered a position for the post war period, and will most certainly be offered others. I feel that he would be a great strength for our department, tending to tie together its teaching, its research and its experimental and theoretical aspects. I may give you two quotations from men with whom he has worked. Bethe has said that he would rather lose any two other men than Feyman from this present job, and Wigner said, “He is a second Dirac, only this time human.”
xkcd: Physics for Entertainment
Physics for Entertainment was written by Yakov Perelman in the 1920’s (in Russian) and updated periodically through the 1930’s. There are actually two parts to it, but Volume 1 is long out-of-print (though findable online — more on that later). The book I have is a 1975 translation of Volume 2. The book is a series of a few hundred examples, no more than one or two pages each, asking a question that illustrates some idea in basic physics.
Rainbow trapped for the first time
In 2007, Ortwin Hess of the University of Surrey in Guildford, UK, and colleagues proposed a technique to trap light inside a tapering waveguide, which is a structure that guides light waves down its length. The waveguide in question would use metamaterials – exotic materials that can bend light sharply.
The idea is that as the waveguide tapers, the components of the light are made to stop in turn at ever narrower points. That’s because any given component of the light cannot pass through an opening that’s smaller than its wavelength. This leads to a “trapped rainbow”.
Goldbogen and his colleagues found that big fin whales are not just scaled-up versions of little fin whales. Instead, as their bodies get bigger, their mouths get much bigger. Small fin whales can swallow up about 90% of their own body weight. Very big ones can gulp 160%. In other words, big fin whales need more and more energy to handle the bigger slugs of water they gulp. As their body increases in size, the energy their bodies demand rises faster than the extra energy they can get from their food.
This scaling may explain some of the weird diving patterns found in lunge-feeding whales. Blue whales are twice as big as humpback whales, for example, but both species dive for the same period of time (about eight minutes) and to the same depth (148 meters). All things being equal, you’d expect that blue whales would be able to dive deeper and longer, because they could store more oxygen in their bigger bodies. Blue whales also make fewer lunges than humpback whales (6 versus 15). It’s possible that the gigantic blue whales are hard up against a size limit. They need so much energy for their lunges that they can’t afford to hold their breath longer, and they can only manage to make a few lunges before they run out of reserves and have to head for the surface.