E1.4 Dynamics (Newton's Laws of Motion):

"I do not know what I may appear to the world; but to myself I seem to have been only like a boy playing on the seashore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me."

~ Sir Isaac Newton

Sir Isaac Newton (1642-1726): A Mathematician, Astronomer and Physicist was born at Wollsthrope in England. He worked in many area of Mathematics and Physics. 

He developed the theories of gravitation in 1666, when he was only 23 years old. After some year later, he presented his three laws of motion in the book: "Principa Mathematica Philosophiae Naturalis" in 1687. 

Click here for: Vedic Laws of Motion

#1. Newton's $1^{st}$ Law:

"A body remains in the state of rest or in the state of uniform velocity if no net force is acting on it." 

It is also known as law of Inertia. 

(If the net force is zero: Object at rest, stay at rest. And Object in motion, continue to move). 

This means that, there is a natural tendency of objects to keep on doing what they're doing. There is no change in the state (rest or motion) without an external force acting. That is, a force is an agent that produces a change in state of a body. 

 

#2. Newton's $2^{nd}$ Law:

"The rate of change of momentum of a body with respect to time is directly proportional to the net external force acting on the body."

However, Newton's second law gives us an exact relationship between force, mass and acceleration. Mathematically,

$F_{net} = ma$

i.e.  net force on object = mass of object * acceleration

If we exert the same force on two objects of different mass, we'll get different acceleration (change in motion).The heavier objects require more force to move than the lighter objects.

Note: In equation, $F = ma$; if net force acting on a mass is zero then, $a=0$.

This means that, if the net force acting on a body is zero, it moves with constant velocity or it will be rest. Which is the statement of the Newton's $1^{st}$ law. 

#3. Newton's  $3^{rd}$ Law:

"To every action there is always an equal and opposite reaction."

This means that, for every action there is equal and opposite reaction. It dose not matter which force we call action and which we call reaction.

All forces always occur in pair. For example: During the simplest act of walking, we push against the road surface with our shoes and road pushes back with an equal force but in opposite direction. It is the force the road exerts on us that causes us to move forward. 

Similarly, at the swimming time a swimmer moves through the water only because he pushes water backward and the water pushes the swimmer forwards.

Again, Let's understand how a rocket works. The rocket's action is to push down on the ground with the force of its powerful engines, and the reaction is that the ground pushes the rocket upwards with an equal force.

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E1.1 Physical Quantities:

The branch of science which deals with the study of natural phenomena is called Physics.

From the middle of the $18^{th}$ century, natural science has made a rapid progress. So, it has been necessary to divide into different sub-branches. Nowadays Physics means only the branch of Natural Science which deals with the properties of matter, energy and relationship between them.

Physics is very huge, interesting, exciting and complex subject. The range of the mass, length and time is very large in physics. For Example:

In Astronomy : Mass (M) = $10^{30}\;kg$; Length (L) = $10^{26}\;m$; Time (T) = $10^{9}\;years$ 

In Sub-Atomic: Mass (M) = $10^{-31}\;kg$; Length (L) = $10^{-5}\;m$; Time (T) = $10^{-20}\;years$

To understand physics we must be able to connect our theoretical description of nature with our experimental observations of nature. This connection is made through quantitative measurements. To know the physical quantity quantitatively, measurement is must. In order to measure & test physical quantities we need to define some standard measures. Without a widely accepted system of units of measurement our civilization could not exist.

The comparison of any physical quantity (units) with its standard unit (known fixed quantity) is called measurement.

A physical quantity is a property of an object that can be measured with a measuring instrument. Length, time, mass, velocity, force etc. are the example of physical quantities (units).

The quantity used as a standard of measurement is called the unit.

Physical quantities can be divided into two categories, are as follows:

1) Base (Fundamental) Quantities:

Basic quantities are the fundamental quantities that are not related to each other and that are used to derive all other quantities. There are seven basic quantities are as follows: Mass, Time, Length, Temperature, electric current, intensity of light & quantity of matter. 

2) Derived Quantities:

Derived quantities are just quantities that are derived from one or more basic quantities. For example: 

Area is a derived quantity because it is derived from the basic quantity length.

i.e. Area = Length * Length.

Force, Velocity, Pressure, Volume etc. are the derived quantities.

This is a recent upgrade about the Standard Units. Click here.

» International System (S. I.) Units;     » Precision & Significant Figures; 

» Dimension of Physical Quantities;     » Main uses and its Limitations.

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