# Why does a body has inertia? [duplicate]

Today I asked a question on this site regarding inertia, and while reading one of the answers, a question popped up

Why does a body have inertia?

I looked up this site and several others, but could not find any resources.

• @YoungKindaichi Nope, that doesn't at all answer my question. Commented Mar 5, 2021 at 12:36
• @YoungKindaichi And that question contains 2 answers, none of which really answers the question. Commented Mar 5, 2021 at 12:38
• What would be the implications if a body didn't have inertia?
– jpf
Commented Mar 5, 2021 at 13:00
• Mach's principle tried to answer this question, and it was one of GR's postulates at its development but was removed by Einstein later. For elementary particles, it was recently described by the interaction between the elementary particle and Higg's field. But for complicated bodies, I believe it has something to do with Mach's principle. Commented Mar 5, 2021 at 13:49

## Philosophical aspect

Physics typically cannot answer "why"-questions very well.

Physics observes phenomena in the world, creates descriptions of the behaviour observed there, and postulates rules (physics laws) stating that under specific circumstances this very same behaviour will happen again.

So, the best answer that physics can give go along the line "because of a well-founded record of behaviours observed previously".

## Inertia

We always observed that it takes some external effort ("force") to change an object's speed, and we call that concept "inertia". Think of a car. To accelerate, you need engine power, to decelerate, you use the brake. Physics found some formulas describing the numerical aspects of that concept.

A body has "inertia" because that's the best explanation we found so far to describe the effect that bodies have a tendency to keep their speed.

If you google it you can find several explanations for this, but I believe using the classical physics, we can clarify it intuitively and simply.

According to the Newton's first law, bodies tend to keep their velocities unchanged. Inertia typically refers to this phenomenon. You can check it by some experimental tests. Therefore, if you want to change their state of motion (i.e., velocities), you need to change their current state of energy by exerting force on each of its constituting particles. More mass is equivalent to more particles that are supposed to be affected by these changes of the energy state. This is why we see this resistance, i.e., inertia, in objects, especially for those with more mass.

@Eisenstein I suspect from some of your replies that @RKleberhoff is closer to the meaning of your question. I think your question is more metaphysical and ontological. The reference of @YKindaichi provides are what I would call descriptive answers of how rather than why. I think we sometimes confuse mathematics with why rather than the how it is. Sometimes people summarize this by saying "the map is not the terrritory".

I think all new physics begins in metaphysics and questions of ontology.

As @AKKassem notes, Einstein was also searching for something more ontological in the beginning in the form of Mach's Principle, but abandoned it eventually. I think physicists like Julian Barbour and Lee Smolin and others are getting closer to Einstein's initial approach, using a relational view of the universe that abandons the concept of a spacetime background. I think Leibniz also contributes to this view,

Einstein starts down this path by tying spacetime to matter. I think if anyone thinks about it for a while what we call space or our concept of volume can't possibly be "real" or "true" in any sense of the word. After all the ultimate paradox in a sense is the idea of a "big bang" "inflation" etc that demands some kind of expansion, but expanding into what? There is something and nothing, and you can't really expand into nothing. Volume and spacetime are probably some kind of relational perspective that just keeps changing of an initial something, but it can't possibly be getting bigger (increasing volume) in any real sense of the word.

Lack of or presence of inertia, in this sense would be the manifestation of a preferred relational perspective, meaning easier to view things along one perspective and harder from another.

When no forces are acting on an object, it is either completely still or moving at a constant velocity at the same direction in a straight line. It is easy to think keeping an object at a constant velocity requires forces acting on it, but that is because we don't live in a vacuum