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Practical Magic

Popular How Things Work class gives away the secrets to the physical world

Class creator Louis Bloomfield demonstrates acceleration. Andrew Shurtleff

The professor yanks hard on the tablecloth, leaving the place setting atop it untouched. It feels more like a vaudeville-era trick than a science demonstration.

Yet that’s what these summer-session students have just seen: a demonstration of Newton’s first law of motion. The bodies at rest stayed at rest—as they tend to do.

“Physics isn’t that hard,” associate professor Jongsoo Yoon tells the students.

That’s not to say it’s easy.

Welcome to How Things Work, an introductory physics class intended for nonscience majors that was created by, and is usually taught by, UVA professor Louis Bloomfield. It teaches students the mechanics of how objects in the physical world interact with one another and why.

“What I tell them from the get-go is that what I do is the opposite of magic,” Bloomfield says, “because the point of every demonstration is to convey understanding—to give away all the secrets.”

Those secrets have proved popular. When Bloomfield started the class in the fall of 1991, he says he was expecting 20 to 25 students; 92 enrolled. The class went on to draw more than 200 students that spring.  A few years later, Bloomfield says, more than 500 enrolled, forcing him to break into separate classrooms and use a live video feed.

These days, Bloomfield caps the class at 236 students—the seating capacity of Room 203 of the Physics Building. Bloomfield, whose textbook, How Things Work, is now in its sixth edition, says he sees a “mixed bag” of students in his classes. “There are people who are scared to death of science and have never taken it to ones who have taken it and learned nothing, to ones who have taken it and have had good teachers and learned a lot. … But they’re almost always nonscience majors.”

Seshi Konu (Col ’20), who hopes to major in architecture, took the class this summer to satisfy that major’s physics requirement. “I had heard about E = mc2 and also knew it was one of Mariah Carey’s album titles,” Konu says with a laugh, “but I never knew where it came from. It was pretty cool learning a little bit about Einstein.”

Bloomfield says he and the course’s other teachers use a case-study approach, which means lots of demonstrations.

In the summer session, Yoon placed two identically sized cans of Campbell’s soup—one beef broth, the other beef vegetable—at the top of a ramp and asked students which one they thought would get to the bottom faster.

The students agreed that the cans would get there at the same time.

However, the beef broth beat the beef vegetable by a wide margin.

“He kept presenting that the weight was the same, the size was the same, so you would think that they would get there at the same time,” says third-year James Levenberg (Arch ’19), “but you forget about what’s actually inside of the cans.”

Levenberg and his fellow students had just been introduced to the concept of rotational mass. “The content of the beef broth does not rotate, while everything in the beef vegetable [can] rotates,” Yoon explains, just as “an empty truck will always win a speed race against a heavily loaded truck.”

Such lessons are what Konu says she enjoyed most. “It’s taught me,” she says, “how to think differently.”

Some Concepts Covered In Class

Q. Why are cars designed to buckle (permanently deform) instead of recoiling (bouncing) when they are in an accident?

Steve Hedberg

A. Buckling dissipates the energy of the crash instead of leaving the energy in the car’s motion, where it can cause injury. If you fall from a height onto a trampoline (or a large air bag or air mattress), the trampoline bends—just like buckling cars at collision—spreading out the impact over a relatively long contact time. If cars recoil (without buckling), then the impact is concentrated over a rather short contact time, and it would be like falling from a height onto a concrete floor.

Q. Why are the curves on a bicycle racetrack steeply banked?

Steve Hedberg

A. To turn a curve is to make a circular motion, and circular motion requires centripetal force—force toward the center of the circle. The banking tips the support force that the track exerts on the bicycle wheels toward the center of the turn and moves the cyclist’s center of gravity toward the center of the curve. Gravity then exerts the centripetal force to allow the cyclist to complete the turn without skidding.

Q. Just like a baseball bat, a tennis racket has a sweet spot. If a tennis ball hits the racket in this spot—its center of percussion—the racket’s handle does not accelerate. Why?

Steve Hedberg

A. As the racket’s center of mass accelerates backward, its handle rotates forward about its center of mass. These two motions cancel each other out. If the ball hits hard at the tip of the racket, the impact will make the handle move away from your hand. If the ball hits too close to the handle, the impact will make the handle move into your hand, making a full swing impossible. The center of percussion is in the middle of these extremes; if the ball hits it, there is no impact at the handle, and the racket’s full swing is focused on the ball.

Q. If you fill a Styrofoam cup with water, poke a hole in the bottom and drop it, will the water leak out the bottom on the way down?

Steve Hedberg

A. It won’t leak, because the cup and the water are both in free fall and locally weightless. Like a group of sky divers who jump together, they will stay together as they fall.