Units Guide You

You don t need to know all possible unit conversions (between meters and inches, for example). Instead of memorizing or looking up conversion factors between all types of units, you can memorize just a handful of conversion factors and use them one after another, letting the units guide you each step of the way. 

Say you want to know the number of seconds in a standard calendar year (clearly a very large number, so don t forget about scientific notation, as we explain in Chapter 1). Very few people have this conversion memorized — or will admit to it — but everyone knows that there are 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day, and 365 days in a standard calendar year. So use what you know to get what you want  

A chemistry student measures a length of 423 mm, yet the lab she s working on requires that it be in kilometers. What is the length in kilometers  

23 X10 km. You can go about solving this problem in two ways. We first show you the slightly longer way involving two conversions, and then we shorten it to a nice, simple one-step problem. 

This conversion requires you to move across the metric-system prefixes you find in Table 2-2. When you re working on a conversion that passes through a base unit, it may be helpful to treat the process as two steps, converting to and from the base unit. In this case, you can convert from millimeters to meters and then from meters to kilometers  

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