This Fire is Cool

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Researchers experimenting with flames onboard the International Space Station have produced a strange, cool-burning form of fire that could help improve the efficiency of auto engines.

Don't You Fly Off the Handle

It’s Plane To See …

Joe runs a popular blog with lots of neat stuff, but I have to pick a nit here. Well, honestly I think it’s more than a nit.

When something spins while being tethered in place by gravity, its mass wants to fly outward into a pizza-like shape, like frosting flying from errantly-aimed eggbeaters.

This is vague, at best, but at worst it reinforces a misconception about circular motion. If by “outward” you mean straight out, i.e. radially, then mass in circular motion does not want to fly outward. The only force on the mass is inward, which is gravity. If you “snipped the rope” or turned gravity off, the mass would want to go in a straight line, tangent to the circle it was traveling in. Now, that line increases in distance from the center, so in that sense the motion is outward, but I think the vagueness of such wording is counterproductive, because it can reinforce a misconception so easily.

You might ask, “What about the centrifugal force?” Well, that’s a fake force we put in place when we are in a rotating coordinate system, but want to pretend. We pretend that we can still apply Newton’s laws, and we pretend our motion is a straight line. As such, the net force has to be zero, so we pretend there’s a centrifugal force that balances gravity. But that force really isn’t there, as forces are defined. It’s an illusion of being used to inertial reference frames and trying to make sense of motion when suddenly you’re not in an inertial frame. (The Coriolis force explanation is similar to this, owing to the rotation of the earth)

And that batter flying off the egg beater? Take a look at the pattern it makes on the bowl. Is it perfectly round dots, which is what you’d expect from flying straight out, or is it somewhat elongated dots or streaks, from something moving in a tangential straight line, hitting a curved surface? (This week on CSI: Kitchen)

This Claim Won't Fly

One of things I like to do, as you know, is to run an order-of-magnitude calculation when I see a claim that seems off, to see if it’s plausible.

The future of flight: Shape-shifting engines and body heat power

This claim caught my eye

Tan Kai Jun, the team leader, envisions cabin seats upholstered with a thermoelectric fabric that can convert a person’s energy into 100 nanowatts of voltage. Alas, that amounts to about one-millionth of what your iPhone needs to stay on standby. Still, Jun maintains that it does ultimately add up.
“It’s a small amount, but imagine this collected from 550 seats throughout 10 hours of flight. A plane has a lifespan of a few hundred flights — over time that’s a big reduction,” he says.

This one’s pretty easy. Ignoring that nanoWatts is a power, not a voltage, let’s assume a 10 hour flight. That gets you a total of 1 microWatt-hour per passenger. Let’s be generous and assume 1000 flights, which gets you to a milliWatt hour. Another further generous assumption of 1000 passengers gets you to a Watt hour. I think you can see where this is going. US grid electricity costs around ten cents per kiloWatt-hour. Even if we assume that the cost of electricity on a plane is ~100x more than commercial US electricity, because of the inefficiency, this system will save, at most, a penny over the lifetime of the plane. If the components and installation are free, that is, and the weight of the system costs no additional fuel.

So, not a big reduction.


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This is a fascinating shot and there is lot going on. Play it at least three times and watch what happens above the water surface, at the water surface and below the water surface!

The explanation over at Fuck Yeah Fluid Dynamics

In air, some of the energy of this pressure wave would be dissipated by compressing the air. Since water is incompressible, however, the blast instead moves water aside as the bubble expands. Eventually, the bubble expands to the point where its pressure is less than that of the water around it, which causes the bubble to collapse. But the collapse increases the gas pressure once more, kicking off a series of expansions and collapses. Each bubble contains less energy than the previous, thanks to the loss of pushing the water aside.