INTRODUCTION TO PHYSICS

INTRODUCTION TO PHYSICS

1. Physics is a science. Science works according to the scientific method. The 

scientific method accepts only reason, logic, and experimental evidence to tell 

between what is scientifically correct and what is not. Scientists do not simply 

believe – they test, and keep testing until satisfied. Just because some “big 

scientist” says something is right, that thing does not become a fact of science. 

Unless a discovery is repeatedly established in different laboratories at different 

times by different people, or the same theoretical result is derived by clear use of 

established rules, we do not accept it as a scientific discovery. The real strength of 

science lies in the fact that it continually keeps challenging itself. 

2. It is thought that the laws of physics do not change from place to place. This is 

why experiments carried out in different countries by different scientists – of any 

religion or race – have always led to the same results if the experiments have been 

done honestly and correctly. We also think that the laws of physics today are the 

same as they were in the past. Evidence, contained in the light that left distant 

stars billions of years ago, strongly indicates that the laws operating at that time 

were no different than those today. The spectra of different elements then and 

now are impossible to tell apart, even though physicists have looked very 

carefully. 

3. This course will cover the following broad categories: 

a) Classical Mechanics, which deals with the motion of bodies under the 

action of forces. This is often called Newtonian mechanics as well. 

b) Electromagnetism, whose objective is to study how charges behave under 

the influence of electric and magnetic fields as well as understand how 

charges can create these fields. 

c) Thermal Physics, in which one studies the nature of heat and the changes 

that the addition of heat brings about in matter. 

d) Quantum Mechanics, which primarily deals with the physics of small 

objects such as atoms, nuclei, quarks, etc. However, Quantum Mechanics 

will be treated only briefly for lack of time. 

4. Every physical quantity can be expressed in terms of three fundamental 

dimensions: Mass (M), Length (L), Time (T). Some examples: 

1

2

2

2 2

1 2

Speed 

Acceleration 

Force 

Energy 

Pressure 

LT

LT

MLT

ML T

ML T

−

−

−

−

− −

 You cannot add quantities that have different dimensions. So force can be added 

 to force, but force can never be added to energy, etc. A formula is definitely 

 wrong if the dimensions on the left and right sides of the equal sign are different.

5. Remember that any function f ( ) x takes as input a dimensionless number x and 

outputs a quantity f (which may, or may not have a dimension). Take, for example, 

the function f ( ) sin . θ = θ You know the expansion: 

3 5

sin

3! 5!

θ θ θ θ = − + −⋅⋅⋅ If θ

had a dimension then you would be adding up quantities of different dimensions, 

and that is not allowed. 

6. Do not confuse units and dimensions. We can use different units to measure the 

same physical quantity. So, for example, you can measure the mass in units of 

kilograms, pounds, or even in sair and chatak! In this course we shall always use 

the MKS or Metre-Kilogram-Second system. When you want to convert from one 

hsystem to another, be methodical as in book

7. A good scientist first thinks of the larger picture and then of the finer details. So, 

estimating orders of magnitude is extremely important. Students often make the 

mistake of trying to get the decimal points right instead of the first digit – which 

obviously matters the most! So if you are asked to calculate the height of some 

building using some data and you come up with 0.301219 metres or 4.01219×106

metres, then the answer is plain nonsense even though you may have 

miraculously got the last six digits right. Physics is commonsense first, so use 

your intelligence before submitting any answer. 

8. Always check your equations to see if they have the same dimensions on the left 

side as on the right. So, for example, from this principle we can see the equation 

2 2 2 2 22 v 2 is clearly wrong, whereas v 13 =+ =+ u at u a t could possibly be a 

correct relation. (Here v and u are velocities, a is acceleration, and t is time.) Note 

here that I use the word possibly because the dimensions on both sides match up 

in this case. 

9. Whenever you derive an equation that is a little complicated, see if you can find a 

special limit where it becomes simple and transparent. So, sometimes it is helpful 

to imagine that some quantity in it is very large or very small. Where possible, 

make a “mental graph” so that you can picture an equation. So, for example, a 

2 2 0 -(v-v ) /

 formula for the distribution of molecular speeds in a

 gas could look like (v) v . Even without 

 knowing the value of a you can immediately see that

 a) (v) goes to zero fo

a f e

f

=

0

r large values of v, and v 0.

 b) The maximum value of (v) occurs at v and the

 function decreases on both side of this value.