# Underlying cause of Inertia?

1. Dec 3, 2005

### Physics101

When dealing with the concept of Inertia (Newton's First Law of motion), is there an explanation of why this is so or do we accept it for what it is based on observations and experiments? While realizing that asking for more and more underlying reasons can be recursive and has to stop somewhere, I was just wondering if Inertia was to be taken as a given.

2. Dec 3, 2005

### Danger

As far as I know, it's based upon very well established measurements. The underlying reason might be known to the biggies in physics, but I've never seen it explained. Okay, this wasn't really helpful, but at least it's a response.

3. Dec 3, 2005

### Physics101

LOL. At least I got a response to this very mundane question.
Okay. Here's my take on it followed by another related question. If I'm off by a lot, then someone please stop and correct me.

If I were to take an educated guess at the reason for Inertia, I suppose it has to do with conservation of Energy (never mind the underlying reason for that for a second). When force is applied to an object (mass) causing it to gain velocity, this Inertia is retained as long as the object doesn't encounter some other external force. I suppose in this sense, it makes perfect sense that an object in motion should stay in motion while at rest, it should stay at rest (speaking relative to any given frame of reference).

The above behavior seems rather obvious when viewed against background devoid of everything (traditional view of vaccum), but what if this weren't the case? If Space-Time isn't as simple as "nothing," but rather much more complex (as suggested by modern physics), can Inertia be equally sensible and explainable?

Consider this scenerio, and once again, stop me if I'm straying into erroneous region. If Space-Time can be thought of as forever expanding (be it in the form of Field or quantized unit that multiplies endlessly -- perhaps the remnant of the Big Bang...) in an isotropic fashion, isn't Inertia simply built-in to the system/background? The force required to "nudge" an object can be equated to moving that mass through this background activity, after which the mass finds itself in a newly shifted point of reference (equilibrium) which will in turn appear as moving away at constant velocity from the original frame of reference (before the force was applied). I believe what I'm trying to describe is somewhat similar to the concept presented in SED.

If my line of thinking so far is not too far off the center, I have few more follow-up questions and thoughts. Any responses and guidances will be appreciated.

4. Dec 3, 2005

### Danger

Overlooking the fact that I've had an awful lot of beer at this point, you seem to present a fascinating question. Unfortunately, you're beyond my field of knowledge. I can't wait to see what Astro, Chronos, Chi Meson and others of their ilk have to contribute here.

5. Dec 3, 2005

### Physics101

LOL. Lots of beer? Some might argue that I've consumed too much alcohol to even suggest this type of notion. If you think this is fascinating or crazy, then read the following thread I started on the QM board. It sort of is a continuation of this concept, but related more to QM.

6. Dec 4, 2005

### Sir_Deenicus

Haha, if I had a conclusive answer to your question then be certain that I would already be preparing my nobel speech for next year.

The nature and origin of inertia is unknown and is a subject to strong debates and arguments among its discussers. Take for example Mach's principle, which (in one of its many forms) states that inertia is a result of a bodies interaction with the rest of the universe; some argue such is incosistent with General Relativity while others point out sensbily that such a conclusion can only be reached if the conceptualization of the principle is [mis]approached in an inconsistent manner.

See D Sciama's On the Origin of Inertia and also The Lense--Thirring Effect and Mach's Principle by the General relativity master, Hermann Bondi and [Joseph Samuel].

There are even more convtraversial claims which argue that inertia is electromagnetic in origin.

Did 20th century physics have the means to reveal the nature of inertia and gravitation?

Update on an Electromagnetic Basis for Inertia, Gravitation, the Principle of Equivalence, Spin and Particle Mass Ratios

Last edited: Dec 4, 2005
7. Dec 4, 2005

### Physics101

Thaks, Sir_Deenicus. I'll be sure to catch up on reading the links you provided. To tell you the truth, I wasn't even aware that there was any controversy associated with the origin of inertia.

8. Dec 4, 2005

### Danger

Me neither. I just figured that I didn't know enough about it.
And just so's you know's... that's 8 Keith's (pronounced Keets), a couple of Canuk's, and a Kokanee.

Yes, I know that it's kind of wimpy, but given my weight/height rato, it's enough to get me pissed.

Last edited: Dec 4, 2005
9. Dec 4, 2005

### Averagesupernova

What inertia actually is has perplexed me for a very long time. I keep trying to relate it to rotational inertia. Take for instance a flywheel rotating on a horizontal axis that is spun up to a fairly good speed that easily demonstrates resistance to rotating it 90 degrees from its axis. Just why is this? What causes this resistance? I would say that it is because the mass in the flywheel has to follow a back and forth path while rotating. As the mass travels UP on one side of the wheel while the rotating wheel is turning on a verticle axis it has to travel sideways also (left for instance). But when it travels DOWN the other side of the wheel it has to travel right. So the faster it turns in the verticle axis the more distance it has to cover in a left to right motion in the same amount of time. Obviously this takes more energy which is noticed by the resistance in turning the flywheel in the verticle axis. So how does this relate to plain old inertia? I don't know, you tell me. But I've often wondered if it has to do with the orbiting electrons themselves since more electrons means more mass which means more inertia. So assuming that what I've (very) briefly suggested is true, then the particles/subatomic particles must be somehow tied in with spacetime. Yes, I know I haven't really said anything here, I've just illustrated another path that leads to the same old conclusion that matter seems to have a direct connection with spacetime. May I be so bold as to wonder if I have sparked something in someones mind who is much smarter than me who will take this a step farther?

