## Measurement: When Students Ask

**Why do I need to learn the metric system?**

The metric system is used by 95% of the world's population. Liberia and Myanmar are the only other countries besides the United States that have not officially adopted the metric system. All scientists, even those in the United States, use the metric system. Although the metric system is not the official measuring system of the United States, the United States does use it. Most items in the United States are measured and labeled using both customary and metric units. (Show students a soup can or other food packaging with quantities expressed in metric units.) Eventually, the United States may need to convert to the metric system to match the rest of the world.**Are measurements exact?**

Measurements are not exact. Measurement tools such as rulers are divided into specific units. The smaller the units, the more precise the measurement. When you measure, you are measuring to the nearest unit on the tool that you are using. To demonstrate this, place a clear centimeter ruler on the overhead. Line up an object such as a pencil above the ruler. Have students identify the 2-mm marks to which the end of the pencil is closest. Discuss the fact that objects aren't exactly as long as a unit of measurement. So a measurement of 145 mm is really an estimate that could be more precise.- Measuring the same object many times—or having many people measure the same object—may also produce slightly different numbers. Have four students measure the same crayon to the nearest millimeter. Don't let them share their results until all four have had a chance to measure. Compare the results and discuss them.
**How do I know which unit of measurement to use?**

Certain units are used to measure certain attributes. There are more units to remember in the customary system than in the metric system. In the metric system the basic unit of length is the meter, the basic unit of capacity is the liter, and the basic unit of mass (weight) is the gram. In the customary system, length is measured in inches, feet, yards, or miles. Capacity is measured in cups, pints, quarts, and gallons and weight is measured in ounces, pounds, and tons.**How do I convert from one unit to another?**

To convert between units, you need to find the relationship between the units. For example, if you are converting from yards to feet, you must know that 1 yard equals 3 feet. If you are converting a larger unit to a smaller unit, multiply. For example, 5 yards = 15 feet. (5 x 3 = 15.) If you are converting a smaller unit to a larger unit, divide. For example, 12 feet = 4 yards. (12 ÷ 3 = 4.)- In the metric system, you will always be multiplying or dividing by a power of 10.
**What kinds of units are used to measure the perimeter of a figure?**

Units of length are used to measure perimeter. Units of length in the customary system are the inch, foot, yard, and mile. Units of length in the metric system are the millimeter, centimeter, decimeter, meter, and kilometer.**How can a measurement be a negative number?**

A thermometer is like a number line. Some numbers are greater than 0 and some are less than zero. Numbers that are less than 0 are called negative numbers. The scientists who created the Fahrenheit and Celsius temperature scales assigned temperatures to fixed points like the boiling and freezing points of water. On the Celsius scale the freezing point of water is 0°C. Temperatures often drop below the freezing point of water. When the temperatures are lower than 0, they are negative numbers. Even on the Fahrenheit scale, where the 32° is the number assigned to the freezing point of water, the temperature sometimes drops into negative numbers. The coldest temperature ever recorded on Earth, in Antarctica, was^{−}129°F, or about^{−}90°C.**What is the difference between***weight*and*mass*?

People often use the terms*weight*and*mass*to describe the same thing. Actually, the terms describe two different things.*Mass*is the amount of matter, or stuff, in an object.*Weight*is a measurement of how much the Earth's gravity pulls on an object's mass. The farther you get from the Earth, the less gravity pulls on you; so, the less you weigh. It is because of the reduced gravitational pull on an object in space that astronauts in space are weightless. If an astronaut stepped onto a scale in space, it would read 0. Does this mean there is nothing left of them—that they no longer have any matter? No, the 0 reading means that the Earth's gravity is no longer pulling on them. Even though the astronauts are weightless, their bodies still have the same mass as they did on Earth.