Plane power really Hertz, ladybugs come and go, space clock ticks slower
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
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
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
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.
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
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
"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
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.
WonderQuest: Ladybugs mate
WonderQuest: Ladybugs too busy eating to nest
Carolina State University: Life tables by John Meyer
Space clock ticks slower
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
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
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.
WonderQuest: The clock beats slowly
by Rod Nave: Centrifugal force
Science by Alistair B. Fraser
(Answered April 16, 2004)