By Patrick Clinton
How do porcupines make love?
Franklin Gerard, Reno, Nevada
Actually, it's easier than you might think. After a courtship ritual that involves a fair amount of what in kinkier human circles might be called water sports, the female simply folds her tail over her back, creating a quill-free platform for the male to rest on. The mating process is repeated until one of the porcupines abruptly breaks away, climbs a tree, and then--for reasons that aren't entirely clear--shrieks abuse at the other. It's not a pretty sight, all in all, but at least it's not painful.
The dirt roads around here get this weird pattern of regularly spaced ridges running across them. What causes this--wind, water, or extraterrestrials?
Monty Olson, Tucson, Arizona
That ribbed pattern you're talking about is called washboarding, and it's caused neither by wind nor by rain, but, ironically enough, by the very thing that's supposed to make for smoother driving: your car's suspension. All that "quiet ride" technology under your car may feel good to you, but it can be hell on unimproved road surfaces.
Imagine you're driving down a gravel road. You hit a bump, and your car dissipates the shock by bouncing in a rhythmic pattern that engineers call a harmonic oscillation. Your car, says Forest Service engineer Tom Pettigrew, a man with a lot of country roads on his hands, becomes "kind of like a trampoline." As your bouncing car digs into the road surface, it displaces gravel and begets even more bumps. Over time, suspensions not varying much from car to car, the bumps turn into a series of evenly spaced ridges that can make your steering column rattle like a jackhammer.
Barring some radical change in how suspensions are made, Pettigrew says, washboarding is almost inevitable on unpaved roads that see heavy use. The Forest Service has been working with logging companies to diminish washboarding by experimenting with a new system called central tire inflation, in which an onboard computer automatically lets air out of the tires so that they're nice and squishy. But there's really only one thing the average car driver can do: slow down.
What makes the world turn?
Erin Cohen, Leucadia, California
Not "makes." you mean "made." Our planet doesn't rotate because of something that's happening now. Something set it going aeons ago, and though it's gradually slowing down, it's got more aeons to go before it stops. What kind of something? Here's the latest theory: The solar system formed out of dust and gas left over from the making of the sun. Little chunks of matter gradually gathered into bigger chunks, as gravity worked its magic. Eventually, about 4.5 billion years ago, a very large chunk--the size of Mars--is thought to have collided with Earth. "It was really a titanic collision," says Luke Dones of NASA's Ames Research Center. "It hit off-center, so it set the Earth spinning." (Some of the debris from this collision, incidentally, is believed to have been thrown into orbit, forming our Moon.)
Earth really got to twirling after that collision; a day may have been as short as four hours. Since then the pace has slowed considerably, because of a process known as tidal friction. The tides, of course, occur when the Moon's gravity raises a bulge in the ocean. To oversimplify a bit, the Moon tries to hold on to the bulge while Earth's rotation tries to move it away, with the net effect that Earth turns a little slower (and the Moon revolves around it a little faster) with each rotation. The slowdown isn't what you'd call drastic--only a few thousandths of a second per century--but forewarned is forearmed: Eventually there's going to be a day that lasts forever. What do you want to bet it's a Monday?
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