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Base Units in SI Help

By — McGraw-Hill Professional
Updated on Sep 4, 2011

Introduction to Base Units in SI

In all systems of measurement, the base units are those from which all the others can be derived. Base units represent some of the most elementary properties or phenomena we observe in nature.

The Meter

The fundamental unit of distance, length, linear dimension, or displacement (all different terms meaning essentially the same thing) is the meter, symbolized by the lowercase nonitalicized English letter m. Originally, the meter was designated as the distance between two scratches on a platinum bar put on display in Paris, France. The original idea was that there ought to be 10 million (10 7 ) meters along a great circle between the north pole and the equator of Earth, as it would be measured if the route passed through Paris (Fig. 6-1). Mountains, bodies of water, and other barriers were ignored; the Earth was imagined to be a perfectly round, smooth ball. The circumference of the Earth is about 40 million (4.0 × 10 7 ) m, give or take a little depending on which great circle around the globe you choose.

Units and Constants Base Units in SI The Kilogram

Fig. 6-1 . There are about 10 million meters between the Earth’s north pole and the equator.

Nowadays, the meter is defined more precisely as the distance a beam of light travels through a perfect vacuum in 3.33564095 billionths of a second, that is, 3.33564095 × 10 −9 second. This is approximately the length of an adult’s full stride when walking at a brisk pace.

The Kilogram

The base SI unit of mass is the kilogram, symbolized by the lowercase non-italicized pair of English letters kg. Originally, the kilogram was defined as the mass of 0.001 cubic meter (or 1 liter) of pure liquid water (Fig. 6-2).

Units and Constants Base Units in SI The Kilogram

Fig. 6-2 . Originally, the kilogram was defined as the mass of 0.001 cubic meter of pure liquid water.

This is still an excellent definition, but these days scientists have come up with something more absolute. A kilogram is the mass of a sample of platinum-iridium alloy that is kept under lock and key at the International Bureau of Weights and Measures.

It is important to realize that mass is not the same thing as weight . A mass of 1 kg maintains this same mass no matter where it is located. That standard platinum-iridium ingot would mass 1 kg on the Moon, on Mars, or in intergalactic space. Weight, in contrast, is a force exerted by gravitation or acceleration on a given mass. On the surface of the Earth, a 1-kg mass happens to weigh about 2.2 pounds. In interplanetary space, the same mass weighs 0 pounds; it is weightless .

The Second

The SI unit of time is the second, symbolized by the lowercase nonitalicized English letter s (or sometimes abbreviated as sec). It was defined originally as 1/60 of a minute, which is 1/60 of an hour, which in turn is 1/24 of a mean solar day . A second was thus thought of as 1/86,400 of a mean solar day, and this is still an excellent definition (Fig. 6-3). However, formally, these days, 1 s is defined as the amount of time taken for a certain cesium atom to oscillate through 9.192631770 × 10 9 complete cycles.

Units and Constants Base Units in SI The Second

Fig. 6-3 . Originally, the second was defined as (1/60)(1/60)(1/24), or 1/86,400, of a mean solar day.

One second also happens to be the time it takes for a ray of light to travel 2.99792458 × 10 8 m through space. This is about three-quarters of the way to the Moon. You may have heard of the Moon being a little more than one light-second away from Earth. If you are old enough to remember the conversations Earth-based personnel carried on with Apollo astronauts as the astronauts walked around on the Moon, you will recall the delay between comments or questions from earthlings and the replies from the moonwalkers. The astronauts were not hesitating; it took more than 2 seconds for radio signals to make a round trip between Earth and the Moon. In a certain manner of thinking, time is a manifestation or expression of linear dimension, and vice versa. Both of these aspects of nature are intimately related by the speed of light, which Albert Einstein hypothesized is an absolute.

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