Newton’s First Law vs Second Law of Motion
In his groundbreaking book Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), Sir Isaac Newton proposed the three laws of motion. Newton’s laws of motion are the corner stones of the classical mechanics. These laws are applied almost everywhere on the field of physics. Newton’s first law describes the motion of an object in a qualitative method. The first law also defines the inertial frame. The second law of motion is a quantitative law, and it also describes the concept of force. It is vital to have a very good understanding in these laws in order to have a proper understanding in classical mechanics and even relativity. In this article, we are going to discuss what Newton’s first law of motion and Newton’s second law of motion are, their definitions, the physical interpretations of these two laws, the similarities between first law and the second law and finally the difference between Newton’s first law and second law of motion.
Newton’s First Law of Motion
The simplest form of Newton’s first law is that the velocity of a body remains unchanged unless the body is acted upon by an external force. If the book is translated to English, it gives the sentence “Every body persists in its state of being at rest or of moving uniformly straight forward, except insofar as it is compelled to change its state by force impressed” as the 1st law of motion. This law implies that to change a certain state of an object an external force must be applied. In other words, the object is unwilling to change the current state. This is known as the inertia of the object. Inertia can be identified as the tendency of an object to stay in its current state. Any frame (coordinate system) that satisfies Newton’s first law is known as inertial frame. In this sense, the first law of motion can be taken as the definition of inertial frames.
Newton’s Second Law of Motion
The simplest form of the second law is “The acceleration of a body is parallel and directly proportional to the net force F and inversely proportional to the mass m”. In other words, F = k m a. The SI unit system is defined so that k is equal to 1. Therefore, the equation becomes F = ma in SI system. The second law can also be taken as the definition of a force. Force can also be expressed using momentum. The rate change of momentum is equal to the net force applied upon the object. Since an impulse acting on an object is same as a sudden momentum change, the force can also be defined using impulse.
What is the difference between Newton’s First and Second Law of Motion? • The first law is a qualitative one whereas the second law is a quantitative one. • The first law is the definition of inertial frame whereas the second law is the definition of force. • When the net force on the object is zero the 2nd law reduces to the first law of motion.

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