Review SI system of units: REM 244 Length, area and volume: •
Length, measured in meters (m). Was originally selected as 1/10,000,000 of the distance from Earth’s equator to the north pole. It is equal to about 3.28 feet or 39.37 inches. Table 1. SI length units as related to meter and English equivalents •
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SI unit Kilometer Abbreviation km Meter equivalent 1000 m Meter m 1 m English equivalents 0.621 mile; 3280 feet 39.37 inches; 3.28 feet 3.94 inches 0.394 inches 0.0394 inches Decimeter dm 0.1 m Centimeter cm 0.01 m Millimeter mm 0.0001 m Area is expressed in cubic meters, cubic centimeters etc… For land area hectares are commonly reported. 1 hectare is 10,000 square meters or about 2.47 acres. Volume is expresses in cubic meters, cubic centimeters and so on. The liter (l) is commonly used as a unit of volume and is equal to 1000 cubic centimeters or 0.264 U.S. Gallons. Mass and Density: •
Mass: The basic Si unit of mass is the kilogram (kg). The kilogram was originally selected because it is approximately the mass of 1 liter of water. The gram is 1/1000 of a kilogram, or approximately the mass of 1 cubic centimeter of water. • Density is generally expressed in grams or kilograms per cubic centimeter, or cubic meter. • Specific gravity is the ratio between the density of a substance to that of liquid water. • Confusion between mass and weight: Weight and mass are different! Weight refers to the force acting on an object because of gravitational attraction. We conveniently use weight to measure mass on earth at sea level. However if an object was on the moon, it would weigh only about 1/6th of its weight on Earth, but it’s mass would be the same. The mass of on abject is the sum of the masses of its atoms. Table 2. SI mass units and English equivalents SI unit Metric ton Kilogram Gram Abbreviation T Kg g Kilogram equivalent 103 kg 1 kg 10‐3 kg English equivalents 1.10 U.S. tons 2.2 pounds 0.0353 onces Time Units: • SI units of time are the same as English units of time: Unit Abbreviation Hour H Minute Min Second S Millisecond ms Force and Pressure Units: • Force: The basic unit of force in the SI system is the newton (N). A newton is the force needed to accelerate a mass of 1 kg at a rate of 1 meter per second per second. In the English system, 1 pound of force is the force that will accelerate 1 pound of mass at the rate of 32.2 feet per second per second. This definition was selected such that one pound of mass at sea level would feel one pound of gravitational force. One newton is equal to 0.224 pounds of force, and the gravitational force of one kilogram at sea level is 9.81 newtons. • Pressure is force per unit area: The basic SI unit of pressure is the pascal (Pa), which is 1 newton per square meter. One pascal is a very low pressure so we will often times use a unit called a bar. A bar is defined as 100,000pascals or 100 kilopascals. It so happens that 1 bar is only 1.3% greater than normal atmospheric pressure at sea level, so one bar is about equal to 1 atmosphere. • Pressure in the English system is often expressed in pounds per square inch (psi) or in inches of water. One bar equals 14.89 pounds per square inch. One kilopascal equals 4.02 inches of water. Energy Units: •
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The basic SI unit of energy is the joule (J). A joule is the quantity of energy expended when a force of 1 newton pushes something a distance of 1 meter. Thermal energy as well as mechanical energy can be expressed in joules. One joule equals 0.239 calorie, or 4.187 joules equals 1 calorie. A calorie is the energy needed to heat 1 gram of water 1 degree C. 1 joule is equal to 1 watt‐second. In English units energy is expressed in foot‐pounds or BTU’s. 1 BTU is equal to 1055 joules or 255 calories. Power Units: •
Power is the rate at which energy is expended. In SI units power is expressed in watts. One watt is 1 joule per second. The kilowatt (1000 watts) is frequently used. •
In English units, horse power is a common measure of power. One horse power equals 745 watts. And 1 BTU per second is equal to 1.055 kilowatts. (roughly 1 killowatt = 1 btu per second) Temperature Units: • Two temperature scales are used in the SI system. The Celsius scale and the Kelven scale. On the Celsius scale (at sea level), water freezes at 0 degrees and boils at 100 degrees. On the Kelvin scale (sometimes called thermodynamic temperature) negative numbers do not occur. The zero point on the Kelvin scale is called absolute zero and equals ‐273.15 degrees on the Celsius scale. • To convert from Kelvins to degrees Celsius you subtract 273. • To convert from degrees Celsius to Kelvins add 273. • The English system uses Fahrenheit scale, where zero degrees Fahrenheit is the temperature reached by mixing ice with salt, while 100 degrees Fahrenheit is the body temperature of a chicken. Water freeze sin this scale at 32 degrees and boils at 212 degrees. • To convert from degrees Fahrenheit to Celsius you subtract 32 from the temperature and then divide by 1.8. Conversion Factors: •