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Physics Units and Measurements


A unit is an internationally accepted standard for measurements of quantities.

  • Measurement consists of a numeric quantity along with a relevant unit.
  • Units for Fundamental or base quantities (like length, time etc.) are called Fundamental units.
  • Units which are combination of fundamental units are called Derived units.
  • Fundamental and Derived units together form a System of Units.
  • Internationally accepted system of units is SystèmeInternationale d' Unites (French for International system of Units) or SI. It was developed and recommended by General Conference on Weights and Measures in 1971.
  • SI lists 7 base units as in the table below. Along with it, there are two units - radian or rad (unit for plane angle) and steradian or sr (unit for solid angle). They both are dimensionless.

Base Quantity












Electric Current



Thermo dynamic Temperature



Amount of Substance



Luminous intensity



Measurement of Length

Length can be measured using metre scale (10-3m to 102m), verniercallipers (10-4m) and screw gauge and spherometer (10-5m).

Range of Length

Size of object or distance

Length (m)

Size of proton


Size of atomic nucleus


Length of typical virus


Wavelength of light


Thickness of paper


Height of Mount Everest above sea level


Radius of earth


Distance of moon from earth


Distance of sun from earth


Distance of pluto from sun


Size of our galaxy


Distance to Andromeda Galaxy


Distance to observable universe boundaries


Measuring large Distances – Parallax Method

  • Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. Distance between the two viewpoints is called Basis.

Measuring distance of a planet using parallax method

Similarly, α = d/D

Whereα = angular size of the planet (angle subtended by d at earth) and d is the diameter of the planet.αis angle between the direction of the telescope when two diametrically opposite points of the planet are viewed.

Measuring very small distances

To measure distances as low as size of a molecule, electron microscopes are used. These contain electrons beams controlled by electric and magnetic fields.

  • Electron microscopes have a resolution of 0.6 Å or Agstroms.
  • Electron microscopes are able to resolve atoms and molecules while using tunneling microscopy, it is possible to estimate size of molecule.

Estimating size of molecule of Oleic acid

The steps followed in determining the size of molecule are:

  • Dissolve 1 cm3 of oleic acid in alcohol to make a solution of 20 cm3.Take 1 cm3 of above solution and dissolve in alcohol to make a solution of 20 cm3 Concentration of oleic acid in the solution will be (1/(20x20)) cm3.
  • Sprinkle lycopodium powder on the surface of water in a trough and put one drop of above solution. The oleic acid in the solution will spread over water in a circular molecular thick film.
  • Measure the diameter of the above circular film using below calculations.
  • If n –Number of drops of solution in water, V – Volume of each drop, t – Thickness of the film, A – Area of the film

Total volume of n drops of solution = nV cm3

Amount of Oleic acid in this solution = nV(1/(20x20)) cm3

Thickness of the film = t = Volume of the film / Area of the film

t =  nV/(20x20A) cm

Special Length units

Unit name

Unit Symbol

Value in meters



10-15 m




astronomical unit(average distance of sun from earth)


1.496 X 1011 m

light year(distance travelled by light in 1 year with velocity 3 X 108 m/s)


9.46 X 1011 m

parsec(distance at which average radius of earth's orbits subtends an angle of 1 arc second)


3.08 x 1016 m

Measurement of Mass

Mass is usually measured in terms of kg but for atoms and molecules, unified atomic mass unit (u) is used.

1 u = 1/12 of the mass of an atom of carbon-12 isotope including mass of electrons (1.66 x 10-27 kg)

Apart from using balances for normal weights, mass of planets is measured using gravitational methods and mass of atomic particles are measured using mass spectrograph (radius of trajectory is proportional to mass of charged particle moving in uniform electric and magnetic field).

Range of Mass


Mass (kg)





Red Blood Cell


Dust particle


Rain drop










Boeing 747 aircraft








Milky way Galaxy


Observable Universe


Measurement of Time

Time is measured using a clock. As a standard,atomic standard of time is now used, which is measured by Cesium or Atomic clock.

