Random Facts

In my research for this blog as well as my own musings and curiosity, I run across some little factoids that aren't substantive enough to make a post all on their own, so I'll talk about a few of those here.

Faster Than Light Poke

I was thinking about the idea that "information can not travel faster than the speed of light," and I thought I was quite clever when I came up with a way to do it, theoretically at least. I imagined a wooden pole, say one light-year long. If you wanted to transmit information across a light year, it would take at least 1 year to send a radio signal. That's where my pole came in. If one person 'poked' in Morse code then the person one light year away would be able to receive that information immediately. I thought I had cracked it, information traveling faster than light.

It turns out I made an incorrect assumption however. I assumed the pole would all move as one, as with a broom. Poke someone with a broom and they feel it immediately. This is not so with exceptionally large brooms (nor is it with small ones, but the delay is so short it is all but immediate).

There is a 'speed of poke' or a 'poke wave' in the broom. The speed of poke turns out to be the speed of sound through that medium. Your poke would travel at about 3500 meters / second if your pole was made of wood, brass, or concrete. This is far less than the speed of light. It turns out it's about how fast GPS satellites orbit the Earth. The 'poke wave' (I love that phrase) comes about because when you move the pole, individual atoms in the pole have to move to alert their neighbors that they’re moving. Each atom must push its neighbor which must in turn push its neighbor and this is how the poke is transmitted through the object. So no faster than light pokes, science won this round.

Helicopter Speed Limit

It turns out that most helicopters have a speed limit imposed not by the FAA or the maker of the helicopter, but by physics. On helicopter, there is a “retreating blade” (#1, left side) and a “retreating blade” (#2, right side) due to the direction of the blades' rotation.

Image: AVStop.com

When the helicopter is flying, the advancing blade is traveling at the speed of the helicopter PLUS the speed of the rotor. The retreating blade is traveling at the speed of the rotor MINUS the speed of the helicopter. This creates something called "dissymmetry of lift." Because one blade has a higher airspeed than the other one, the central hub of the helicopter changes whats called the angle of attack on the retreating blade, giving it more lift.

Image: Johnathan M. Stern

The increased angle of attack compensates for the slower air speed or the retreating blade. This only works up to a point. If the retreating blade requires more lift than can be compensated for by altering the angle of attack, the helicopter pitches back and slows itself down. This gives most helicopters a physics-induced speed limit.

Image: SPC Glenn Anderson

Most helicopters

Melting Ice and Boiling Water

It is really hard to get water to change temperature. This is because of a property called specific heat capacity that I wont get into much here. Suffice it to say that it requires a whole lot of energy. But the true champion of temperature stubbornness is ice. Ice takes more energy to change by a few degrees than water takes to change by dozens of degrees.

The experiment:

Put two glasses in the microwave for about two minutes. Fill one with water and one with ice and start your microwave. Go do it, it only takes two minutes, and your dishes are still clean afterword. As it turns out the glass filled with water will boil before most of the ice has even melted. This means the water changed temperature from around 70°F to 212°F, or 142°F verses the ice which changed from the temperature in my freezer, 15°F to about 32°F, a change of 17°F. Both the ice and the water absorbed the same amount of energy from the microwave.

The way a microwave works is by creating a standing wave that oscillates about 2.5 billion times per second. This means a charged molecule like water...

...will act like a magnet and try to align itself to the current direction of the field, and this effort causes it to vibrate at about 2.5 billion times per second, increasing thermal energy, heating the water up.

Red is slightly negative and blue is slightly positive

The reason ice doesn't play along and do the same is because it's molecules are in a solid crystal arrangement:

Image: Danski14

In order to align with the field, the molecules must be free to rotate, and in a solid crystal structure, the molecules must break their bonds before they can vibrate and increase the thermal energy. This takes a lot of energy and happens very slowly, which is why the ice takes so long to change temperature and melt.

Bonus fact: When writing this I thought to myself “because fat molecules are non-polar they should heat up less in a microwave.” I tried the experiment, and I heated up identical volumes of water and vegetable oil for one minute, and sure enough, the water changed by 61°C and the oil changed by only 32°C. Science wins another round.

Falling Out of Orbit

When something in orbit loses power or is disabled in some dramatic fashion, often it is portrayed to "fall out of orbit." Almost every science fiction franchise is guilty of portraying this. Even if the writers know the physics, portraying something falling out out of orbit is climactic, while reality in this case is rather underwhelming.

Orbits themselves are often misunderstood. Getting into orbit is not about going up quite as much as it is about going sideways. Space is only 100 km away, so getting there is easy. Staying there takes some work. As you can see in this amazing video (click it) a US space shuttle launch doesn't go straight up, but turns at a steep angle a few seconds into it's flight. By the end of the main fuel tanks' supply, the shuttle will be going about 7.8 kilometers per second sideways and it will not be be changing height at all.

If the shuttle were to get shot down by aliens or lose power, it would not dramatically fall back to Earth but simply stay in orbit. The only way to get out of orbit is to change your velocity dramatically so you're going slower relative to the Earth's surface. In order to change your velocity in orbit you have to take some of your mass and accelerate it away from you. This is all rocket engines do; the fuel is the mass, and they accelerate it by igniting it and directing the outflowing material opposite the direction you want to travel. Without this process occurring you would remain in orbit because your velocity didn't change.

Put simply, it takes nearly as much energy to get out of orbit than it does to get into orbit, so losing power does send your craft careening towards the thing you are orbiting. Orbiting isn't an active process, it is a passive one.

Cheers,

   - Scott

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