A Few Tricks of the Trade Help Teachers Sell Physics : Education: Professor’s light-hearted stunts help explain scientific principles.


Wearing a hat that says “Physics Coach,” Bob Ehrlich hammers a stake against his chest, spins a hula hoop, and blows a quarter into a cup, all to show that, gee whiz, physics can be fun.

Ehrlich is a physics professor at George Mason University and author of “Turning the World Inside Out and 174 Other Simple Physics Demonstrations.” He taught a class to 15 high school physics teachers recently to show them easy and inexpensive ways to make physics easy to learn.

Hammering the steel stake against his chest without flinching demonstrates the relationship between mass and inertia. “This gets across the concept of mass,” Ehrlich said. “The bigger something’s mass is, the more it resists.”


Ehrlich puts steel weights on two opposite points on a hula hoop to show that when an object is rotated, its resistance to having its motion change depends on both its mass and how far from the rotation axis the mass is located. When he holds the hula hoop so that the weights are under his hand at 12 o’clock and 6 o’clock, the hoop rotates easily from side to side. When he holds it so that the weights are at 3 o’clock and 9 o’clock, the hula hoop is hard to move.

He said he can win bar bets by blowing over a quarter and getting it to jump into a cup. The high speed air flow over the quarter creates a partial vacuum and sucks the quarter into the air stream.

But Ehrlich isn’t doing tricks just for fun. He said the same effect accounts for lift on an airplane wing.

“You can show these as a ‘gee whiz’ kind of thing to get people interested in physics or as a result of mathematical calculations,” Ehrlich said. For example, a student could do a calculation to show how fast someone would have to blow to make the quarter jump in the cup, he said.

“This is the kind of thing teachers like to use to help illustrate principles of physics in their classes,” he said.

“I can essentially develop things a company would sell for $500 or $1,000” and “show a teacher how to make it for a couple of dollars,” he said.


William Entley, a physics teacher at James Madison High School in Vienna, Va., has used several of the demonstrations with his students.

Judy Ng, another James Madison physics teacher, took Ehrlich’s course last summer.

“Half the session, we the teachers acted like students. We actually did the experiments. On top of that, we had a chance to talk. We were able to exchange ideas,” she said.

Ng and Entley originally were chemistry teachers. Ehrlich said that was one reason for his seminars--to help teachers who may have studied different subjects in college.

“There is very definitely a shortage of qualified physics teachers,” he said. “Whatever students graduate with a degree in physics, almost all of them find they can do better getting a job in industry rather than teaching.”