Units

Working as a nurse, one  of the most common measurements Jolene takes is heart rate.  Heart rate is often measured by counting the number of pulses that occur in the wrist or the neck over a specified amount of time.  In order to compare heart rates measured by different people we need to be sure that everyone is using the same measurement units. The medical field uses beats per minute (BPM) as the standard unit for heart rate measurements.

A YouTube element has been excluded from this version of the text. You can view it online here: https://openoregon.pressbooks.pub/bodyphysics/?p=251

Unit Conversion

Rather than waiting and counting pulses over a full minute, you can make the measurement more quickly by counting pulses for six seconds and then multiplying the number by ten, to get the number of pulses that would have occurred in sixty seconds, or one minute. This process is known as unit conversion and the number ten was the conversion factor for this example.

The chain-link method of unit conversion prevents mistakes by keeping track of all the values, units, and conversion factors.

To apply the chain-link method:

1. Write down the original value and units.
2. Set this equal to itself, only now with units written as a fraction.
3. Multiply by conversion factors to cancel undesired units and leave only desired final units.
4. Invert some conversion factors to get the undesired units to cancel, if needed.
5. Multiply the numbers across the top.
6. Multiply the numbers across the bottom.
7. Divide the top result by the bottom result.
8. Record the final value.
9. Add on the desired final units (top and bottom) that are left over after cancelling.

Applying the chain link method to the previous example gives us the same answer, only now we don’t need to just know ahead of time that we should multiply by ten, we only needed to know there are 60 seconds in one minute, which we use as our conversion factor.

Applying the chain link method to the previous example gives us the same answer, only now we don’t need to know ahead of time that we should multiply by ten, we only needed to know that were 60 seconds in one minute, which we used as our conversion factor.

The act of ensuring that your answer to a problem has the correct units is called unit analysis. The term chain-link method is often used interchangeably with the terms unit analysis or dimensional analysis, such as in this helpful video demonstrating unit analysis with the chain-link method. Let’s practice some more unit conversion using the chain-link method with multiple conversion factors:

Standard Units

Similar to medical professionals, scientists use standard scientific (SI) units when reporting measurements so we can all stay on the same page. For example, the fundamental SI unit of time is seconds. In this course we will primarily use seconds for time, meters for length, kilograms for mass, and Kelvin for temperature. All of the other units we use will be combinations of these few fundamental SI units. The table below shows all seven of the fundamental SI units and their abbreviations^[1]. All other standard scientific units are derived units, meaning they are combinations of those seven fundamental units.  Throughout this book abbreviated units will be bold for clarity.  The seven fundamental units and their abbreviations are summarized in the following table. Visit the National Institute for Standards and Technology (NIST) for  more information on standard units.

As with heart rate, the standard medical units and standard scientific units don’t always match up, which means that we will need to be skilled in unit analysis and unit conversion if we want to use physics to analyze the human body. Let’s practice again.