Standards Of Length Mass And Time
A unit for a physical quantity must be defined in order to communicate the outcome of a measurement of that quantity. If our basic unit of length is defined as 1.0 meter, and someone who is familiar with our measurement system reports that a wall is 2.0 meters high, we know that the wall's height is twice the mendasar unit of length. Similarly, if our basic unit of mass is defined as 1.0 kilogram, and we are told that a human weighs 75 kilograms, that person weighs 75 times the mendasar unit of mass.
SI (Sistem Internasional) yakni metode tolok ukur untuk besaran pokok dalam sains yang disepakati oleh komite internasional pada tahun 1960. Meter, kilogram, dan sekon yakni satuan panjang, massa, dan waktu, masing-masing.
Length
The meter, defined as one ten-millionth of the distance between the equator and the North Pole, became the legal standard of length in France in 1799. The official length of the meter was defined until 1960 as the distance between two lines on a specific kafe of platinum-iridium alloy stored under controlled conditions. This standard was dropped for a variety of reasons, the most important of which being that measurements of line spacing aren't exact enough. In 1960, 1 650 763.73 wavelengths of orange-red light emitted from a krypton-86 lamp were defined as the meter. This definition was likewise dropped in October 1983, and the meter was renamed to reflect the distance traveled by light in vacuum over a time interval of 1/299 792 458 second. The speed of light is now defined as 299 792 458 meters per second, according to the most recent definition.
Mass
The kilogram, a SI unit of mass, is defined as the mass of a certain platinumiridium alloy cylinder stored at the International Bureau of Weights and Measures in Sèvres, France (similar to that shown in Fig. below). As we'll discover in Chapter 4, mass is a unit of measurement for an object's resistance to change in motion. Changing the motion of a large mass object is more difficult than changing the motion of a small mass object.
Time
The time standard was defined before 1960 in terms of the average duration of a solar day in the year 1900. (A solar day is the interval between the Sun's successive appearances in the sky at its highest point each day.) The second, the mendasar unit of time, was defined as (1/60)(1/60)(1/24) = 1/86 400 of a solar day. The second was redesigned in 1967 to take use of the high precision possible with an atomic clock, which employs the distinctive frequency of light emitted by the cesium-133 atom as its "reference clock". The period of oscillation of radiation from the cesium atom is presently defined at 9 192 631 700 times the second. The most recent generation of cesium atomic clocks shown in figure below.
Tables 1, 2, and 3 show approximate values for some lengths, masses, and time intervals, respectively. The large ranges of values should be noted. Examine these charts to obtain a sense of a kilogram of mass, a time period of 1010 seconds (one century is approximately 3 x 109 seconds), or two meters of length (the approximate height of a forward on a basketball team). Appendix A examines the notation for powers of ten, for as the statement 5x104 for the number 50 000.
Some of the most frequently used “metric” (SI and cgs) prefi xes representing powers of 10 and their abbreviations are listed in Table 4. For example, 10-3 m is equivalent to 1 millimeter (mm), and 103 m is 1 kilometer (km). Likewise, 1 kg is equal to 103 g, and 1 megavolt (MV) is 106 volts (V).
Sumber http://keluargasepuh86.blogspot.com