  • In Cesium clock, a second is equal to 9,192,631,770 vibrations of radiation from the transition between two hyperfine levels of cesium-133 atom.
  • Cesium clock works on the vibration of cesium atom which is similar to vibrations of balance wheel in a regular wristwatch and quartz crystal in a quartz wristwatch.
  • National standard time and frequency is maintained by 4 atomic clocks. Indian standard time is maintained by a Cesium clock at National Physical Laboratory (NPL), New Delhi.
  • Cesium clocks are very accurate and the uncertainty is very low 1 part in 1013 which means not more than 3 μs are lost or gained in a year.


Range of Time


Time Interval (s)

Life span of most unstable particle


Period of x-rays


Period of light wave


Period of radio wave


Period of sound wave


Wink on an eye


Travel time of light from moon to earth


Travel time of light from sun to earth


Rotation period of the earth


Revolution period of the earth


Average human life span


Age of Egyptian pyramids


Time since dinosaur extinction


Age of Universe


Accuracy and Precision of Instruments

  • Any uncertainty resulting from measurement by a measuring instrument is called error. They can be systematic or random.
  • Accuracy of a measurement is how close the measured value is to the true value.
  • Precision is the resolutionor closeness of a series of measurements of a same quantity under similar conditions.
  • If the true value of a certain length is 3.678 cm and two instruments with different resolutions, up to 1 (less precise) and 2 (more precise) decimal places respectively, are used. If first measures the length as 3.5 and the second as 3.38 then the first has more accuracy but less precision while the second has less accuracy and more precision.

Types of Errors- Systematic Errors

Errors which can either be positive or negative are called Systematic errors. They are of following types:

  1. Instrument errors: These arise from imperfect design or calibration error in the instrument. Worn off scale, zero error in a weighing scale are some examples of instrument errors.
  2. Imperfections in experimental techniques: If the technique is not accurate (for example measuring temperature of human body by placing thermometer under armpit resulting in lower temperature than actual) and due to the external conditions like temperature, wind, humidity, these kinds of errors occur.

3. Personal errors: Errors occurring due to human carelessness, lack of proper setting, taking down incorrect reading are called personal errors.

These errors can be removed by:

  • Taking proper instrument and calibrating it properly.
  • Experimenting under proper atmospheric conditions and techniques.

Removing human bias as far as possible

Random Errors

Errors which occur at random with respect to sign and size are called Random errors.

  • These occur due to unpredictable fluctuations in experimental conditions like temperature, voltage supply, mechanical vibrations, personal errors etc.
  • Smallest value that can be measured by the measuring instrument is called its least count. Least count error is the error associated with the resolution or the least count of the instrument.
  • Least count errors can be minimized by using instruments of higher precision/resolution and improving experimental techniques (taking several readings of a measurement and then taking a mean).

Errors in a series of Measurements

Suppose the values obtained in several measurement are a1, a2, a3, …, an.

Arithmetic mean, amean = (a1+ a2 + a3+ … + an)/n=

  • Absolute Error: The magnitude of the difference between the true value of the quantity and the individual measurement value is called absolute error of the measurement. It is denoted by a| (or Mod of Delta a). The mod value is always positive even if Δa is negative. The individual errors are:

Δa1 = amean - a1, Δa2 = amean - a2, ……. ,Δan = amean – an

  • Mean absolute error is the arithmetic mean of all absolute errors. It is represented by Δamean.

Δamean = (|Δa1| + |Δa2| +|Δa3| + …. +|Δan|) / n =

      For single measurement, the value of 'a' is always in the range amean± Δamean

                So, a =  amean ± Δamean Or amean - Δamean< a <amean + Δamean

  • Relative Error: It is the ratio of mean absolute error to the mean value of the quantity measured.

Relative Error = Δamean / amean

  • Percentage Error: It is the relative error expressed in percentage. It is denoted by δa.

δa = (Δamean / amean) x 100%

Combinations of Errors

If a quantity depends on two or more other quantities, the combination of errors in the two quantities helps to determine and predict the errors in the resultant quantity. There are several procedures for this.

Suppose two quantities A and B have values as A ± ΔA and B ± ΔB. Z is the result and ΔZ is the error due to combination of A and B.


