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Law of Inertia

The Law of Inertia Explained With Examples

What is the law of inertia? How is it connected with the definition of force? Read to know all about it.
Omkar Phatak
Last Updated: Jul 22, 2017
After I was first introduced to the law of inertia, I was relieved to know that my 'Laziness' made sense after all. It's not just me who doesn't feel like changing things about myself, but it's in the very nature of matter itself, to resist any kind of change. The law associated with inertia is the central axiom on which the whole edifice of Newtonian mechanics is based.
The word 'inertia' is derived from the Latin word 'iners', which means 'idle'. Here is its definition, as stated by Sir Isaac Newton in his book, fondly referred to as Principia,
"The vis insita, or innate force of matter, is a power of resisting by which every body, as much as in it lies, endeavors to preserve its present state, whether it be of rest or of moving uniformly forward in a straight line."
To simplify the definition, what Newton refers to as inertia is the property of matter, which resists all kinds of change and makes it stay in its state of rest or uniform motion, in a straight line. This idea was then used to define Newton's first law of motion, which states:
"Every body prefers to stay in a state of rest or keeps moving at a constant velocity, unless an external unbalanced force acts upon it."
The law makes a connection between inertia and the concept of force. Rather, it states what happens in the absence of a force. The inertia of a body is its inherent tendency to resist changes of any kind. Left to itself, a particle will continue to be at rest or move at a constant velocity, in an environment devoid of any unbalanced forces.
The mass of a body is the measure of its inertia. More the mass, more is its inertia and more is its resistance to change. On Earth, being in the grip of our planet's gravitational force, electromagnetic forces, and in the presence of frictional forces, we cannot see its effects.
The only way you can truly see the effects of inertia in full measure, is in the absence of forces, which is only in outer space. Still you can see its effects, though they are masked by gravitational and frictional forces. When an object is thrown from a flying aircraft, it continues to move with the same velocity as the aircraft, for some time, till it hits the ground, under the effect of gravity. This linear velocity, that it gets from its inertia, makes it fall in a trajectory that is not a straight fall, but a curved one.
In outer space, where there is little or no effect of gravity, a ball thrown away or a bullet fired will continue to move in a straight line forever until it comes in contact with a gravitational or electromagnetic force which halts or changes its direction.
Thus, it is in the nature of matter, to stay in its state of rest or uniform motion, if left undisturbed by the action of any force. Matter overcomes inherent inertia to stay at rest or moves with a constant velocity, when an external force acts on it.