Reference: The Book of Universe
Reference: The Book of Physics
There is more to inertia than inertia being just the property of matter by which it retains its state of rest or its velocity along a straight line so long as it is not acted upon by an external force.
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Definition of INERTIA
The properties of consistency and motion appear to be in a precise inverse relationship. In other words, for any stable form of substance, the consistency and motion will always be in precise balance. This explains that the speed of light is constant only because it is in a precise balance with its consistency.
Similarly, in the material region, the velocity of any material object will be precisely balanced by its mass. Only when this balance is disturbed that a restoring force shall be felt. We call this restoring force inertia.
Therefore, inertia is the force that tries to bring the velocity and mass of an object back into balance.
Newton stated that if you accelerate an object to a higher velocity and let go, it keeps on moving at that higher velocity. That will happen only when there is a permanent reduction in mass in balance with that higher velocity; otherwise the inertia will bring the velocity back to its original value.
When an external force is applied to an object, it is not only changing its velocity, but it is also changing its mass. That change in mass is, however, too small to be perceived at normal accelerations, and it is ignored in Newtonian mechanics. But inertia is a manifestation of change in mass. As that force of inertia is permanently overcome the mass is changed. There is now a new balance between reduced mass and higher velocity.
Thus, there is an equivalence between mass and force. Mass is like the condensed force. Force is like the un-condensed mass. It is this un-condensing of mass that appears as the force of inertia.
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Notes
When a satellite is continually being pulled forward by the gravity of a planet its mass starts to get converted into motion. When the satellite moves past the planet it is then continually being pulled back by the gravity of the planet and its motion starts getting converted back into mass. But, in between these two events, there is a certain point where neither the push nor pull of gravity exists, and the satellite floats in space at its natural velocity commensurate with its mass. In this case no inertia exists.
In the above example, when the satellite is floating free of the influence of the gravity of the planet, its motion is balanced by its mass, and there is no resistance due to inertia either against increasing the speed or against decreasing it. This is also the situation in deep space where the external gravitational forces are at a minimum, and the major influence on motion is just the mass of the object. The speed remains constant.
Inertia is manifested as a resistance to change in motion either during acceleration or during deceleration. If no inertia is felt, there cannot be any push or pull. For example, you can neither push nor pull light because in order to do that you have to be able to move faster than the speed of light. So you do not feel any inertia with respect to light; and, you say that light has no mass. The constant velocity of light means that the motion of light is balanced by its consistency. If light had no consistency its speed would keep on increasing.
Therefore, the force of inertia would manifest whenever a change is imposed either on motion or on the consistency of substance. Thus, there exists a conservation of consistency, motion and force. This conservation goes deeper than the conservation of mass and energy. Both mass and energy refer to substance. So, in the law of conservation, any attendant force must be included along with the conservation of substance (mass and energy).
This was the conservation alluded to by Faraday in his talk On the Conservation of Force.
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Key Misconception
A key misconception is that inertia is not seen as directly related to a change in mass of an object in reaction to change in its motion.
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More Misconceptions
You may discover more misconceptions on your own, if you contemplate on each sentence of the above definition with mindfulness. Please see:
Or, you may end up improving upon this definition.
Good luck!
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