Newton’s Law (s) are the bedrock of classical mechanics. On 05 July 1687 Sir Issac Newton, an English physicist and mathematician published a set of three laws of motion. These (laws) laid the foundations of classical mechanics and described the relationship between an object, force and resulting motion. These laws of motions were subsequently called Newton’s law (s). We shall now discuss each of the three Newton’s law (s) in detail here.

Newton’s law No. 1

When viewed in an inertial reference frame, an object either remains at rest or continues to move at a constant velocity, unless acted upon by an external force.

In his first law, Newton expanded on the concept of Inertia as expounded by Galileo. According to the first Newton’s law, all objects that are moving will continue to move unless an external force acts on it. The first law is similar to the concept of inertia put forth by Galileo.

Imagine a moving car. According to this (Newton’s law) law, the car should keep moving unless an external force acts on it. But in practice, the car will come to a stop once it runs out of fuel. How do we explain this discrepancy?

This discrepancy can be explained if we account for the frictional forces that are acting on the car. We will discuss friction in more detail in another article.

Newton’s Law No. 2

The vector sum of the external forces F on an object is equal to the mass (m) of that object multiplied by the acceleration vector (a) of the object: F = ma.

The second Newton’s law is a powerful tool that allows for quantitative calculation of force and acceleration. In the above equation force on an object is a product of its mass and acceleration. As Newton’s law (s) are valid only for constant mass systems, we can conclude that acceleration of an object is proportion to the force acting on it. Here both force and acceleration are vectors, and the direction of the force is the direction of the acceleration. The first and second Newton’s law differ from the ideas of Aristotle significantly. Aristotle propounded that force maintains the velocity of an object. Aristotle’s views seem to be more plausible based on common sense. However, they (Aristotle’s views) are not true. Aristotle erred to account for the frictional forces. Therefore, his observations are at variance with that of Newton.

Newton’s law No. 3

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

The third Newton’s law can be summarised in a simple sentence- “every action has an equal and opposite reaction”. This is the law that has made most of our air, space and water travel possible.

Do you know how a jet engine works?

A jet engine is similar to a balloon filled with air. As long as the mouth of the balloon is shut, there is no net force acting on the balloon. However, as soon as the mouth of the balloon is opened, air from inside the balloon escapes through the hole. The balloon now experiences a force or thrust that propels it forward. The force pushing the balloon forward is equal to the force exerted by the escaping air.

newton's law

Image from howthingsfly.si.edu

In a jet engine, fuel is burnt at a rapid pace, and a large quantity of hot gas (mainly carbon dioxide) is produced. This (hot gas) is extruded through a small nozzle at the back of the engine. The force exerted by this gas is equal to the force acting on the engine to push it forward.

So how does a rocket fly?

Rockets operate on the same principle. However, as rockets fly in space, they have to carry their source of oxygen. Most sophisticated rockets carry liquid oxygen as the oxidising agent.