Sum or Difference


Raised to Power

Resultant value Z

Z = A ± B

Z = AB

Z = Ak

Result with error

Z ± ΔZ = (A ± ΔA) + (B ± ΔB)

Z ± ΔZ = (A ± ΔA) (B ± ΔB)

Z ± ΔZ = (A ± ΔA)k

Resultant error range

± ΔZ = ± ΔA ± ΔB

ΔZ/Z = ΔA/A ± ΔB/B


Maximum error

ΔZ = ΔA + ΔB

ΔZ/Z = ΔA/A + ΔB/B

ΔZ/Z = k(ΔA/A)


Sum of absolute errors

Sum of relative errors

k times relative error

Significant Figures

Every measurement results in a number that includes reliable digits and uncertain digits. Reliable digits plus the first uncertain digit are called significant digits or significant figures.These indicate the precision of measurement which depends on least count of measuring instrument.

Example, period of oscillation of a pendulum is 1.62 s. Here 1 and 6 are reliable and 2 is uncertain. Thus, the measured value has three significant figures.

Rules for determining number of significant figures

  • All non-zero digits are significant.
  • All zeros between two non-zero digits are significant irrespective of decimal place.
  • For a value less than 1, zeroes after decimal and before non-zero digits are not significant. Zero before decimal place in such a number is always insignificant.
  • Trailing zeroes in a number without decimal place are insignificant.
  • Trailing zeroes in a number with decimal place are significant.

Cautions to remove ambiguities in determining number of significant figures

  • Change of units should not change number of significant digits. Example, 4.700m = 470.0 cm = 4700 mm. In this, first two quantities have 4 but third quantity has 2 significant figures.
  • Use scientific notation to report measurements. Numbers should be expressed in powers of 10 like a x 10b where b is called order of magnitude. Example, 4.700 m = 4.700 x 102 cm = 4.700 x 103 mm = 4.700 x 10-3 In all the above, since power of 10 are irrelevant, number of significant figures are 4.
  • Multiplying or dividing exact numbers can have infinite number of significant digits. Example, radius = diameter / 2. Here 2 can be written as 2, 2.0, 2.00, 2.000 and so on.

Rules for Arithmetic operation with Significant Figures


Multiplication or Division

Addition or Subtraction


The final result should retain as many significant figures as there in the original number with the lowest number of significant digits.

The final result should retain as many decimal places as there in the original number with the least decimal places.


Density = Mass / Volume


if mass = 4.237 g (4 significant figures) and Volume = 2.51 cm3 (3 significant figures)


Density = 4.237 g/2.51 cm3 = 1.68804 g cm-3 = 1.69 g cm-3 (3 significant figures)

Addition of

436.32 (2 digits after decimal),

227.2 (1 digit after decimal) &  .301 (3 digits after decimal) is

= 663.821


Since 227.2 is precise up to only 1 decimal place, Hence, the final result should be 663.8

Rules for Rounding off the uncertain digits

Rounding off is necessary to reduce the number of insignificant figures to adhere to the rules of arithmetic operation with significant figures.

Rule Number

Insignificant Digit

Preceding Digit

Example (rounding off to two decimal places)


Insignificant digit to be dropped is more than 5

Preceding digit is raised by 1.

Number – 3.137

Result – 3.14


Insignificant digit to be dropped is less than 5

Preceding digit is left unchanged.

Number – 3.132

Result – 3.13


Insignificant digit to be dropped is equal to 5

If preceding digit is even, it is left unchanged.

Number – 3.125

Result – 3.12


Insignificant digit to be dropped is equal to 5

If preceding digit is odd, it is raised by 1.

Number – 3.135

Result – 3.14

Rules for determining uncertainty in results of arithmetic calculations

To calculate the uncertainty, below process should be used.

  • Add a lowest amount of uncertainty in the original numbers. Example uncertainty for 3.2 will be ± 0.1 and for 3.22 will be ± 0.01. Calculate these in percentage also.
  • After the calculations, the uncertainties get multiplied/divided/added/subtracted.
  • Round off the decimal place in the uncertainty to get the final uncertainty result.

Example, for a rectangle, if length l = 16.2 cm and breadth b = 10.1 cm

Then, take l = 16.2 ± 0.1 cm or 16.2 cm ± 0.6% and breadth = 10.1 ± 0.1 cm or 10.1 cm ± 1%.

On Multiplication, area = length x breadth = 163.62 cm2 ± 1.6% or 163.62 ± 2.6 cm2.

