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we know now about what matter is and the various forms it can take. But how do we measure it so that we can categorize it, learn about how it behaves, and then manipulate it into useful things like airplanes or rocket engines. There are several different qualities we can measure and for each different quality we assign a unit of measurement. This sounds much more technical than it really is. You already use units everyday in your life. When you talk about time, you speak about seconds, minutes, hours, days, etc... These are just some of the units of measurement for time. You keep track of, or measure, time with a clock or a calendar and say things like "there is 20 minutes until recess" or "It takes 10 days to drive across the country." These are measurements just like any an engineer or scientist would make and they always include two things: a number, (20 and 10 in the two statements above) and a unit (minutes and days) that puts the number in context. The following pages talk about other physical qualities that are important in aeronautics and give some examples of the units of measurement that go with them Length: Length is another basic measurement that we all use in everyday life. We say things like "I have to ride 3 miles to school" or "I am 5 feet tall." The units of length are inches, feet, meters, miles, etc... picture of a ruler Area: Area is a measure of how much room there is on a surface like the floor space in a classroom or the size of something that is mostly flat like a wing. Area is usually measured in square feet (a one foot by one foot square) or square meters in science and engineering. Other units of area you know are acres and square miles. picture of a square foot (two rulers) Volume: Volume is how much apace there is in a container like in a fuel tank or an airplane cabin. Cubic feet (one foot by one foot by one foot cubes) or cubic meters, gallons, liters, and cubic centimeters (CC's for short) are all commonly used units of volume picture of a cubic foot (three rulers) Velocity: How fast you travel is measured by velocity. If you run 10 miles in 1 hour your velocity was 10 mile per hour. In aeronautics we usually use feet per second or meters per second. Like these examples, units of velocity are usually expressed as a unit of length per a unit of time. An exception to this rule are knots, which are also often used in aeronautics. One knot is about 1.7 feet per second. A speedometer. Accelleration: Accelleration is a measure of how quickly velocity is changing If your car goes from zero to sixty in 5 seconds, the acceleration would be (60 miles per hour) per (5 seconds) or 12 miles per hour per second. A more typical unit that an engineer would use is "meters per second per second" or "meters per second squared". Accelleration is also a measure of how fast you slow down even though we use the word deceleration in conversation. An accelerating car? Mass: Mass is the measure of how much stuff (matter) is in an solid object or contained in a volume. Mass resists being lifted. It also resists acceleration. If you put a lot of people in your car you are adding a lot of mass and you will not be able to accelerate as quickly. Mass is measured in kilograms or sometimes as slugs. Mass is very different from weight. Two things with the exact same mass will have different weights on the Earth and the Moon. Dumbell? or the "one ton weight"? Density: Density is a measure of how tightly packed together the mass in a volume or object is. If you have a 10 gallon fish tank with water in it and another 10 gallon fish tank with air in it the one with the water is heavier because water is much more dense than air. The units of density are always mass per volume. kilograms per cubic meter, slugs per gallon, slugs per cubic foot.... water vs air. Force: Force is how hard an object is pushed or pulled. The most well known force is the force of gravity called weight. Forces are most often measured in pounds or Newtons. Pressure: Pressure is a force spread out over an area versus one that is applied at a single point. Pressure is a very important concept in aeronautics as it is a source of the lift that gets an aircraft off the ground. Pressure can be a deceivingly powerful quantity. A small pressure applied over a large area can add up to a very large force. pressure is measured in units of pounds per square foot, Newtons per square meter (or Pascals for short), atmospheres, and bars among other things. A force vs. pressure. Temperature: Temperature is a measurement of how hot something is. It is the same thing you hear about on the news every day. It is measured in degrees Fahrenheit or degrees Celsius. Everything has a temperature - the air around you (That's what they are talking about on the weather every morning), your body, rocks, trees, clouds...everything. A thermometer. Viscosity: Viscosity is probably the most obscure physical quality on this list. It is a measure of how much a fluid will resist flowing. Honey is very viscous, water is less viscous. Flowing fluids exert a force on what they are flowing over. This force (measured in Newtons) is a factor of how fast the fluid is flowing (measured in meters per second), how thick the layer of fluid is (measured in meters), and the viscosity. When you put all these factors together you find that the units of viscosity are "Newton-seconds per square meter". Cross-section of flowing water.
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