By: Beth Post

The weather is cooling a bit, but the exhibition at Explorit is still hot - Hot Wheels(TM), that is. Come to the exhibition and try your hand at making a roller-coaster ride for miniature cars. Or try crashing cars of various weights and velocities to watch potential energy change to kinetic energy - right before your eyes! You're sure to have a bang-up time when you visit "The Way the Ball Bounces: The Science of Toys and Games!" For more information, call Explorit at (530) 756-0191.

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This is the third and final week of Explorit's series of articles about water toys and the science behind them. Today's article explores the physics behind moving in water.

You don't necessarily need toys to explore water physics, or fluid dynamics, and have fun in the water at the same time. Splashing, swimming, diving, and generally cavorting in cool H2O all have their own fun merits, and provide a good chance to immerse yourself in a little science at the same time.

Of course, floating in water can really help you lighten up. You actually "lose weight" when you're immersed in water. Due to the force of buoyancy and Archimedes' principle, you'll "lose" the same amount of weight as the weight of water that you displace when you float (For more on Archimedes' principle, see Explorit's article of July 17, 1998).

But did you know playing in a pool of water can be a real drag? Well, it can be if you're trying to move through it quickly! Water is more than 700 times denser and 55 times more viscous than air (viscosity is a measure of how resistant a fluid is to flowing, or to having something move through it). Just wading takes a lot more energy than walking through air because you have to move all that heavy water aside to get anywhere. But there are other things "dragging you down," literally, as you move through water.

Drag is created when fluid flows around an object, generating turbulence behind the object and slowing it down. As an object moving through water, a swimmer creates a lot of drag. But the drag can be reduced in many ways, most of which are aimed at streamlining the body.

One way to reduce drag while swimming is to change your form in the water. Think of how easily a "streamliner" cruise ship, with its sharp, water-slicing bow, cuts through the water compared to a blunt-front barge. The human body can be made similarly streamlined by placing the arms and hands together above the head, creating a sharp wedge.

Kicking reduces drag because it keeps your legs up, further streamlining your body. Researchers have also found that rolling your shoulders as you make a swimming stroke reduces the frontal area presented by your body, further reducing drag. Olympic swimmers take advantage of these and other findings about the dynamics of water to refine their swimming techniques. Such knowledge can help them "shave" seconds off their race times. For example, many swimmers shave the hair off their heads and the rest of their body in order to reduce "surface drag" - the tendency for a fluid to "stick" to an adhesive boundary layer around a moving object.

One swimmer has combined an understanding of fluid dynamics and marine biology to revolutionize her swimming stroke. Misty Hyman, a butterfly swimmer, used results from a study of how dolphins swim to change her technique and lead her team to swimming gold in this week's Goldwill Games. Research indicates that dolphins swim impossibly fast given their mass, shape, and the amount of drag produced in water. It seems a dolphin uses its tail in that characteristic up-down motion to kick off the turbulent vortices formed behind it as it moves. Misty does the same, by turning sideways as she dives into the water, arms over her head, and kicking like a dolphin. This method of entry has proven effective in giving Misty a huge lead over her competitors before she even begins her stroke.

If you know of a good, safe body of water nearby (Davis has several public swimming pools), you can test out many of the concepts discussed here. To find out more about the science behind water (and other) sports, check out Susan Davis and Sally Stephens' book, The Sporting Life, part of the Exploratorium's Accidental Scientist series. Much of the information presented today was gleaned from this fun publication, an Owl Book published by Henry Holt and Company Inc.

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Explorit Science Center is located at 3141 5th Street in East Davis. The current exhibition is The Way the Ball Bounces: The Science of Toys and Games. Public hours are Saturday from 11:00 a.m. to 4:30 p.m., Sunday from 1:00 to 4:30 p.m., and Tuesday through Friday from 2:00 to 4:30 p.m. Regular admission is $3; members, teachers (with school ID), and children under 4 are free.