Units and Measurement 1

Question 1

define units

Answer 1

Measurement of any physical quantity involves comparison
with a certain basic, arbitrarily chosen, internationally
accepted reference standard called unit. The result of a
measurement of a physical quantity is expressed by a
number (or numerical measure) accompanied by a unit.
types of fundamental units:
fundamental
derived

Question 2

what is a physical quantity

Answer 2

All those quantities that can be measured directly/indirectly and in terms of which the laws of physics can be expressed is called a physical quantity

Question 3

what are fundamental units

Answer 3

Physical quantities that can be treated as independent of other physical quantities and are not defined in terms of any other physical quantiity

Question 4

why are there very little fundamentalunits?

Answer 4

Although the number of physical quantities appears to be
very large, we need only a limited number of units for
expressing all the physical quantities, since they are interrelated with one another. The units for the fundamental or
base quantities are called fundamental or base units.

Question 5

what are derived units?

Answer 5

Quantities that can be derived from the fundamental quantities are called derived units.
Such units obtained for the derived quantities are called derived units.
Theunits of all other physical quantities can be expressed as
combinations of the base units.

Question 6

what is a system of units

Answer 6

A complete set of units (for measuring physical quantities), with both fundamental and derived units is called a system of units

Question 7

what were the three system of units used in the previous years?

Answer 7

In earlier times, scientists in different countries used differenti systems of units for measurement. The three main ones were the CGS system, FPS system ( or English system) and the MKS system. the base units for kength, mass and time in these are:
* In CGS system they were centimetre, gram and second
respectively.
* In FPS system they were foot, pound and second
respectively.
* In MKS system they were metre, kilogram and second
respectively( what makes this different from SI unit system is because the units of energy (mechanical,heat,electric) vary from what is used in SI system ( common for all type of energy,joule)

Question 8

which system of units is currently in use?

Answer 8

The system of units which is at present internationally
accepted for measurement is the Système Internationale
d’ Unites (French for International System of Units),
abbreviated as SI. The SI, with standard scheme of symbols,
units and abbreviations, was developed and recommended
by General Conference on Weights and Measures in 1971 for international usage in scientific, technical,
industrial and commercial work. Because SI
units used decimal system, conversions within
the system are quite simple and convenient

Question 9

what are the units for measuring angles?

Answer 9

thereare two more units that are defined for
(a) plane angle dθ as the ratio of length of arc ds to the radius r
the unit for plane angle is radian with symbol rad

(b) solid angle dΩ as the ratio of the intercepted area dA of the spherical surface, described about the apex O as the centre, to the square of its radius r.
the unit for solid angle is steradian with symbol sr

both radian and steradian are dimensionless quantities.

Question 10

Give the advantages of SI units?

Answer 10

• It is a coherent system of units- The units of all derived quantities can be obtained my simple multiplication/division of fundamental units without introducing any numerical factor

-It is a rational system of units- there is only one unit for one physical quantity

-The SI units do not vary with time, place or atmopsheric conditions like pressure, temperature etc