Newton’s Second Law of Motion

By John Roeder, Upper School Physics Teacher
(from The Calhoun Chronicle, winter 2003)

“Never underestimate the creativity of a student!” If I didn't believe that before, I certainly do now. Here's what happened in my physics class last November. Four groups of students had just finished responding to the challenge of measuring the acceleration of what we call a “modified Atwood's machine”—a cart pulled by a string, suspended over a pulley and connected to a descending weight on the other end. By determining how the acceleration depended on such variables as the descending weight and the weight on the cart, they had been led experimentally to Newton’s Second Law of Motion.

“It’s time for another challenge,” I announced. I got out a device to measure force by the bending of a spring (for lack of a better term we call it a “force measurer”), a wooden block and some weights, and I challenged my students to measure the friction force between the wooden block and the table as it was pulled across the table—with at least three different weights on it.

In almost no time the students were off and running, getting equipment from previous experiments—carts and pulleys from the modified Atwood's machine and, for one group, a “buggy” whose motion had been studied earlier in the year for its constancy. Concerned that they might be grabbing equipment without sufficient prior thought, I canvassed each group to ascertain how they intended to measure the friction force. Without exception, each group came up with a well-thought-out answer—and each one a different approach.

I was overjoyed! My four groups of students were meeting this challenge in four different ways , and three of those ways I had never thought of before! It was as if they were back in ninth grade responding to the challenge at the end of an Active Physics chapter, except for one thing: they were all being asked to measure the same quantity. And that was the most beautiful part of it all. When it came time to report their results, I was able to compile their data onto a single graph. Almost all their data points (of friction force vs. weight) lie on essentially the same line, thus confirming the well-known relationship between the two. My students had been not only creative, but also consistent. Now I’ve got to go out in search of another challenge!

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