College Math Teaching

May 10, 2011

Non-portability of mathematical skill

On my calculus final exam, I gave two questions about a metal plate of uniform density. The plate was easy to describe: it’s boundary was the x axis and the parabola y = 1-x^2 . In the first question, I asked for M_x (the moment about the x axis and in the second question, I asked for the center of mass (they could use symmetry to deduce \overline x = 0 ). So to find \overline y , they needed to find the area (mass) and M_x .

What astonished me is that a number of students missed the question “find M_x  ” completely but then went on to solve for \overline y correctly!

This says something about the intellectually immature mind, but I am not sure what is says.

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