# Newton's laws of motion

## Newton's third Law of Motion

### Introduction

Sir Isaac Newton was a mathematician and physicist scholar who transformed our scientific world. In 1666, Sir Isaac Newton developed the theories of gravitation when he was just 23 years old. Then, in 1686, he presented three laws of motion in the "Principia Mathematica Philosophiae Naturalis." It is believed that he first started studying the effects of gravity after watching an apple fall. Why did it fall, and what determined the speed at which it fell? It is believed that this incident, as well as his curiosity for seeing stars and planets above without them falling to the ground, led him to develop the laws of motion.[1]

#### Why was Newton's Discovery Important

We use Newton's Laws of Motion to explain various motions and reactions we observe on Earth and in space. His discoveries were important because they helped define rules of physics and contributed to the development of many modern technologies, including air planes and development of rockets and space ships. His discoveries have also been implemented by physicists for over 300 years, and have become the main theories studies in modern day physics.

### What are Newton's Laws of Motion

Newton developed three laws of motion.

1. The first is the definition of inertia: any object will remain at rest or in motion in a straight line unless forced to change by the application of an external force. Objects in motion tend to stay in motion, and objects at rest tend to stay at rest, unless acted upon by an outside force.
2. The second law explains how the velocity of an object can changes when an additional force is applied. The second law simply states that the force needed to accelerate an object equals the mass of the object multiplied by its acceleration. The formula is: Force = Mass X Acceleration.
3. The third law states that for every action (or force), there is an equal and opposite reaction. For example, when you stand on something soft, the object caves in or is pushed down. This is the reaction of your force on the soft object, and the soft object's reaction to you standing on it.

### Newton's Third Law

All forces in the universe occur in equal but oppositely directed pairs. There are no isolated forces; for every external force that acts on an object there is a force of equal magnitude but opposite direction which acts back on the object which exerted that external force. In the case of internal forces, a force on one part of a system will be countered by a reaction force on another part of the system so that an isolated system cannot by any means exert a net force on the system as a whole. A system cannot "bootstrap" itself into motion with purely internal forces - to achieve a net force and an acceleration, it must interact with an object external to itself.

Newton's third law is one of the fundamental symmetry principles of the universe. Since we have no examples of it being violated in nature, it is a useful tool for analyzing situations which are somewhat counter-intuitive. For example, when a small truck collides head-on with a large truck, your intuition might tell you that the force on the small truck is larger.

Let’s consider some of the examples of third law of motion. When a person jumps off a boat, it moves in the backward direction. This can be explained using third law of motion. When the person applies a force on the boat in backward direction, the boat also applies a force on the person which helps him to move forward. As the boat also experiences a force in the backward direction so it moves backward. In this case can we say that acceleration of boat is equal to that of the person? We know that the force acting on the boat and the person is equal but their masses are different. Hence their acceleration will also be different. One of the similar example is the recoil of a gun. The gun exerts some force on the bullet, so the bullet also exerts some force on the gun and hence we feel the recoil.[2]

#### Newton's Third law and Gravitational Force

Is Newton's third law of motion applicable to gravitational force? Yes it is. Let us now see how Newton's third law of motion is applicable to the gravitational force existing between the various objects in the universe. According to Newton's third law of motion “to every action, there is an equal and opposite reaction”. That is, if a body A exerts a force on another body B, then the body 'B' also exerts an equal and opposite force on the body 'A'. Two bodies A and B exerts an equal and opposite force on each other

For example, Let us consider the gravitational force existing between a stone and the Earth. What do you observe when a stone is dropped from a height? It falls towards the Earth. This is because the Earth exerts a force on the stone and according to the universal law of gravitation, the stone also exerts an equal force on the Earth. Now the question is why we do not see the Earth moving towards the stone. Newton's second law of motion gives a correct explanation for this. According to the second law of motion, whenever a force is acting on an object, it produces acceleration in it. The gravitational force of attraction produces acceleration both in the Earth and in the stone. Gravitational force of attraction produces acceleration both in the Earth and in the stone. According to Newton's second law of motion,[3]

Force acting on an object = mass of the object x acceleration produced in the object.

1. https://www.biographyonline.net/scientists/isaac-newton.html
2. "Newton's Laws". hyperphysics.phy-astr.gsu.edu. Retrieved 2017-11-17.
3. http://richannel.org/the-earth-the-moon-and-newtons-third-law