A unit is defined as a standard or fixed quantity of one kind used to measure other quantities of the same kind.
Classification
Fundamental units and derived units are the two classifica tions.
Fundamental units
Units of basic quantities of length, mass and time.
Derived units
Units which are derived from basic units and bear a constant relationship with the fundamental units. E.g. area, volume, pressure, force etc.
Systems of units
– F.P.S system is the British system in which the basic units of length, mass and time are foot, pound and second respectively.
– C.G.S system is the metric system in which the basic units of length, mass and time are centimeter, gram and seconds respectively.
M.K.S system is another metric system in which the basic units of length, mass and time are metre, kilo gram and second respectively.
– S.I. units are referred to as Systems International units which is again of metric and the basic units, their names and symbols are as follows. systems (i.e) F.P.S, C.G.S, M.K.S and S.I. systems.
Length: What is the length of copper wire in the roll, if the roll of copper wire weighs 8kg, the dia of wire is 0.9cm and the density is 8.9 gm/cm³?
Solution
mass of copper wire in the roll = 8kg (or)8000grams Dia of copper wire in the roll = 0.9cm Density of copper wire = 8.9 gm/cm³
Area of cross section of copper wire
π d² 4 π × (0.92) 4
Volume of copper wire
8000grams
8.9 gm/cm³
Mass of copper wire Density of copper wire
=898.88cm
Length of copper wire
Volume of copper wire
898.88cm ³
= Area of cross section of copper wire
0.636cm²
= 1413.33 cm
Length of copper wire=1413cm.
Time: The S.I. unit of time, the second, is another base units of S.I., it is defined as the time interval occupied by a number of cycles of radiation from the calcium atom. The second is the same quantity in the S.I. in the British and in the U.S. systems of units.
= 0.636cm²
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