BOOK REVIEW : Simple Look at the Puzzling World of Physics : TURNING THE WORLD INSIDE OUT AND 174 OTHER SIMPLE PHYSICS DEMONSTRATIONS<i> by Robert Ehrlich</i> Princeton University Press $50 cloth, $14.95 paper, 216 pages

If you toss a heavy boulder out of a floating rowboat, does the water level in the pond rise or fall?

Here is a simple physics demonstration that provides the answer:

Float a four-inch length of 2-by-4 in a tray not much larger than the block. Next, put enough pennies on the block to make its top surface just even with the water line. It will probably take about 50 pennies to do this. Mark the water line in the container.

Finally, allow all the pennies on the barely floating block to slide into the water. The water level will drop slightly, as does the water level in the pond when a heavy boulder is tossed overboard from a floating rowboat.

The reason, consistent with Archimedes’ principle (“Eureka!” Remember?), is that the amount of water displaced by the sunken pennies is equal only to the volume of the pennies, while the amount of water displaced by the pennies on the floating block was equal to the weight of the pennies.

This is one of 175 easy, illustrative and insightful demonstrations devised and presented by Robert Ehrlich in “Turning the World Inside Out,” which should be required reading for all teachers of introductory physics and maybe for some advanced teachers as well. It is also a useful and engaging book for anyone interested in understanding the simple principles that underlie the physical world.

In his introduction, Ehrlich, a physicist at George Mason University, says that he has found that everybody likes to see demonstrations, but they are usually too complicated or too expensive or too time-consuming for most teachers to bother with.

Also, he says, as often as not, classroom demonstrations don’t work right, which is embarrassing for the teacher and tends to put the kibosh on further efforts at show and tell.

Ehrlich’s book is an effort to remedy those problems, and it succeeds very well. Most of the demonstrations require minimal equipment, which is easy to transport and doesn’t take much time to set up. I tried several of them, and they all worked like charms.

Each of the demonstrations begins with a clear statement of the principle of physics at issue, a description of the procedure to be followed and a page or so of comment that includes simple equations where applicable.

For reasons that bedevil philosophers of science, the physical world is describable mathematically. Mathematics and physics are inextricably intertwined, though no one knows why. Ehrlich shows that those mathematical descriptions need not be forbidding. At the same time, it is possible to do the demonstrations and learn from them while ignoring the mathematics.

For example, one of the demonstrations is called “Balancing a Stick on Its End.” Ehrlich explains: “Newton’s second law for rotation explains why long sticks are easier to balance than short ones, and why a weight on the top end of a stick makes balancing a stick easier.” The equipment required: “a 12-inch ruler; a meter stick; a pencil; and some clay.”

Ehrlich explains: “Your ability to balance a stick on your finger depends on the stick’s length, as you can easily verify by trying sticks of different lengths. You should find it easy to balance a meterstick, hard to balance a 12-inch ruler, and impossible to balance a pencil. A directly related demonstration is that long sticks placed on their end on a table take longer to topple over than short ones.”

The mathematics explaining Newton’s second law of rotational motion is then given in a paragraph, followed by this:

“Suppose we now consider a nonuniform stick--for example, a stick with a clay ball on one end. If the clay ball is on the top end, the stick is easier to balance, while if it is on the bottom end the stick is harder to balance. This can be verified by trying to balance the 12-inch ruler with a clay ball on its end. Since it is hard to balance without the clay ball, adding the ball to the top makes balancing easier, while adding it to the bottom makes it impossible.”

Unlike many how-to science books that I remember from high school, Ehrlich’s book could actually be used by students and teachers alike. The principles it illustrates are deep, but its techniques are simple. It is a most welcome addition to a physics library for beginners and for accomplished practitioners.

Next: Jonathan Kirsch reviews Deborah Tannen, “You Just Don’t Understand: Women and Men in Conversation.”