10. Dec 4, 2005

### Danger

The only thing about that that strikes me as maybe a little 'off' is the contribution of electrons to mass. They barely make themselves noticeable in an atom from that standpoint. It's the protons and neutrons that provide the mass.

11. Dec 5, 2005

### vaishakh

Please guys, correct where my concepts are wrong? Inertia as far as understood by me is due to the mass of an object. It is just a ratio between force and acceleration. If there were no inertia all objects would have moved with equal acceleration when provided with equal forces. Does that sound funny or this? Anyway according to me inertia cannot disallow an object from moving. The definition of inertia as the tendency of an object to remain at its state is just a twist of the statement that a net external force needs to be applied on an object to accelerate it. Anyway whatever may be the inertia of an object and may force be as small as it be, still an object should have some displacement. If no then that is not due to inertia. So that way a question that why does objects have mass or what is mass is more meaningful.

12. Dec 6, 2005

### Boffin

There are different competing explanations for this out there. My own view is largely based on Einstein's original concepts, but not necessarily the modern interpretations of them. Inertia is the resistance of energy wavicles changing in the amount of energy they contain as they travel relative to the space continuum. Some people believe that the space continuum is totally empty, but even Einstein believed that the space continuum was a system in which energy moved relative to. When a body changes velocity relative to the space continuum it's energy must change due to the relative change in frequency of the energy, and for this to happen there must be energy supplied. If no energy is supplied then there must of course be a resistance to change in velocity. How the space continuum works is still a great mystery. If it is a granular or grid based system, how does it differentiate between moving in a straight line or arc. Does this make sense?

13. Dec 6, 2005

### Danger

I've never heard that before, Ben. Thanks for the info.

14. Dec 7, 2005

### DaTario

Hi,

I think the author Andre Koch Torres Assis has done an interesting work linking inertia with global gravitational scenario.

I guess this reference (googling his name its enough) will be very interesting to this discussion.

Best Wishes

DaTario

15. Dec 11, 2005

### Wishbone

Can't intertia be defined as an objects tendency for a mass to remain at a certain velocity unless force is acted upon it? Couldn't you say that it is due to the fact that the equation for acceleration does not take into account the time that the partcile has been moving at a certain speed? i.e. it doesnt matter how long something has been moving, it needs a force to accelerate?

16. Dec 11, 2005

### Physics101

Well, that is more or less how inertia behaves, but I was asking a more fundamental question of why it behaves that way. For example, we know that light bends under the influence of gravity (takes geodesic path), so we can explain such behavior. Now, inertia dictates that an object in motion tends to stay in motion and object at rest tends to stay at rest (all relatively speaking, of course). Why the heck is that? Is it some force (dark energy?) or some space-time curvature that causes this? I would tend to go with some form of isotropic dark energy as the underlying cause, but this is pure speculation (and we know how dangerous that can be on these boards).

17. Dec 11, 2005

### Wishbone

Well I think when you ask why at that fundamental of a level, you are really getting into more of philisophical or religious questions, something that is out of the boundraies of physics. I always looked at physics as descripitions of events, unable of answering the question why at that fundamental of a level

Last edited: Dec 11, 2005
18. Dec 11, 2005

### Staff: Mentor

http://en.wikipedia.org/wiki/Inertia
Nice summary of the matter. [pun unintended]

Inertia is "a body's resistance to change" in motion. Mass is a measure of inertia, by vitue of $\vec{F}$ = m$\vec{a}$ = $\dot{\vec{p}}$, where $\vec{a}$ is acceleration, and $\vec{p}$ is momenum ($\vec{p}$ = m$\vec{v}$), or m = |$\vec{F}$|/|$\vec{a}$|.

Weight and Mass
http://theory.uwinnipeg.ca/mod_tech/node25.html

Presumably GR and String Theory would attempt to explain Interia, but I am not familiar with any theories.

I tend to agree with Wishbone, that the understanding of some things is possibly beyond science. The question "What is charge?", would also be as difficult to answer as "What is Inertia?", or more generally, "Why does charge/inertia behave the way it does?"

http://en.wikipedia.org/wiki/Physics - a nice discussion of physics.

We tend to define things in terms of change or how things influence/interact with one another, and most of the time that is sufficient, i.e. for the most part, it is more important to know how things work than why, but sometimes it is important to know why.

Last edited: Dec 11, 2005
19. Dec 11, 2005

### X-43D

No one knows. I think it has to do with the properties of the vacuum.

20. Dec 11, 2005

### vaishakh

Inertia is due to a body having mass. That is the physical reason behind a body's tendency to remain at its mechanical state.