David is measuring the length of a textbook so he can cover his book with paper for a project. He normally uses a customary ruler to measure, but he does not have one with him, and the only tool he has measures in metric units. David needs help figuring out which metric unit of measurement he should use to measure his text book so he can finish his project.

In this concept, you will learn how to determine the best use of each metric unit of length.

### Metric System

In science classes, and anywhere outside of the United States, length is measured with the **Metric System .** The most common units that we use to measure length in the metric system are the millimeter, centimeter, meter, and kilometer.

The **millimeter** is the smallest commonly used unit in the metric system. The abbreviation for millimeters is mm (for example, 3 mm). A millimeter would be used to measure something that is very small, like a seed.

The **centimeter** is the next smallest unit of measurement. The abbreviation for centimeters is cm (for example, 3 cm). Centimeters are smaller than inches. One centimeter is roughly the width of a staple. There are 10 millimeters in one centimeter.

On most rulers, both the customary units of measurement and the metric units of measurement are shown, so you can see millimeters marks, centimeter marks and inch marks.

The next common metric unit is the **meter**. The abbreviation for the meter is m (for example, 8 m). A meter is longer than a foot; in fact, a meter is roughly about the same length as a yard. One way to get an idea of the length of a meter is to stretch your arms out to your sides and your fingertip to fingertip shows about a meter. One meter is much longer than one centimeter. There are actually 100 centimeters in one meter. Meters are used to measure bigger objects or longer distances, such as the depth of a pool or length of a hallway.

**Kilometers** are the longest unit of metric measurement. The abbreviation for kilometers is km (for example, 12 km). Like miles, kilometers are used to measure long distances, such as the distance from your house to the store or from one town to another. Kilometers are a little more than 1/2 a mile, and they are much longer than meters. There are 1,000 meters in a kilometer.

### Examples

#### Example 1

Earlier, you were given a problem about David and his text book he was covering for a project.

He needs to measure the length of the book so he can get the right amount of paper to cover it. He needs to use metric units and his options are millimeters, centimeters, meters or kilometers. Which unit of measurement should David use to measure his textbook?

First, consider the object David is measuring. David needs to get his textbook out in front of him as a reference.

Next, David needs to think about the metric units and decide which ones make sense as possibilities. He now knows kilometers are used to measure long distances, so that is ruled out. He also knows a meter is much longer than a text book, so he can rule out meters as well. He needs to decide between using millimeters or centimeters.

Then, David will match the appropriate choice with the object he is measuring. He knows millimeters measure very small objects like seeds or staples. His textbook is much larger than those types of objects, so centimeters are the right choice for measuring his book.

The answer is centimeters.

David will use centimeters to measure his textbook to complete his project.

#### Example 2

Which unit of metric measurement would you use to measure the length of a piece of paper?

First, take out a sample of the object you are measuring, or picture it in your mind.

Next, think about your metric units and consider which units make sense with this object. There are millimeters, centimeters, meters and kilometers as the most commonly used units of metric measurement. If possible, rule out units that are definitely not appropriate to measure the object. Kilometers are about a half of a mile and are used to measure long distances, so that unit is not appropriate. Meters are about the size of a yard, or your wingspan, so that is also not appropriate. That leaves us with millimeters and centimeters.

Then, match the appropriate unit with the object you are measuring. Millimeters are tiny units used to measure very small objects that are even sometimes hard to see. You can see a piece of paper quite well, so you would not use millimeters for this size of an object. Centimeters are the most appropriate unit to measure the length of a piece of paper.

The answer is centimeters.

#### Example 3

What unit of measurement would you use to measure the distance around a track?

First, consider the object you are measuring and look at it or picture it in your mind.

Next, consider your unit of measurement choices. A track is a large object to measure so you would definitely rule out millimeters and centimeters. That leaves meters and kilometers.

Then, match the object with the best appropriate unit of measurement. Long distances like a track or distance around a block etc. are often measured by kilometers. Meters are used to measure things that are closer together, like lengths of houses or pools. The appropriate unit in this example is kilometers.

The answer is kilometers.

#### Example 4

What unit of measurement would you use to measure the length of an apple seed?

First, get the object out to measure or picture it in your mind.

Next, think about the reasonable measurement choices. Kilometers are much to large to measure such a small object, as well as meters. That leaves centimeters and millimeters are the two possibilities.

Then, match the object with the best unit of measurement. Centimeters are 10 times bigger than millimeters, and could be difficult to get a close measurement with the larger unit; therefore, millimeters would give the most accurate measurement.

The answer is millimeters.

#### Example 5

What unit of measurement would you use to measure the length of a paper clip?

First, get out a paper clip or picture one in your mind.

Next, consider the unit choices that make sense with the object. A paper clip is very small, so kilometers and meters do not make sense. That leaves two choices: millimeters and centimeters.

Then, match the object with the appropriate unit. A paper clip could be measured with either unit (millimeter or centimeter) but a centimeter is the more appropriate size to get an accurate measurement.

The answer is centimeter.

### Review

Choose the appropriate unit of length using metric units for each item listed below.

- The depth to plant a seed in the soil
- The height of a tree
- The area of a garden plot
- The distance from a garden to the local farm store
- The length of a carrot
- A stretch of fencing
- The length of a hoe
- The distance between two seedlings planted in the ground
- The height of a corn stalk
- A road race
- A grub collected from the garden
- The width of a garden row
- The length of a garden row
- The size of a small seed
- The distance that a tractor can travel on a large farm per day

### Review (Answers)

To see the Review answers, open this PDF file and look for section 2.2.