Teaching Physics with the Physics Suite

Edward F. Redish

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Dollar and Penny

A student makes the following argument: "I can prove a dollar equals a penny. Since a dime (10 cents) is one-tenth of a dollar, I can write:

10 = 0.1 $

Square both sides of the equation. Since squares of equals are equal,

100 = 0.01 $.

Since 100 = 1 $ and 0.01 $ = 1 it follows that 1$ = 1 ."

What's wrong with the argument?


Page last modified September 30, 2002: G06