To solve the mathematical problems of physical quantities, it is important to have a brief knowledge about units and dimensions. The basic concept of dimensions is that, only those quantities can be added or subtracted which have the same dimension. This concept helps us to derive relationships between physical quantities.
Dimensional analysis is the study of the relation between physical quantities based on their units and dimensions. It is used to convert a unit from one form to another. While solving mathematical problems, it is necessary to keep the units the same to solve the problem easily.
The conversion factor used is based on the unit that we desire in the answer.
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Dimensional analysis is also called a Unit Factor Method or Factor label method, because a conversion factor is used to evaluate the units.
For example, suppose we want to know how many meters there are in 4 km.
Normally we calculate as.
1 km = 1000 meters
4 km = 1000 × 4 = 4000 meters
(Here the conversion factor used is 1000 meters)
This principle depicts that, “the dimensions are the same for every equation that represents physical units. If two sides of an equation don’t have the same dimensions, it cannot represent a physical situation.”
For example, in the equation
[M]a[L]b[T]c = [M]x[L]y[T]z
As per this principle, we have
a = x,
b = y, and
c = z
For using a conversion factor, it is necessary that the values must represent the same quantity. For example, 60 minutes is the same as 1 hour, or 1000 meters is the same as 1 kilometer, or 12 months is the same as 1 year.
Let us try to understand it in this way. Imagine you have 15 pens and you multiply that by 1, now you still have the same number of 15 pens. If you want to find out how many packages of the pen are equal to 15 pens, you need the conversion factor.
Now, suppose you have a packaged set of ink pens, in which each package contains 15 pens. Let's consider that you have 6 numbers of packages. To calculate the total pens, you have to multiply the number of packages with the number of pens in each package. This comes out to be
15 × 6 = 90 pens
Some other examples of conversion factor which are used in day to day life are:
A Simple Example: the time period of a harmonic oscillator.
A More Complex Example: the energy of a vibrating conduction or wire.
A Third Example: demand versus capacity for a disk that is rotating.
Dimensional Analysis is an important aspect of measurement, and it has many applications in physics. Dimensional analysis is used mainly because of five reasons, they are:
To check the correctness of an equation or any other physical relation based on the principle of homogeneity. There should be dimensions on two sides of the equation. The dimensional relation will be correct if the L.H.S and R.H.S of an equation have identical dimensions. If the dimensions on two sides are incorrect, then the relations will also be incorrect.
Dimensional analysis is used to convert the value of a physical quantity from one system of units to another system of units.
Dimensional analysis is used to represent the nature of physical quantity.
The expressions of dimensions can be manipulated as algebraic quantities.
Dimensional analysis is used to derive formulas.
Some major limitations of dimensional analysis are:
● The dimensional analysis doesn't provide information about the dimensional constant.
● Dimensional analysis cannot derive trigonometric, exponential, and logarithmic functions.
● Dimensional analysis doesn't give information about the scalar or vector identity of a physical quantity.
Kinetic energy has a dimensional formula of,
[M1 L2 T-2]
● M = Mass of the object
● L = Length of the object
● T = Time taken
Kinetic energy (K.E) is given by = ½[Mass × Velocity2]------ (i)
The dimensional formula of Mass is = [M1 L0 T0]-------------- (ii)
We know that,
Velocity = Distance × Time-1
= [L] × [T]-1 (dimensions formula)
Velocity has a dimensional formula of, = [M0 L1 T-1]---------- (iii)
On substituting equation (ii) and iii) in the above equation (i) we get,
Kinetic energy (K.E) = ½[Mass × Velocity2]
Or, K.E = [M1 L0 T0] × [M0 L1 T-1]2 = [M1 L2 T-2]
Therefore, the dimensional formula of kinetic energy is represented by [M1 L2 T-2].
1. Find Out How Many Feet Are in 300 Centimeters (cm).
We need to convert cm into feet.
Firstly, we have to convert cm into inches, and then inches into feet, as we can't directly convert cm into feet.
The calculation of two conversion factors is required here:
Then, 300 cm = 300× 1/30.48 feet
= 9.84 feet
2. Check The Consistency Of Dimensional Equation Of Speed.
Dimensional analysis is used to check the consistency of an equation.
Speed = Distance/Time
[LT-1] = L/T
[LT-1] = [LT-1]
This equation is correct dimensionally, because it has the same dimensions of speed on both sides. This basic test of dimensional analysis is used to check the consistency of equations, but it doesn't check the correctness of an equation.
By this method, the constants of some physical quantities cannot be determined.
Sine of angle = Length/Length, and hence it is unit-less.
So, it is a dimensionless quantity.
3. How to Check For Dimensional Consistency
Let us consider one of the equations, let say, constant acceleration,
The equation is given by
s = ut + 1/2 at2
s: displacement = it is a unit of length,
Hence its dimension is L
ut: velocity x time, its dimension is LT-1 x T = L
1/2at2 = acceleration x time, its dimension is LT-2 x T2 = L
All these three terms must have the same dimensions in order to be correct.
As these terms have units of length, the equation is dimensionally correct.