phyz:

Why is it Dark at Night?

Have you ever wondered why you look up and see a dark sky at night?

What a good video!

likeaphysicist:

How wings really work

It’s one of the most tenacious myths in physics and it frustrates aerodynamicists the world over. Now, University of Cambridge’s Professor Holger Babinsky has created a 1-minute video that he hopes will finally lay to rest a commonly used yet misleading explanation of how wings lift.

“A wing lifts when the air pressure above it is lowered. It’s often said that this happens because the airflow moving over the top, curved surface has a longer distance to travel and needs to go faster to have the same transit time as the air travelling along the lower, flat surface. But this is wrong,” he explained. “I don’t know when the explanation first surfaced but it’s been around for decades. You find it taught in textbooks, explained on television and even described in aircraft manuals for pilots. In the worst case, it can lead to a fundamental misunderstanding of some of the most important principles of aerodynamics.”

To show that this common explanation is wrong, Babinsky filmed pulses of smoke flowing around an aerofoil (the shape of a wing in cross-section). When the video is paused, it’s clear that the transit times above and below the wing are not equal: the air moves faster over the top surface and has already gone past the end of the wing by the time the flow below the aerofoil reaches the end of the lower surface.

Read more.

(via phyz)

science:

This is taken from the excellent PBS/BBC documentary Absolute Zero, which you can view in its entirety on YouTube. This particular segment concerns the strange properties of helium at temperatures just above absolute zero.

Helium becomes liquid at around 4 K. Below 2.18 Kelvin, or -270.97 degrees Celsius, it turns into a superfluid. Now, ordinary fluids always have some measure of viscosity, or resistance to flow: if you drop a heavy object into water, it’ll make a big splash and sink relatively quickly; in thicker fluids like honey, it will make a smaller splash and sink more slowly, since honey has higher viscosity. A superfluid has zero viscosity. In the video, you see an “eternal fountain” of helium that keeps flowing until the temperature goes above 2.18 Kelvin, and the way a thin film of liquid will escape unsealed containers, seemingly defying gravity.

Superfluids have another interesting property: something called second sound. In superfluid helium, heat transfer doesn’t occur in the usual way, by diffusion. Rather, it occurs in a wave-like fashion: where pressure waves make up sound, waves of entropy make up second sound. This allows extremely high thermal conductivity: heat flows through superfluid helium a thousand times faster than through copper. There is no known material that can conduct heat faster than superfluid helium.

The entire program is well worth a watch. It starts four hundred years ago, with the first scientific theories of just what heat is, and moves up to present day experiments at tiny fractions of a degree above absolute zero. It runs 1h 40 min, but if you don’t have that much time, the above clip is one of the highlights.

I LOVE this video, I saw it my freshman year and now I’m working towards making this subject my life’s work.