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# Physics: Newton's Laws of Motion

## Newton’s Laws of Motion

Sir Isaac Newton is considered one of the greatest scientists in the history of science. He was born in 1643 on a Christmas day. While he was at home, having nothing to do, he formulated the most popular theories in the field of physics. His theories and discoveries included *Three Laws of Motion*, *Universal Law of Gravitation* and invention of *Calculus* to solve mathematical problems. In 1687, Newton published his three laws of motion in his Latin book *Philosophiae Naturalis Principia Mathematica.* The title of the book means *Mathematical Principles of Natural Philosophy *in English*.* He explains the reasons as to why some objects remain at rest, while some objects remain in motion. These laws are explained below:

## Newton's First Law of Motion

Newton’s First Law of Motion states that** “in the absence of external force, an object will remain at rest and a moving object will remain in motion in a straight line at a constant speed.”**

Newton’s First Law of Motion deals with two important points with a condition. The first point is that any object at rest will remain at rest in the absence of any external force. Consider a book lying on the table. The book will remain on the table if no external force is exerted on it. If external force is exerted on the book, the book will never remain at rest on the table. Rather, it will change its position.

The second point of Newton’s First Law of Motion is of immense importance. This part of Newton’s First Law of Motion means that a moving object will remain in motion in the same direction with a uniform speed if no external force is exerted on it. It means, in the absence of force, a moving object will continue its movement until some external force is applied to change its direction or prevent its movement.

If you throw a stone in the air, it falls quickly on the ground. Why? The reason is that there is some external force, which compels the stone to change its direction and fall on the ground. These forces are the friction between the air and the stone along with the gravitational force of the earth. The gravitational force of the earth pulls the stone towards it, while, the friction slows down the speed of the stone. If these forces are removed, the stone will never fall on the ground. Rather, it will continue its movement in the same direction with a constant speed.

Newton’s First Law of Motion is also called the *Law of Inertia*. WA Redmond defines *inertia* as “the property of matter that causes it to resist any change of its motion in either direction or speed.”

## Newton’s Second Law of Motion

Newton’s Second Law of Motion shows us the relationship between acceleration, force and mass. It says that ** “acceleration of an object is directly proportional to the magnitude of force in the direction of force, while inversely proportional to the mass of the object.” **Mathematically, Newton’s Second Law of Motion can be expressed as

*a= F/m**.*This expression can be rearranged as:

**wherein**

*F=ma***is the total force exerted on the object,**

*F***is the mass, while**

*m*

*a**is the acceleration of the object.*

Newton’s Second Law of Motion is very simple. In everyday life, you might have observed that moving a heavy object is difficult than moving a light object. Similarly, greater the force, the greater will be acceleration of the object. If you exert 50 newton force on an object having 1000 kgs mass, it will move with a speed of .05 m/s^{2}.

## Newton’s Third Law of Motion

Newton’s Third Law of Motion states that “**every action has reaction but in opposite directions.” **It means if an object A exerts force on another object B, object B will also exert force on the first object i.e. object A with the same force but in opposite direction. What happens when you hit a wall with a ball? Indeed, the ball will bounce back with the same force with which you throw the ball. Greater the force, the greater will be reaction.

Why does the rocket move up? The reason is that the hot gas produced by the fuel in the rocket is trapped inside the rocket. When it is released, the rocket moves upward. Emission of gas from the rocket is an action, while moving up of a rocket is a reaction. Similarly, the balloon also goes upward, when the air is released from it. The greater the release of the air, the greater will be the distance travelled by the balloon in the air.

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**© 2015 Muhammad Rafiq**

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