Newton’s First Law

Sir Isaac Newton first presented his three laws of motion in 1686 in Principia Mathematica Philosophiae Naturalis. His first law states that any object, whether at rest or in uniform motion, will remain in a straight line unless compelled to change its state by the action of an external force. This is usually used as the definition of inertia. The key point here is that when there is no net force resulting from the unbalanced forces acting on an object (when all external forces cancel each other out), the object will maintain a constant velocity. If this velocity is zero, the object remains at rest. And when an additional external force is applied, the speed changes due to the force. The amount of change in velocity is described by Newton’s second law of motion.

Definition

Newton’s first law of motion states that any object, whether is at rest or is in uniform motion, will remain in a straight line in the same state unless it is forced to change state by the action of an external force. The tendency to resist changes in a state of motion is inertia. No net force acts on an object (when all external forces cancel each other out). Therefore the object maintains a constant speed. If this velocity is zero, the object will remain at rest. When an external force acts on an object, the speed changes because of the force.

There are two conditions on which the first law of motion depends. 

Objects at rest

When an object is at rest, the speed (v = 0) and the acceleration (a = 0) are zero. Therefore, the object continues to be at rest. 

Moving objects

When an object is moving, the speed is not zero (v ≠ 0) while the acceleration (a = 0) is zero. Therefore, the object will continue to move at a constant speed and in the same direction.

External Force

In our daily lives, we are exposed to many external physical forces in the performance of our activities. These forces vary in magnitude and direction and therefore vary in effect. For example, when you feel a nice spring morning breeze, it feels different than the effect of wind loads on high-rise buildings. In structural analysis, external forces are forces applied to the structure from outside the structure. Examples of external forces are dead loads such as the weight of the structure itself and non-structural materials and traffic loads, which include, but are not limited to, moving loads such as people, property, and fixtures, as well as wind loads, seismic loads, and impact loads. In addition, despite the lack of connection between two or more objects, forces such as gravitation and electromagnetic force can also be classified as an external force.

Inertia

The principle of inertia is one of the fundamental principles of classical physics, still used today to describe how things move and how forces act on them. The term inertia can be defined as “the amount of resistance of an object to a change in velocity or resistance to a change in motion”. This includes changes in the speed or direction of movement of the object. One aspect of this property is the tendency of things to move in a straight line at a constant speed when no force is acting on them. 

There are three Types of Inertia 

Inertia of rest 

An object stays where it is placed and stays there until you or something else moves it. 

Inertia of Motion

An object will continue at the same speed until a force acts on it. 

Inertia of Direction 

An object keeps moving in the same direction when no force acts on it.

Conclusion

Newton used these three laws to explain and study many physical phenomena. Newton proved that these laws, in addition to the law of universal gravitation, are capable of explaining Kepler’s laws of planetary motion, and these laws are still among the most important physical laws till date. These laws were used in every aspect of our day to day life, from walking to driving a car, from falling from a height to swimming in the water; everything can be described using Newton’s Law of Motion.