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Measurements | |
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Measurements In order to determine if the aerodynamic forces on an object are sufficient to allow flight, the forces must be measured. The different contributions of the air moving around the object must be categorized and defined, and they must be quantified. There are specific qualities that are used to categorize and define these contributions. In addition, these qualities need a guideline to measure them by. The guidelines are called units, and the qualities are mass, time, length, and temperature. Unit Units are used to define measurements so that everyone knows exactly how much. They sound very technical, but they are used all over the world in every day examples. It is important to always include the units when talking about measuring a quality. If a bridge designer develops a bridge across a river that is supposed to be 1000 meters long (a measure of length), and one of the builders tells him or her that the bridge is short by 1, what does that mean? If it is 1 centimeter short (1/100 of a meter), it won't be a problem - it can't even be seen. If it is 1 meter short, it may cause a problem years later if the river bank erodes away. If it is short by 1% (10 meters), the bridge may not reach the other side of the river! So units are very important! A measurement should always include 2 things: a number and a unit. Some examples everyone may know include things like: there are 20 minutes until recess; it takes 10 days to drive across the country; a desk top is 20 inches wide and 25 inches long; a recipe uses 2 cups of flour; it is 85 degrees outside today. Each of these measurements includes a number and a unit. Mass Everything, whether it is a solid, liquid, or a gas has mass. It is a measure of how much of the substance is involved - how many molecules. Mass is sometimes used interchangeably with weight (more on that in the properties section), even though they are not the same. In the metric system, the units for mass are grams, kilograms (1000 grams) or milligrams (1/1000 grams). In the American units (called the English system), the weight of the substance is used, in pounds or ounces. A pound is 16 ounces. Often, abbreviations are used for the units: a gram is g, a kilogram is kg, a milligram is mg, a pound is lb, and an ounce is oz. Time The easiest way to think of time is how long it takes something to happen. It may take 10 minutes to drive to school; it may take an hour to eat dinner. The units for time are the same around the world: seconds, hours, days, years. In aerodynamics, a common time measurement is how long it takes an object to go from one point to another, or from point A to point B. Length How long is it? How far is it? These are questions heard every day. Length is a quality used by many people to define an object. A pencil is 7 inches long. A student is 4 feet tall. A swimming pool is 2 meters deep. The most common units for the metric system are a centimeter, a meter (100 centimeters), and a kilometer (1000 meters). In the English system that most Americans use, common units are the inch, a foot (12 inches), or a mile (5280 feet). These units may be abbreviated: centimeter as cm, meter as m, kilometer as km, inch as in, foot or feet as ft, and a mile as mi. In addition to the length of an object, it is often useful to know the area or volume of the object in question. The area is how much room is on a surface like the floor of the classroom or the surface of a wing. It is found by multiplying one length by another (one side of the room by the front for example), and the units are length squared. So common measurements for area in science and engineering would include square feet or square meters. Other common measurements include an acre (approximately 40,000 square feet) or square miles. The volume of an object can be illustrated by how much there is in a container like a fuel tank or an airplane cabin. Volume is in all three dimensions, so it is calculated by multiplying a length by a length by a length. The units are length cubed. A 12 inch long section of a 2 by 4 board (2 inches by 4 inches) would have a volume of 72 cubic inches, or 72 in3. Cubic feet, cubic meters, gallons, liters, and cubic centimeters (cc for short) are all common units for volume. Temperature The quality of temperature is a measure of how hot or cold something is. A thermometer is commonly used to determine the temperature of an object. Everything has a temperature - the rocks, trees, people, air. The weather report in the newspaper usually gives the high and low temperatures of the air each day. The common units for temperature are degrees Fahrenheit or degrees Celsius (what used to be centigrade). In America, almost everyone uses the Fahrenheit scale, so temperatures are reported just as degrees. In science and engineering, however, temperatures can be reported using either scale, so a temperature will always be properly defined as 85 degrees F or 85 degrees C.
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