Unit 1- Mechanics

Chapter 1 – Physical Quantities


The process of comparing an unknown physical quantity with known fixed unit quantity is called measurement.

Physical Quantity

Those quantity which can be measured and can be expressed in numerical value are called physical quantity. Example : Mass, Length, Time etc

A physical quantity is expressed as; Physical quantity = Numerical value (N) x Unit (U)

The physical quantity can be divided into two parts

  1. Fundamental Quantity
  2. Derived Quantity  

Fundamental Quantity :

The basic physical quantity which is taken as standard to measure other physical quantities known as fundamental quantity .

For example: Length, Mass, Time, Temperature, Electric current, Luminous intensity , Amount of chemical substance.

Derived Quantity :

A quantity obtained from fundamental quantities is called a derived quantity.

For example: Area, Volume, Density, Speed , Electric intensity

Unit :

The standard quantity in terms of which the given physical quantities can be compared is called unit. For example: Second, Newton etc.

There are two types of unit :

  1. Fundamental Unit
  2. Derived Unit

Fundamental Unit:

The units of fundamental quantities are called fundamental units.

For example: m, s, k etc

Derived unit

The unit of derived quantities are called derived unit.

For example: m/s, m/s2, etc

System Units:

F.P.S System

In this system Length is measure in foot, mass in pound, time in second.

M.K.S System

In this system Length is measure in meter, mass in kilogram, time in second.

C.G.S System

In this system Length is measure in centimeter, mass in gram, time in second.

Characteristics of standard Unit

  1. It should be well defined and of a suitable so.
  2. It should be easily available, so that easily reproduce in laboratories.
  3. It should not change with time and place.
  4. It should not change with change in physical condition.
  5. It should be universally agreed, so that result of measurement at different places can be compared .

S I System :

The internationally Standardize unit of measurement is called SI system.

In SI system of unit 7 fundamental units and 2 supplementary units were proposed. These units are written below :

Fundamental QuantitiesS.I. UnitsSymbol
Luminous Intensitycandelacd
Electric CurrentampereA
Amount of Substancemolemol

Supplementary Quantities

Plane angleradianrad
Solid Anglesteradiansr

Dimensions of physical quantity: The power raised to the fundamental quantities which are involved in derived physical quantities is called dimensions of physical quantity

Dimensions formula of different physical quantities:

The dimensional formula of speed = [M0L T1]
The dimensional formula of velocity = [M0L T1]
From the above Both speed and velocity has same dimensional formula, But speed is scalar quantity and velocity is vector quantity. Hence, from dimensional formula we cannot say physical quantity is scalar or vector.

Dimensional Equation:

An equation containing physical quantities, each quantity is represented by its dimensional formula the resulting equation is known as dimensional equation.

Alternative Definition:

When the dimension of a quantity is found and expressed in the form of an equation, the equation is called dimensional equation.

Principle of homogeneity of dimension:

The physical relations must obey the principle of homogeneity. According to this concept: “every term in physical relation must have the same dimension.”

Suppose in physical relations, S=ut+12 at 2 in the equation there are three terms:
S,ut,and 12 at2  All the terms must have the same dimensions i.e
[S]=[ut]=[ 12 at2] to obey the principle of the homogeneity.

Table of Units and Dimensions of Physical Quantities:

S.NFundamental Physical QuantityDimensional   formulaSI unit of physical Quantity
4Temperature[K] or [θ]kelvin
5Electric current[Ɪ] or [A]ampere
6Amount of substance[N]Mole(mol)
7  Luminous intensity[J]Candela(cd)
A + B = C + D / or A + B – C – D = 0
According to homogeneity
Dimension of A = Dimension of B = Dimension C = Dimension of D

Derived Physical Quantities:

S.NPhysical quantityFormula RelationDimension formulaSI unit
1Densitymass/volume[M1 L-3 T0]Kg m-3
2Speed or Velocitydistance/time[M0 L1 T-1]m/s
3Accelerationvelocity/time[M0 L1 T-2]m/s2
4momentummass x velocity[M1 L1 T-1]Kg m s-1
5Forcemass x acceleration[M1 L1 T-2]Newton (N)
6PressureForce/area[M1 L-1 T2]Nm-2 or Pa
7WorkForce x displacement[M1 L2 T-2]J (joule)
8EnergyWork(E=mc2)[M1 L2 T-2]J
9Gravitational constant (G)Force Xd2(mass)2[M-1 L3 T-2]Nm2kg-2
10Surface TensionForce/length[M1 L0 T-2]Nm-1
11Moment of inertiaMass x (distance)2[M1 L2 T0]Kg m2
12Angular MomentumMoment of inertia x angular velocity[M1 L2 T-1]Kg m2 s-1
13Torque or coupleForce x 1r distance[M1 L2 T-2]Nm
15Angular velocity (w)Velocity/radius[T-1]S-1
16Specific HeatEnergy/mass x tempr[M0 L2 T-2    k-1]J kg-1  0C-1
17StressForce/Area[M1 L-1 T-2]N/m2
19Refractive indexV1/v2Dimensionless 
20Mechanical AdvantageLoad/EffortDimensionless 
21Electric ChargeI x t[M0 L2 T1 I1]Coulomb
22Electric resistanceV / I[M1 L2 T-3 I-2]Ohm(Ω) or volt
23Boltzmann’s constant (K)Energy/temperature[M1 L2 T-2 K-1]J/K
24Plank’s Constant (h)E = hf[M1 L2 T-1]J.S or eV.S.
25Power of lens (P)P = 1/f[L-1]dioptre

Uses of Dimensional formula:

Check whether the physical equation v2 = u2 + 2as is dimensionally correct or not.

