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Plane power really Hertz, ladybugs come and go, space clock ticks slower

Q: My colleague today asked me a question that puzzles me a lot!! Why do aircraft use 115 volts, 400 hertz (Hz) while our domestic supply is 115 volts, 60 Hz! Why so different? I tried to search for the answer for the whole day but still to no avail. (Allan, Irving, Texas)

Cockpit lights gleam, powered by 400-Hz power  [Jim Ross, NASA, Dryden Flight Research Center]

A: Aircraft use 400-Hz power for the same reason that Charles Lindbergh didn’t take a radio when he flew across the Atlantic — to save weight and volume. We can make electrical generators smaller and lighter if they operate at higher frequencies because alternators running at those frequencies need less iron and copper, says Jeff Scott, aerospace engineer at AerospaceWeb.org.

High-frequency power losses, however, is a drawback, says Aspi Wadia, head of the compressor group at GE Aircraft Engines. The inductive properties of cables carrying alternating current at 400 Hz frequency cause losses that are up to 7 times greater than the same cable carrying current at 60 Hz.

By the way, Lindbergh’s plane was "a two-ton flying gas tank" according to biographer Scott Berg. No parachute, no radio, no brakes, not even a forward-facing window (a small periscope would do). The only weight that counted was fuel.

Further Surfing:

Aerospaceweb.org: Ask a rocket scientist

Aircraft electrical systems and why they operate at 400-Hz frequency by Aspi Wadia of GE Aircraft Engines

Berg, Scott, Lindbergh, Berkeley Pub Group, 1999.

Ladybugs come and go

Q: One of your answers states that ladybugs mate in spring. Another states that they mate in autumn. You state also that they have a 4-week lifetime but they hibernate in the winter. Pick one — delete one. (Cliff, USA)

Ladybug eating [© Scott Camazine 2003, used with permission]

A: Actually, I didn’t state that they mate in autumn. You do, however, bring out an aspect worth mentioning. Ladybugs (Ladybird beetles) are a diverse bunch.

"According to Ross H. Arnett's count in American Insects: A Handbook of the Insects of America North of Mexico there are 399 species of ladybugs (assigned to 53 genera) recorded in the US and Canada," says John Meyer, entomologist professor at North Carolina State University.

They mate at various times, have a variety of life spans, and differ widely in the number of generations per year. Most species have multiple generations that mate and die through the summer. Yet the species must live on. So, some live through the winter to start the whole business over again by mating in the spring. Over-wintering ladybugs live longer than the typical life span of their species.

Further Surfing:

WonderQuest: Ladybugs mate

WonderQuest: Ladybugs too busy eating to nest

North Carolina State University: Life tables by John Meyer

Space clock ticks slower

Q: Your answer on the International Space Station (ISS) clock neglects the fact that a clock in orbit at any altitude is at zero gravity because centrifugal force cancels gravity. Thus at the ISS, the clock slowdown due to speed must first cancel the zero gravity speedup. (Dan)

Diving off a ship, he falls “weightlessly” [NOAA]

A: The original question concerns which effect dominates — a slower clock because the ISS clock speeds around Earth at 20,000 mph (32000 km/hr) and therefore goes slower relative to a clock on Earth. (This is the same effect that allows a returning spacefarer twin to be younger than her earthbound sister.) Or, a faster clock because the ISS clock (being higher) has a smaller gravity field (88% smaller) than an earth clock.

It is true, as you say, that the clock, like the astronauts, is in free fall and we often call that state "microgravity" or zero gravity. The term misleads. A ball dropped off the Leaning Tower of Pisa, a floating astronaut, or a person jumping into a pool is falling because of Earth’s attraction. The gravity field at the ISS (253 miles, 407 km, high) is almost as strong as at the Earth’s surface. It is 88% of full strength. So, gravity affects the ISS clock 88% as much as it does an earthbound clock.

It looks like astronauts are floating in space — unaffected by gravity — only because they and the loose objects around them are falling at the same speed as their orbiting craft. Gravity is almost the only force acting on them, much like the two balls — one light and one heavy — that fell from the Leaning Tower of Pisa. They fell at the same speed and hit the ground at the same time. Only when they hit, did they experience their weight. But gravity is always a large force present as long as a massive body like Earth is nearby.

Furthermore, centrifugal force cannot cancel the effects of gravity. It may seem like centrifugal "force" is really a "fly away force". Some teachers even teach beginning physics as if it were a force. This is wrong. "It’s just a useful framework to describe the inertial tendency of an object to move in a straight line," says Rod Nave, physicist professor at Georgia State University.

If I tie a tin can to a string and swing it about my head, the string exerts a force (directed radially inward) on the can. That’s called "centripetal" force and it is real. Now, I let go of the string and the can zooms off in a straight line. That’s Newton’s law of inertia — not a real force. Newton said that a body continues in a straight line at constant speed unless a net force compels it to change that state.

So, centrifugal force is not an applied force but a way to describe the inertial tendency to travel in straight lines. Thus it can’t act to cancel any real force like gravity.

Speed wins. The ISS clock ticks slower.

Further Surfing:

WonderQuest: The clock beats slowly

HyperPhysics by Rod Nave: Centrifugal force

Bad Science by Alistair B. Fraser

(Answered April 16, 2004)