Table of Contents

**Introduction:**

In the 20th century, Newton’s laws were replaced by quantum mechanics and relativity as the most fundamental laws of physics. Newton’s laws continue to give an accurate account of nature, except for very tiny bodies such as electrons or for the bodies moving close to the speed of light. The quantum mechanics and relativity reduce to Newton’s laws for the larger bodies or for the bodies which move very slowly.

Newton’s laws of motion are the three statements that describe the relations between the forces acting on a body and the motion of the body. These are initially formulated by the English physicist and mathematician Isaac Newton and these are the foundation of classical mechanics. This law can also be referred to as the law of inertia.

Newton’s first law states that if a body is at any kind of rest then it will continue to be in a rest position unless and until an external force is applied. In classical Newtonian mechanics, there is no important differentiation between the rest and uniform motion in a straight line. These may be regarded as the same state of motion observed by the different observers, where one moves at the same velocity as the particle and the other moves at constant velocity with respect to the particle. This is the postulate which is called the law of inertia.

**A brief outline of the topic: **

If a body contains a net force that is acting on it then it undergoes accelerated motion according to the second law. If there is no net force acting on a body then either because there are no forces at all or because all forces are precisely balanced by the contrary forces and the body does not accelerate and may be said to be in equilibrium. On the contrary, a body that is observed not to be accelerated may be deduced to have no net force acting on it.

**History**

Newton’s laws first come under his masterpiece the Philosophiae Naturalis Principia Mathematica (1687) which is commonly called the Principia. In the year 1543, the scientist Nicolaus Copernicus suggested that the Sun might be at the center of the universe rather than the earth. In the years of intervening Galileo, Johannes Kepler, and Descartes set the foundations of new science that would both replace the Aristotelian worldview and inherited from the ancient Greeks and can explain the workings of a heliocentric universe. The Principia Newton created the new science. He developed his three laws in order to explain why the orbits of the planets are ellipses rather than that of circles at which he succeeded but it turned out that he explained much more. The series of events from Copernicus to Newton is collectively called the Scientific Revolution.

## Newton’s first law of motion

The law of inertia also called Newton’s first law of motion was the first formulated by scientist Galileo Galilei for the horizontal motion on Earth and was later generalized by the scientist René Descartes. The principle of inertia is the starting point and the fundamental assumption of classical mechanics; it is less than intuitively obvious to the untrained eye. In Aristotelian mechanics and in ordinary experience the objects that are not being pushed tend to come to rest. Newton’s first law of motion was deduced by the scientist Galileo from his experiments with balls rolling down on inclined planes.

According to the scientist Galileo the principle of inertia was fundamental to his central scientific task: he had to explain how it is possible that if Earth is spinning on its axis and orbiting the Sun and we do not sense that motion.

**Central Issue **

The principle of inertia helps to give the answer since we are in motion together with Earth and our natural tendency is to retain that motion hence Earth appears to us to be at rest. Therefore the principle of inertia far from being a statement of the obvious was once a central issue of the scientific contention. By the time Newton had sorted out all the details and it was possible to accurately account for the small deviations from this picture caused by the fact that the motion of Earth’s surface is not uniform motion in a straight line.

In the Newtonian formulation, the common observation that bodies that are not pushed tend to come to rest is attributed to the fact that they have unbalanced forces acting on them such as friction and air resistance.

**Newton’s second law**

It is an explanation of the changes that a force can produce in the motion of a body. This law states that the time rate of change of the momentum of a body is equal in both magnitude and direction to the force acting on it. The momentum of a body is always equal to the product of its mass and its velocity. The momentum is a vector quantity similar to the velocity with having both magnitude and direction. A force applied to a body is able to change the magnitude of the momentum or its direction or both.

Newton’s second law is one of the most important in all laws of physics. If a body has mass m which is constant then it can be written in the form F = ma, where F is the force and a is the acceleration and both these are vector quantities. If a body with the net force acting on it then it is accelerated according to this equation. On the contrary, if a body is not accelerated then there is no net force acting on it.

**Newton’s third law **

It states that when two bodies interact then they apply forces to each other which are equal in magnitude and opposite in direction. The third law is also referred to as the law of action and reaction. This law is important in analyzing problems of static equilibrium where all the forces are balanced but it also applies to the bodies in uniform or accelerated motion.

The forces it describes are the real ones. For example, a book that is at rest on a table applies a downward force equal to its weight on the table. According to the third law, the table applies an equal and opposite force to the book. This force occurs due to the weight of the book causing the table to deform slightly so that it pushes back on the book like a coiled spring.

**FAQs (Frequently Asked Questions):**

**Que 1: List down the major concepts involved in the topic?**

**Ans:** The major concepts involve the Frame of reference, Newton’s First law of motion, Newton’s Second law of motion, Newton’s Third law of motion, and the constraint equations.

**Que 2: Write the characteristics of the normal force.**

**Ans: **The normal force is a force that always acts perpendicular to the surface of contact.

**Que 3: ****Define the constraint equation?**

**Ans: **When the motion of one body is governed by another or in other words when the motion of two or more than two bodies is interlinked. The equation governing such motion is called the constraint equation.