Solution here,

Given formula

 v2 = u2 + 2as

[L.H.S] = [v2] = [LT-1 ]2 = [L2T-2]…………………… (i)

[R.H.S] = [ u2 + 2as] = [L T-1]2  + [L T-2 .L] = [LT-2] + [L2 T-2]

Since, the number ‘2’ is dimensionless,

[R.H.S] = [L2 T-2]  + [L2 T-2] ………………………. (ii)

From equation (i) and (ii), we have

[L.H.S] = [R.H.S]

Hence, the given equation is dimensionally correct.

Example :

Derive the dimensional relation of time period of pendulum with length and acceleration due to gravity.

T  la gb
T = K la gb.....(i)
Where k is dimensionless constant.
[T]= [L]a [L T2]b
[M0 L0 T1]=[M0 La+b T2b]
Equating dimension on both sides, we get
(i) 1=2b
putting the value of b in equation
(ii) a+b=0
or, a=12
putting the value of a & b in equation (i)
T=k l12 g12
T=k l12 g12
T=k lg

# To convert a unit from one system into another:

Convert 10 dyne into Newton.


Let 10 dyne = N2 Newton.

Where, dyne is the unit of force in CGS, and Newton in SI.

Dimension formula of force as

[Force] = [M L T-2]

: a = 1, b = 1, and c = -2 in mass, length and time respectively,

CGS systemSI system
N1 = 10 dyneN2 = ?
M1 = 1 gM2 = 1 kg
L1 = 1cmL2 = 1m
T1 = 1sT2 = 1s

 According to conversion formula,

N2 = N1 [M1M2]a  [L1L2]b   [T1T2]c
Putting the above values, we get
N2 = 10 [1g1kg]1  [1cm1m]1   [1s1s]2
=N2 = 10 [1g1000g]1  [1cm100cm]1   [1s1s]2
= 10×103×102×1= 104N
Hence, 10dyne=104Nproved

# Determine the dimension of universal gravitational constant (G)

We have
F = Gm1m2r2
G =Fr2m1m2
G =[MLT2][L2][M][M]
G =[M1L3T2]
Hence the dimension of G are -1 in a mass, 3 in length and -2 in time.

Limitation of Dimensional analysis:

  1. It cannot give information about dimensionless constant involved in a physical relation.
  2. It is very difficult to derive the dimensional relation between more than three physical quantities.
  3. It cannot be use to derive the relation involving trigonometric and logarithmic functions.
  4. It can’t tell us whether a quantity is vector or scalar.
  5. The dimensionally correct relation may not be physically correct.


The difference between standard value and observed value of physical quantity is called error.

                        i.e    Error = Standard value – observed value


 The measurement of a physical quantity is expressed as x  y where x is observed value and y is error which means the standard value of this measurement lies between (x + y) to (x – y).

Q. The measurement of resistance of a conductor is expressed as (10  5%) Ω. What does it means?

ans: The measurement of resistance of a conductor is expressed as (10  5%) Ω which means 10 Ω is observed value and 5% is error, so the standard value of this resistance lies between (10 – 0.05) Ω to (10 + 0.05) Ω.

Types of error :

Systematic error

      The error introduced due to the Limitation of the formula used and fault in the instrument is called systematic error.

Random error

The error introduced due to the carelessness of the experiment and unfavorable condition of the environment such as temperature, pressure, and humidity etc is called  Random error.

Accurate Measurement:

            The measurement in which observed value of any physical quantity is closer to the standard value of that physical quantity is called accurate measurement.

            Example: Standard value of g in lab is 9.8 m/s2. The measured value of g is obtained as 9.67 m/s2 and 9.79 m/s2. The value so, it is more accurate than the value 9.67 m/s2.

Precise Measurement:

            The measurement in which the observed value of a physical quantity can be reproduced again and again by repeated experiment and produce is called precise measurement.

If the reading are very close to each other, they are called precise reading. The thickness of a glass plate measured by spherometer are 2.43mm, 2.44mm, 2.42mm and 2.44mm. These reading are very closed to each other. So they are précised measurement.

Significant Figure :

            The meaningful digits of a number are called significant figure. Significant figure depends on the least count of a measuring device.

Let us take the length of rod by a ruler for three times. Reading are obtained as 10.2cm, 10.3cm and 10.3cm. so mean length
= 10.2666666 cm
Hence, all digits are not significant only first three digits are significant. The Length of rod is taken 10.2cm or 10.3 cm
Q. The length of rod is exactly 1 cm. An observe records the reading as 1.0cm, 1.00cm and 1.000cm which is most accurate measurement?
Hence, the length of rod = 1.0 or 1.00

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