Cracking the Story of Fracture

A crack slicing through a brittle material may be a more complicated process than researchers previously thought. A team reporting in the 29 January Physical Review Letters wedged apart a piece of Plexiglas and saw three different fracture processes, depending on the speed of the moving crack. Their results help to further the basic physical understanding of material fracture and, ultimately, material failure.

Update: How could I have missed that the story was written by sciencegeekgirl?

Uncertain Principles: Entanglement Happens

(Oops. I accidentally hit publish instead of save yesterday, and didn’t add my comment. Hence the time travel on the post.)

There are several application people try and use entanglement to pull off some neat trick (e.g. clock synchronization) and the problems Chad points out are often glossed over. It’s almost like the famous Sidney Harris cartoon, “Then a miracle occurs,” when the step in the procedure is “we take our entangled particles and move them an arbitrary distance apart,” which ignores the little demons Chad describes, that compromise your ability to know the measurement basis after your system has been exposed to the real world. It’s not quite at the level of ignoring the second law of thermodynamics and proposing a perpetual motion machine, but it’s almost as naive to propose it for an arbitrary system, without working out the details.

And even to the bad ones.

People in this area generally don’t know how to drive in the snow. The generally slow clearing of snowy conditions definitely does not help, but folks just aren’t helping things by sucking at driving. Between the time I was excavating my car and the recent trudge over to a local fast-food restaurant (I won’t blatantly advertise the chain — for free — but according to Concrete Blonde, their iconic spokesperson is going to die tomorrow), I have seen and heard dozens of people slip and slide their tires over the snow and ice.

Today’s public service lesson is Friction.

The details of friction on the microscopic level are quite complicated, but the general idea is this: the frictional force is proportional to the normal force (N) on an object. The normal force is that force the surface exerts on it, so for an object which is not accelerating vertically, the normal force and weight (W) cancel (the net force being zero). The proportionality constant for the frictional force (f) is the greek letter mu; I’ll just use u since I don’t want to muck around with LaTex. f = uN (Who said physics isn’t fun? It says it is, right there!)

It turns out that there are two categories of friction: static friction, for when an object is moving, and kinetic, or sliding friction, for when two surfaces are moving over one another. Generally speaking, the coefficient of kinetic friction is smaller than that of static friction, and the implication of this is that (all things being equal) once two surfaces start to slide over each other, they will continue to slide. They won’t suddenly “catch,” unless there is a change in the surface conditions.

So when your tire starts to slide on the snow or ice, or any other surface for that matter, it is going to continue to slide. This is the concept behind pumping your brakes when you start to skid, and the reason that anti-lock brakes are a popular safety feature. Once you start to slide, continuing that action is the wrong thing to do, much less gunning your engine to make the wheels spin really fast (coefficients of friction can decrease with speed, and melting the snow makes for a slipperier surface). Your best bet is easing up off the gas and starting over. Rock back and forth a little so there’s some momentum, and you don’t need quite as much force to get going faster. Once you start to skid, you’ve lost.

Electric Charge Can Change Freezing Point of Water

[P]revious experiments to understand whether electric fields can influence freezing were complicated by the materials used. The best materials for holding electric charge are metals, but as anyone who has tried to open a car door after a snowstorm knows, ice forms easily on metals even without a charge.

“If you try to do it with metal, you don’t know what is from the electric field and what is from the metal itself,” Lubomirsky says. “We wanted to know whether it is the charge that does it, or something special in metal.”

Instead of metal, Lubomirsky and his colleagues used a pyroelectric material, which can form a short-lived electric field when heated or cooled. The researchers used four pyroelectric crystals, each of which was placed inside a copper cylinder. The bottom surfaces of two crystals were coated with chromium to conduct an electric charge, and the other two were coated with an aluminum oxide to keep the surface uncharged.

An obvious question, not addressed by the article, is why you couldn’t (or rather, why they didn’t) create an external field separate from the surface being used for the condensation and freezing, and see how that affected the freezing point. This doesn’t seem to differentiate between surface and bulk effects.

Turtle Universe: Neutrinos again

So now you’ve got this ice, you’ve got these muons made by muon neutrinos, you’ve got this blue glow. The ice below the South Pole is probably the purest and clearest in the world. There’s nothing to compete with this blue light, and it just lights up that ice, traveling a great distance through the crystal clear solid water. And when it comes to a detector (called a DOM for Digital Optical Module), that detector grabs the blue glow and stores it away. You’ve just detected a neutrino!

OK, so what? So you’ve just detected a neutrino. Big deal.

It is a big deal, and here’s why. Neutrinos weigh almost nothing. Almost. We now know that they have a tiny, but real, mass. Why? Because of Einstein again. Any particle with zero mass travels at the speed of light, but any particle with a real mass, no matter how tiny, travels slower.

Of course, any post entitles Neutrinos Again must have a prequel: Neutrinos