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Introduction to Principles of Motion

Sir Issac Newton in his book "Philosophiæ Naturalis Principia Mathematica" states about the principles of motion which are the related to Inertia in his first principle, Acceleration and Force in second law, and in the third principle he wrote about Action-Reaction phenomenon.

Principles of motion is generally known as Newton's Laws of Motion, apart from this there are lot more to know about different Laws and Principles of Motion, such as circular motion, gravitational effect (free fall acceleration), spacial motion of heavenly objects etc.

Newton's first Law of Motion

Newton in his 1st Law of Motion explains about the Inertia also called as “The Law of Inertia”.

A body continues to be in motion if it is moving or will stay at rest if it was initially at rest unless acted upon by external force.

Another definition could be:

“Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces impressed on it.”

For instance, a book lying on the table will stay as it is unless someone picks up or moves it by pushing or pulling.

A day to day practical experiment: In this demo we will learn about Inertia. A coin is placed on the card which is again placed on an empty glass as shown in figure. The coin being in rest continues to be in rest, and any changes around it will not affect its position and motion unless the coin comes into direct contact. We are applying force only on the card because of which coin is not affected much and falls into the glass while card moves away.

  Demonstrate Inertia

The above experiment and definition explains almost everything about the first principle of motion.

Newton's Second Law of Motion

Newton's Second Law: The acceleration produced by force acting on a body is directly proportional to the magnitude of the force and inversely proportional to the mass of the body.

`F/m = a`


The acceleration of an object is proportional to the force acting upon it and is directed in the direction of the force.

`F = m*a`

For example, when the rocket is sitting on the launch pad, its weight is exactly opposed by the upward "supporting" force generated by the launch pad. These diametrically opposed forces result in a net external force of zero. Since the rocket is initially at rest and no net external force exists, it will stay at rest. Once the rocket motor is ignited, the external force is no longer balanced and the rocket begins to move. In this case, the external thrusting force overcomes the rocket's "desire" to say motionless.


The above experiment and definition explains almost everything about the second principle of motion.

Check this topic Motion Graphs it might be helpful to you for more help keep reading my blogs.

Newton's third Law of Motion

Newton’s Third Law: In a two body system when a first body exerts a force F on a second body, then second body exerts an equal and opposite force –F on the first body. Both the forces are equal in magnitude and opposite in direction. This law is actually called Action-Reaction Law.

  Rocket Engine Thrust

The above experiment and definition explains almost everything about the third principle of motion.