Units are very important in engineering, especially in mechanical engineering. If you are a very good engineer, you must understand the general unit’s systems rule the engineering systems in use or that you are dealing with. So, you need to start from the fundamentals of units in engineering and science.
Dimensions and Units
Dimensions are used to characterize the physical and engineering systems. The magnitudes of dimensions are defined by units. There are two classes of dimensions which are primary and derived dimensions.
If we take a look at the primary dimensions, they are;
- meter(m) for length,
- kilogram(kg) for mass,
- seconds(s) for time,
- Kelvin(K) for temperature,
- ampere(A) for electrical current,
- candela(cd) for amount of light,
- mole(mol) for the amount of matter.
These are actually for SI units. The secondary or derived units are derived from these primary units. For example, ‘V’ is the velocity that has the unit of ‘meter/second’, which is derived from two primary units; meter and second.
Difference Between SI and English Units
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In the SI unit system, there is a decimal relationship between the successive dimensions such as 1 kilogram is equal to 1000 grams. But in English units, there is no regular relationship between the successive units. For example, again, 12 inches is equal to 1 foot, 1 mile is equal to 5280 feet… This controversy is valid for other primary units in the English system. So, adapting the English system to physical systems or engineering systems is very hard.
For engineering students, it is very hard to learn and apply the engineering and physical systems in English units.
All the industrialized countries are adopted the SI metric system over the English system, except for the United States. In the United States, English units are still in use, and there are lots of studies and works to convert this system to SI systems today. Because of this uncertainty in the United States, engineering books and publications are made in both unit systems.
But, you can realize that there are great changes in the United States converting the English system to SI units. The automotive industry in the United Stated fully converted to the SI metric system.
Unit Conversion Between SI and English Systems
Conversion of the successive same primary units in the English system makes some things hard. But there is another factor that makes dealing with units in physical and engineering systems hard. Conversion of the units between SI and English systems. Again, there is no logical relation between the same primary and secondary units in SI and English systems. Check the examples below.
- 1 lbm = 0.45359 kg
- 1 Btu = 1.0551 kJ
To be a different and good engineer, it will be very advantageous to learn both SI and English units at the same time. But also, there are lots of very good unit converters that you can use to convert different units between the SI and English system.
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