Newton’s law of motion is the between forces acting on a body and motion of the body, postulated by Sir Issac Newton. It is first mentioned in his masterpiece, commonly known as Principia in 1687. Inspired by famous philosophers like Copernicus, Galileo, Kepler, and Descartes, Newton laid the foundation of new science. This modern science changed the Aristotelian and heliocentric worldview.

In the Principia, he developed three laws of motion known as Newton’s law of motion. These laws explain why the orbits of planets are elliptical rather than circular. In the 20th century, quantum mechanics and relativity replaced newton’s law of motion and became the fundamental laws of physics. Now let us discuss what the laws of motion are.

**Newton’s First Law of Motion**

- If a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at a constant rate, unless acted upon by a force
- Also called the law of inertia, formulated by Galileo Galilei
- It explains the horizontal motion of the Earth

**Newton’s Second Law of Motion**

- The rate of change of momentum of a body is equal to the magnitude and direction of the total force imposed on it
- It is the quantitative description of the changes that a force made on the moving body
- A force applied to the body can change its magnitude of momentum, or direction
- It is the most important law of physics
- The expression for a body, whose mass (m) is constant, force (F) and acceleration (a) are also constant, is F=ma
- If a body has no acceleration, then there will be no force acting upon it

**Newton’s Third Law of Motion **

- When two bodies interact, they apply a force on one another which is equal in magnitude and opposite in direction
- Newton’s third law of motion is commonly referred to as the law of action and reaction
- The law analysis problems of static equilibrium
- It is also applicable to the bodies which are at rest or in motion
- The forces which this law describes are the real ones
- E.g., A book kept on the table applies equal and opposite force on each other, thus keeping the balance.
- The force occurs because the weight of the book deforms the table slightly, so it pushes back the force on the table like a spring.