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Walking on water, Blue ice, Magnetic poles flip but life chugs on

Q: How do insects walk on water?

[Robert Suter, Vassar College] A quarter-size (leg tip to leg tip) water strider dimples the water as she walks.

A: If you're a little bug with big waxy feet (like the water strider in the figure), walking on water is a snap because the water acts like a taut elastic sheet. Water behaves this way because of surface tension: the surface's tendency to contract.

Why does a surface contract? Ah, we dip into a universal truth. Water molecules cling to one another to reduce their overall potential energies. All physical systems do this. Systems arrange themselves so they end up with the lowest possible potential energy--as if they've "gone downhill."

Potential energy is the stored energy of position. It's called "potential" energy because of its potential to do work. When we draw a bow, we store potential energy in the bow. For more information about potential energy see Conceptual Physics by Paul G. Hewitt.

Back to our pond with water striders zipping about on its taut-elastic-sheet surface. The water molecules throughout the pond cling to their neighbors to reduce potential energy. The poor molecules at the surface, though, have no neighbors above. They end up with the highest potential energy and there's not a darn thing they can do about it.

But the water system still has to get into an arrangement that has the lowest overall potential energy. So, it tries to minimize the trouble--that is, it tries to minimize the surface area where the potential energy is highest: by contracting.

A water strider steps and thereby pokes the pond surface. The surface deforms (see figure) but springs back as soon as she lifts her foot. That's why she can walk on water--surface tension makes the water a springy membrane.

By the way, quarter-size water striders walk on water with ease and so fast they escape predators. That's the real world but get this: according to Robert Suter, biology professor at Vassar, a pond's surface tension is so strong that it could support a 1-pound water spider as big as your fist. His legs, though, would have to span 18 inches--six times the span of a real-life spider.

Floating a sewing needle is a really good demonstration of surface tension that anybody can do. I don't know if you folks are familiar with it, but it really isn't difficult (I got it on my second try tonight). Just use a pair of tweezers to gentle place (drop) a sewing needle onto the water surface. It floats. Click HERE to see what you can do in your very own kitchen.

Further Surfing:

How things work

Q: Why is some ice blue? Sherie, Albuquerque, New Mexico

An iceberg and blue ice, Gerlache Strait, Antarctica.  Photo courtesy of NOAA.

A: Ice is blue for much the same reason that water is blue--it absorbs a bit more of the red-frequency part of light that shines on it than it does the blue.

Poke a hole in snow or ice and look down it. You'll see blue-green light because the emerging light has bounced around through many snow-particle passages. At each snow collision, the snow absorbs more red than blue. Eventually, the reflected light is noticeably blue. The white light fades to blue as it bops its way out. The deeper the ice hole, the bluer the returning light.

Sometimes icebergs look green instead of blue. Icebergs contain more stuff than ice---suspended sediments, algae, and air bubbles. These particles contribute to the green color.

See the book, Color and Light in Nature by David K. Lynch and William Livingston, for more information on ice and its colors.

Q: The Earth's magnetic poles switch about every 200,000 years. When the poles switch, space radiation that reaches Earth increases. What is the effect on life? --John C, Albuquerque, New Mexico

A: Not much. Radiation increases such a small amount that it is no cause for alarm.

Earth's magnetic field shields her from incoming cosmic rays and the field decreases to 20% of its normal strength during a reversal. That sounds fairly alarming. But wait: the atmosphere is our main protector from radiation and it steadfastly shields us no matter what the magnetic field does.

Earth's magnetic poles switch at random intervals--as short as tens of thousands of years apart to more than a million years--an average of once every 200,000 years. It's hard to imagine such an event: the south magnetic pole becoming the north and visa versa. Unsettling. That's the way it happens too--Earth's field becomes disorganized during the time of reversal, which can stretch out 5,000 years. Maybe that's short on a geological scale but plenty long on a human one.

So, the flip causes no significant bad effect on life. How about any effect? Mutations might increase. Birds and other animals that depend on the magnetic field for orientation might get confused during migrations. However, five thousand years gives them much time to adapt so maybe they wouldn't notice.

Further Surfing:

WonderQuest: Compasses and why the poles switch

Astronomy Magazine: Effect of Earth's magnetic field reversal

(Answered Dec. 13, 2002)