UNITS OF MEASUREMENTS - FLUID MECHANICS (StudyCivilEngg.com)

UNITS OF MEASUREMENTS

SUBJECT : FLUID MECHANICS

Units may be defined as those standards in terms of which the various physical quantities like length, mass, time, force, area, volume, velocity, acceleration etc., are measured. The system of units used in mechanics are based upon Newton’s second law of motion, which states that force equals mass times acceleration or F = m × a, where F is the force, m is the mass and a is the acceleration. There are in general four systems of units, two in metric (C.G.S. or M.K.S.) system and two in the English (F.P.S.) system. Of the two, one is known as the absolute or physicist’s system and the other as the gravitational or engineer’s system. The difference between the absolute and gravitational systems is that in the former the standard is the unit of mass. The unit of force is then derived by Newton’s law. In the gravitational system the standard is the unit of force and the unit of mass is derived by Newton’s law. Below table lists the various units of measurement for some of the basic or fundamental quantities.

Table : Units of Measurement

The units of measurement for the various other quantities may be readily obtained with the help  of the above table. Further table below illustrates all the four systems of units in which the units  are defined so that one unit of force equals one unit of mass times one unit of acceleration.

Table : Systems of Units - their relationships

Further the following relationships may be utilized to affect the conversion from one system to  another.

1 gram-wt. = 981 dynes:

1 metric slug = 9810 gm (mass)

1 lb-wt.= 32.2 poundals

1 slug = 32.2 lb (mass)

The use of the different systems of units by the scientists and the engineers and also by the different  countries of the world often leads to a lot of confusion. Therefore, it was decided at the Eleventh  General Conference of Weights and Measures held in Paris in 1960 to adopt a unified, systematically  constituted, coherent system of units for international use. This system of units is called the International System of Units and is designated by the abbreviation SI Units. More and more countries of the world are now adopting this system of units. There are six base units in SI system of units which are given in table below.

Table : Base units in SI System

The unit of a derived quantity is obtained by taking the physical law connecting it with the basic  (or primary or fundamental) quantities and then introducing the corresponding units for the basic  quantities. Thus in SI units the unit of force is newton (N) which according to Newton’s second law  of motion is expressed as 1 N = 1 kg × 1 m/s2, i.e., a force of 1 N is required to accelerate a mass of  1 kg by 1 m/s2. The units for some of the derived quantities have been assigned special names and  symbols. Some of the important derived units with special names, commonly used in Fluid mechanics,  in SI and metric gravitational systems of units are given in below table

Table : Commonly used units in SI and Metric

Certain units though outside the International System have been retained for general use in this  system also. These units are given in below table.

In using SI units certain rules and conventions are to be followed which are as noted below

• Names of units, even when they are named after persons, are not written with first letter capital when written in the spelled form e.g., newton, joule, watt etc.
• The symbols for the units which are named after persons are written with capital first letter of  the name e.g., N for newton, J for joule, W for watt, etc.
• The symbols for all other units are written in lower case (small letters), e.g., m for metre, s for  second, kg for kilogram, etc.
• A dot is inserted in the space between the symbols for the compound (or combined) units  e.g., N.m (for newton metre), kW-h (for kilowatt hour) etc.
• Numbers with more than three digits should be written in groups of three with narrow space  in between consecutive groups. However, a sequence of four digits is usually not broken e.g.,  
• 50000 should be written as 50000
• 72345.685 should be written as 72345.685
• 0.13579 should be written as 0.13579
• 9810 may be written as 9 810 or 9810
• The decimal multiples and sub-multiples of the units are formed by using the prefixes. The  various prefixes and the corresponding symbols are given in below table

The prefixes hecto (h), deca (da), deci (d) and centi (c) are not commonly used and these should  be used only when special necessity arises. Further as far as possible prefixes in steps of 10±3 should  only be used to form multiples and sub-multiples of the units. It may also be noted that compound prefixes (or combination of prefixes) are not to be used. For example the correct form for 10–9 is the prefix nano (n) and not the combination of prefixes such as milli micro (mµ) or any other combination  which is incorrect and should not be used. 

FAQs COVERED IN THIS POST

What are the basic units of measurements?

What are the units of measurements used in Fluid Mechanics?

What are the System of units and their relationships?

What are system of units used in Fluid Mechanics?

What are the commonly used units in SI system?

What are the commonly used units in Metric System?

What are the prefixes of various symbols in Fluid Mechanics?

ALSO READ : INTRODUCTION, HISTORY AND DEFINITION OF FLUIDS

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