Units and Standards

Standard of Measurement It is an exact quantity that people have agreed upon to use for comparison. A standard of measurement always remains constant (It does not change each time you use it.) It can be readily duplicated (So everyone who is using it will get the same results)

Earlier forms of measurement did not work well because they were not good standards. The “Foot” may actually have been the size of someone's foot.

Shoes and barleycorn A barleycorn is simply one grain from a barley plant. During the time of the Romans, shoemakers in that society determined that three barleycorns could be used as a unit of measure—an inch. One barleycorn represented one third of an inch, and became a standard in shoe measurement.

Two major systems of measurements in use are the English system (pounds, inches, feet etc.) and the Metric System, based on multiples of 10. (This was devised by a group of French scientists in 1791.)

Originally, the French commission that created the metric system decided that the basic unit of length should be one ten millionth (10 -7 ) of the distance from the equator to the pole.

Originally the length was carefully determined and marked off by means of two parallel scratches on a bar of platinum-iridium alloy. The distance between the these two scratches, when the bar was kept at 0 o Celsius (32 o F) was then officially declared to be the standard meter. (The original bar has been kept at the International Bureau of Weights and Measures in France.)

After the Treaty of the Meter had been signed in 1875, the International Bureau of Weights and Measures (BIPM) in Sevres, France made 30 bars of platinum- iridium as meter standards. The bars had a modified X cross section named for the French scientist, Henri Tresca, who proposed it. The Tresca section was designed to provide maximum rigidity. United States received National Prototype Meters No. 27 and No. 21 in 1890.

As science and technology advanced, the standard of length was redefined. In 1960 it was defined in terms of a wavelength of orange light from Kypton 86 gas. This was more precise that the bar of metal.

The standard measure of length eventually became the distance light travels in 1/299,792,458 second.

Astronauts set up light reflectors on the moon during exploration from 1969 –early 1970’s. The time it took pulses of laser light, aimed at these reflectors, to travel to the moon and back to Earth, showed the distance of the moon to be 378,000,000 meters (approx.234,878 miles).

International System of Measurement (SI units): This is the system of standard measurement units used worldwide. Force Newton N

Accuracy and Precision

Accuracy can be affected by: Method errors – errors in taking the measurement (such as “line-of-site” -pg 16 Serway/Faughn Physics - Holt ) Instrumental error – problems with the measuring device (such as a damaged or warped meter stick, or a balance that has not been adjusted properly.)

Precision Poor accuracy involves errors that can be corrected. On the other hand, precision describes how exact a measurement can possibly be. A lack or precision is typically due to limitations of the measuring instrument and not the result of human error or lack of calibration. (If a meterstick is divided only into centimeters, it will be difficult to measure something only a few millimeters thick.)

Significant figures (digits) The significant digits in a measurement consist of all the digits known with certainty plus one final digit, which is uncertain or is estimated. It demonstrates the precision of the measuring instrument used.