Standard Units And System Of Units


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What are Standard Units and system of units:

What is standard Units

To describe nature we make different measurments and express these measurments in terms of the magnitures of units. Units enable us to describe things in a concrete way, that is numerically. Suppose that you are given the following directions to find the way to campus wheny you first arrive in town. keep going on this street for a few distances or few blocks, and then turn left at a traffic light, go a little ways and you are there. certainly some units or numbers would be helpful.

Many objects and phenomena can be described in terms of the fundamental physical quantities of length, mass and time (fundamental because they are the most basic quantities or properties we can imagine). In fact the topics of mechanis- the study of motion and force require only these physical quantities. Another fundamental quantity, electric charge, will be discussed. For now lets focus on the units of length, mass, and time.

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To measure these fundamental qunatities , we compare them with a references or standard, that is taken to be a standard unit. Thait is a standard unit is a fixed and reproducible value for the purpose of taking accurate measurments. Traditionallu, a government or international body establishes a standard unit.

A group of standard units and their combinations is called a system of units.

Two major systems of units in use today are the metric system and the British system. The latter is used primarily in the United States but is gradually being replaced in a favor of in favor of the metric system, which is usd throughout most of the world.

The Image shows that the canadian sign warns drivers that the metric system is in use. Notice the difference in the magnitude of the speed limit you’d better not go in 90mi/h.


The decsription of spaces might refer to locaton or to hte size of an object (amount of space occupied). To measure these properties, we use the fundamental qunatity of length. The measurment of space in any direction.

Spaces has three dimensions, each of which can be measured in terms of length. The three diemensions are easily seen by considering a rectangular object such as a bathtub. It has length, width, and height, but each dimension is actually a length. The dimensions of space are commonly represented by a three dimensions Cartesian coordinate system (named in honor of french mathematician Rene Descartes, 1596-1650, who developed the system).

The Standard unit of length in the metric system is the meter(m), from the Greek metron, “to measure”. It was defined originally as one ten-millionth of hte distance from the Earth’s equator to the geographic North Pole.

A portion of the meridian between Dunkrik, France and Barcelona, Sapain, was measured to determine the meter length and that unit was first adopted in Frabce in the 1790s. One meter is slightly longer than 1 yard, as illustrated above.
From 1889 to the standard meter was defined as the length of a platinum-iridium bar kept at the Inernational Bureau of weights and Measures in Paris, France. However, the stability of the bar was questioned ( for example, length variations occur with temperature changes). So new standards were adopted in 1960 and again in 1983. The current definition links the meter to the speed of ;ight in a vacuum, Light travels at speed pf 299,792,458 meters/second (usually listed as 3.00x10sup8m/sec. So by definition 1 meter is the distance light travels in 1/2999,792,458 of a second.

The standard unot of length in the British system is the foot, which historically was referenced to the human foot. As noted in the Physics Facts at the beginning King Henry I used his arm to define the Yard. Other early units commonly were referenced to parts of the body. For example, the hand is a unit that even today is used to measuring the heights of horses (1 hand is 4in).


Mass is the amount of matter an object contains. The more massive an object, the more matter it contains. (More precise definition of mass in terms of force and acceleration and in terms of gravity).

The Standard metric unit of mass is the kilogram(kg). Originally, this amount of matter was related to length and was defined as the amount of water in a cubic container 0.10m or 10cm, on a side. However, for convineicne the mass standard was referenced to a material standard (an artifact or human-made object). Currently, the kilogram is defiend to be the mass of a cylinder of platinum=irridim kept at the international Bureau of Weights and Measures in Paris. The U.S prototype (copy) is kept at the National Institure of Standard Technology(NIST) in washington, D.C.

This stanadard is based on the artifact rather than on a natural phenomenon. Even though the cylinder is kept under controlled conditons, its mass is subkect to slight changes because of contaminiation and loss from surface cleaning. A property of nature, by definition is always the same and in the theory can be measured anywhere. Scientists have yet to agree on an appropriate fundamental constant, such as the speed of light for the meter on which to base the kilogram.
The Unit of mass in the British system is the slug, a rarely used unit. We will not use this unit in our study neacaues a quantity of matter in the British system is expressed in terms of the weight on the surface of the Earth and in units of pounds. (The Bristh system is sometimes said to be a gravitational system.) Unfortuantely, weight is not a fundamental quantity and its use often gives rise to confusion. Of cousrse a fundamental quantity should be the same and not change. However, weight is the gravitational attraction on an object by a celestial body, and this attraction is different for different celestial bodies. The gravitational attraction of a body depends on ots mass.

For example, on the less massive Moon, the gravitational attraction is 1/6 that on the earth. So , an object on the Moon weighs 1/6 less than on the Earth. For example, a suited astronaut who weighs 300 pounds on the Earth will weigh 1/6 that amount or 50 pounds on the moon, but the astronaut’s mass will be the same.

A fundamental quantity does not change at different locations. The Astronaut has the same mass, or quantity of matter, whenever he or she is. As will be, mass and weight are related, but they are not the same. For now, Mass, not weight, is the fundamental quantity.

Time:Each of us has an idea of what time is , but when asked to define it, you may to ponder a bit.
Some terms often used when referring to time are Duration, period and interval. A commong descriptive definition is that time is the continuious, forward flow of events. Without events or happenings of some sort, there would be no perceived time. The mind has no innate awareness of time, only an awareness of events taking place in time. In other words we do not perceive time as such, only events that mark locations in time, similar to marks on a meterstic indicating length intervals.

Note that time has only one direction and it is forward. Time has never been observed to run backward. That would be like watching a film run backward in a projector.

Time and Time Again : What is time ?

Time is a difficult concept to define. The common definition that time is the continuios forward flow of events is more of an observation that a definiton. Others have thought about time :

Time is a strong river of passing events, and strong is its current.

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