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# Integers that Represent Different Situations

## Write variable expressions to represent word problems including positives and negatives

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A Grain of Sand

Credit: Conrad Johnson/U.S. Army
Source: http://www.flickr.com/photos/rdecom/9083154620/
License: CC BY-NC 3.0

Did you know that the sediment found on a piece of land determines what the land can be used for?

#### Why It Matters

To understand the Earth's sediments and minerals, geologists measure the size of sediment grains. Grain size is one of the most important characteristics of sediment. Sediment is classified based on the diameter of its grains. This classification helps geologists figure out how environmental factors such as weather and erosion affect the area, information that enables them to determine the best uses for the land.

Credit: U.S. Forest Service
Source: http://www.flickr.com/photos/usdagov/10020846614/
License: CC BY-NC 3.0

There are various scales for grain size classification, including the Krumbein phi scale and the Wentworth scale. After a geologist measures the diameter of the grain in millimeters, he looks up that measurement in a table to find the classification of the sediment. The phi scale converts measurements to special integers, while the Wentworth scale assigns classes to size ranges. For example, a pebble with a measurement of >4 millimeters has a phi scale classification of -2 and is classified as "fine gravel" on the Wentworth scale. If you are a geologist, then using integers is at the heart of your work—especially when classifying sediment.

Check out this video on measuring sand: http://www.youtube.com/watch?v=6LfSHmSav-E

#### Explore More

Read the first article below to find out how measuring sediment led to the discovery of a huge storm from the past. Use the next link to learn more about the Wentworth and Krumbein phi scales.

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