Therefore after rounding off, area = 164 ± 3 cm2.

Hence 3 cm2 is the uncertainty or the error in estimation.


  1. For a set experimental data of 'n' significant figures, the result will be valid to 'n' significant figures or less (only in case of subtraction).

Example 12.9 - 7.06 = 5.84 or 5.8 (rounding off to lowest number of decimal places of original number).

  1. The relative error of a value of number specified to significant figures depends not only on n but also on the number itself.

Example, accuracy for two numbers 1.02 and 9.89 is ±0.01. But relative errors will be:

For 1.02, (± 0.01/1.02) x 100% = ± 1%

For 9.89, (± 0.01/9.89) x 100% = ± 0.1%

Hence, the relative error depends upon number itself.

  1. Intermediate results in multi-step computation should be calculated to one more significant figure in every measurement than the number of digits in the least precise measurement.

Example:1/9.58 = 0.1044

Now, 1/0.104 = 9.56 and 1/0.1044 = 9.58

Hence, taking one extra digit gives more precise results and reduces rounding off errors.

Dimensions of a Physical Quantity

Dimensions of a physical quantity are powers (exponents) to which base quantities are raised to represent that quantity. They are represented by square brackets around the quantity.

  • Dimensions of the 7 base quantities are – Length [L], Mass [M], time [T], electric current [A], thermodynamic temperature [K], luminous intensity [cd] and amount of substance [mol].

Examples, Volume = Length x Breadth x Height = [L] x [L] x [L] = [L]3 = [L3]

Force = Mass x Acceleration = [M][L]/[T]2 = [MLT-2]

  • The other dimensions for a quantity are always 0. For example, for volume only length has 3 dimensions but the mass, time etc have 0 dimensions. Zero dimension is represented by superscript 0 like [M0].

Dimensions do not take into account the magnitude of a quantity

Dimensional Formula and Dimensional Equation

Dimensional Formula is the expression which shows how and which of the base quantities represent the dimensions of a physical quantity.

Dimensional Equation is an equation obtained by equating a physical quantity with its dimensional formula.

Physical Quantity

Dimensional Formula

Dimensional Equation


[M0 L3 T0]

[V] = [M0 L3 T0]


[M0 L T-1]

[υ] = [M0 L T-1]


[M L T-2]

[F] = [M L T-2]

Mass Density

[M L-3 T0]

[ρ] = [M L-3 T0]

Dimensional Analysis

  • Only those physical quantities which have same dimensions can be added and subtracted. This is called principle of homogeneity of dimensions.
  • Dimensions can be multiplied and cancelled like normal algebraic methods.
  • In mathematical equations, quantities on both sides must always have same dimensions.
  • Arguments of special functions like trigonometric, logarithmic and ratio of similar physical quantities are dimensionless.
  • Equations are uncertain to the extent of dimensionless quantities.

Example Distance = Speed x Time. In Dimension terms, [L] = [LT-1] x [T]

Since, dimensions can be cancelled like algebra, dimension [T] gets cancelled and the equation becomes [L] = [L].

Applications of Dimensional Analysis

Checking Dimensional Consistency of equations

  • A dimensionally correct equation must have same dimensions on both sides of the equation.
  • A dimensionally correct equation need not be a correct equation but a dimensionally incorrect equation is always wrong. It can test dimensional validity but not find exact relationship between the physical quantities.

Example, x = x0 + v0t + (1/2) at2Or Dimensionally, [L] = [L] + [LT-1][T] + [LT-2][T2]

x – Distance travelled in time t, x0 – starting position, v0 - initial velocity, a – uniform acceleration.

Dimensions on both sides will be [L] as [T] gets cancelled out. Hence this is dimensionally correct equation.

Deducing relation among physical quantities

  • To deduce relation among physical quantities, we should know the dependence of one quantity over others (or independent variables) and consider it as product type of dependence.
  • Dimensionless constants cannot be obtained using this method.

Example, T = k lxgymz

Or [L0M0T1] = [L1]x [L1T-2]y [M1]z= [Lx+yT-2y Mz]

Means, x+y = 0, -2y = 1 and z = 0. So, x = ½, y = -½ and z = 0

So the original equation reduces to T = k √l/g

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