1 parsec, Parallactic second is the distance at which an arc of length 1AU subtends an angle of 1 second of arc 1 parsec = 3.08\(\times\)10¹⁶ m = 3.26 light year.

Triple point of water is the temperature at which saturated vapour, pure and melting ice are all in equilibrium. The triple point temperature of water is 273.16K.

Mole is the SI unit of Amount of substance. One mole is the amount of substance which contains as many elementary entities as there are atoms in 0.012 kg of pure carbon-12.

"ampere" is the SI unit of electric current. One ampere is the constant current, which when maintained in each of the two straight parallel conductors of infinite length and negligible cross section, held one metre apart in vacuum shall produce a force per unit length of 2\(\times\)10^-7 N/m between them.

Kilogram is the SI unit of mass. One kilogram is the mass of the prototype cylinder of platinum iridium alloy (whose height is equal to its diameter), preserved at the International Bureau of Weights and Measures at Sèvres, near Paris, France.

Astronomical telescopes are used to study the motion of planets and other heavenly bodies in the sky. Radio telescopes have enabled the astronomers to observe distant points of the universe. Studies of the universe are done using physical principles.

Range of masses: from heavenly bodies to electron, 10⁵⁵ kg (mass of known observable universe) to 10^-31 kg (mass of an electron) [the actual mass of an electron is 9.11\(\times\)10^-31 Kg].

(i) Accidental discoveries and well-analysed research outcome in the laboratory based on intuitive thinking and prediction.
(ii) For example, magnetism was accidentally observed but the reason for this strange behaviour of magnets was later analysed theoretically.
(iii) This analysis revealed the underlying phenomena of magnetism. With this knowledge, artificial magnets were prepared in the laboratories.

The study of the discrete nature of phenomena at the atomic and subatomic levels.

The study of the production and propagation of sound waves.

The study of forces acting on bodies whether at rest or in motion.

The scientific method is a step-by-step approach in studying natural phenomena and establishing laws which govern these phenomena.

The relative error expressed as a percentage is called percentage error.
Percentage error = \({{\triangle a_m}\over{a_m}}\times100\%\)

The arithmetic mean of the magnitude of absolute errors in all the measurements is called the mean absolute error.
If am is the true value and \(\triangle a_m\) is the mean absolute error, then the magnitude of the quantity may lie between \(a_m+\triangle a_m\) and \(a_m-\triangle a_m.\)

The uncertainty in a measurement is called an error.
The three possible errors are
(i) Systematic errors
(ii) Random errors
(iii) Gross errors

Units used to measure extremely small distances are
(i) 1 micron or micrometer, 1 \(\mu\)m = 10^-6 im
(ii) 1 nanometer, 1 nm = 10^-9 m
(iii) 1 Angstrom unit 1 \(\mathring{A}\) = 10^-10 m
(iv) 1 Fermi, 1 Fm = 10^-15 m.

(i) The unit in which the quantity is measured
(ii) The numerical value or the magnitude of the quantity (n), the number of times that unit (u) is contained, in the given physical quantity. Q = nu.

(i) Barn,1 barn 10^-28m²
(ii) Acre,1 acre = 4047 m²
(iii) Hectare,1 hectare = 10⁴ m².

(i) It is the time taken by the earth to complete one revolution around the sun in its orbit.
(ii) 1 Solar year = 365.25 average Solar days 
= 366.25 Sidereal days

The mks system is based on metre, kilogram and second as the fundamental units of length, mass, and time respectively.

(i) Mechanics is a branch of physics.
(ii) It is divided into statics and Dynamics.
(iii) Mechanics deals with the study of motion of particles, deformable bodies and general system of particles.

(i) Physics is a branch of science.
(ii) The word comes from a Greek word meaning 'nature'.
(iii) It deals with the study of nature and natural phenomena.

Physical quantities which possess dimensions and have constant values are called dimensional constants. Examples are Gravitational constant, Planck's constant etc

Physical quantities, which possess dimensions and have variable values are called dimensional variables.
Examples: are length, velocity, and acceleration etc

The dimensions of a physical quantity are the powers to which the units of base quantities  are raised to represent a derived unit of that quantity