## American Students Don't Think Metric

### As physics teachers, we know that measuring and calculating with kilograms, meters, and seconds, is better than doing so with pounds, feet, and minutes. However, our country is still an Imperial one. Students have an intuitive idea that 60 mph is fast, but have no idea that 30 m/s is a little faster. For this reason, I still use Imperial units along with metric ones in my class.

There are some complications. Some students mix the systems together; others want to always convert from one system to the other. These are hassles in any class, but part of the learning process.

#### How much horsepower?

Over the Thanksgiving break, I have my students do the classic problem of calculating the average horsepower generated by a car's engine from the 0 to 60 mph time and the curb weight. We have covered work, power, and energy in class, and sometimes have discussed a homework problem where the power output (in watts) of a Porsche is calculated. Since the published data for most cars sold in the US are already in Imperial units, I have my students keep those measurements in feet, pounds, and seconds.

The day before the break, I remind the guys that, while I have not given them a one-line formula to do this calculation, they do have the necessary tools to do a successful analysis. But, the first time or two they try, they will make mistakes. This is normal, part of the learning process. If they get an unrealistic result, they have made a mistake. Set things aside for a couple of hours or overnight, but keep thinking about it in the back of their minds. Come back to the question later and try again. I want them to make mistakes (the typical ones are using 60 mph not 88 ft/s and using weight in place of mass). About this time of year, most of my students start to understand the place of making mistakes in scientific analysis.

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