# Understanding of what inertia is?

1. Apr 19, 2004

### jnorman

another newbie question - what is the current understanding of what inertia is? ie, what is it that provides resistance to acceleration?

2. Apr 19, 2004

### AntiMagicMan

We don't have an understanding of the mechanism behind inertia. A potential solution is the Higgs Field theory. Which hypothesises there is a higgs field with an associated higgs boson that gives objects their inertial mass. Basically a particle creates a disturbance in the higgs field and this increases their resistance to acceleration.

3. Apr 19, 2004

### jnorman

thanks for the straightforward response at the beginning of your answer. as to the higgs field, and higgs boson, again i would ask how does such an interaction operate, in your opinion? i am unaware of any experiment which has discerned a higgs boson, or any indication of whence the higgs field is derived - it is merely conjecture, yes? i think i understand that there simply is no answer to this question - but i would be interested in anyone who might wish to opine on the matter.

4. Apr 20, 2004

### Lorentz

Why is there even a need for the law of inertia?

I thought of it and within classical Physics I don't see the need of the concept inertia at all.

The natural state of an object is to stay at rest or move with a constant speed along a straight line. The law of inertia says mass provides resistance to acceleration, but is this true? This implies the object wants to stay at rest or move at a constant speed along a straight line, but whatever small force I apply to a big mass (in free space) it will accelerate and never stay at rest or move with a constant speed along a straight line. So what I'm saying is the object doesn't resist acceleration at all... it's just as natural for an object to accelerate under a force as it is to stay at rest and move at a constant speed along a straight line without any force applied. Both are natural states.

The law of inertia states mass is a measure of inertia which implies mass is a measure of resistance to acceleration. You could agree with this. It is harder to push an object which has a greater mass then an object with a smaller one. But is this really because the mass provides resistance? I don't think so. I like to use the concept of kinetic energy in this case. Different speeds correspond to different energy levels. There's no resistance there's just more energy needed to reach a higher energy level(which is completely logical). All energy applied is converted into kinetic energy. If you could speak of resistance there surely should have been energy loss. When an object has a greater mass it doesn't have more inertia (in my opinion) it just has more matter to divide it's energy with.
Compare the concept of kinetic energy with filling a cup of water:
Water is the kinetic energy.
The cup is the object.
The surface is the mass.
The change in height is the acceleration.
The height of the water level is the speed.
You could have a cup of water for instance that is empty. Now when I fill the cup could you speak of resistance of the cup (or actually: mass = the surface of the bottom) to the filling? No, the glass just lets the filling happen. There's no way the cup struggles to receiving the water (the object just gets more energy and is gladd to hold this). The surface of the bottom could be bigger which would mean the cup would take longer to fill, but does this resemble resistance or inertia? In my opininion there's just more space to fill.

I would replace the concept of inertia with the following which I think is more correct:

The natural state of an object is -->
a) To stay at rest or move with a constant speed along a straight line if no external force is present.
b) To accelerate if there is an external force working on the object.

Further I would use the concept of kinetic energy and the Impuls-momentum theory.

I just don't see the need of the concept of inertia as it is defined in my textbook: "Inertia is the natural tendency of an object to remain at rest or in motion at a constant speed along a straight line. The mass of an object is a quantitative measure of inertia."

5. Apr 20, 2004

### Javier

With regards to the existence of inertia:
Let's stick to classical mechanics (despite the fact that this is the quantum mechanics forum). There is a slight problem with the above interpretation of "resistance". Of course, in the sense that an object will accelerate when you force it, it is "yielding" to your influence. But the fact that it *requires* your influence and effort means that the object does *resist* the change in it's velocity.
Thus, the statement of Newton's first law is that an object will be at rest or move with constant velocity in a given inertial reference frame if there are no net external forces acting on it. The fact that, e.g., you have to push the front of a rolling car (ignore the fact that there is already a force provided by the ground/tire rubbing) to stop it from moving means that the car does not want to stop...we then title this resistance to change "inertia".
Then Newton's 2nd law says that we can quantify this relationship via
F(net)=ma (for constant mass).

With the regards for why objects have inertia: In classical mechanics, there is clearly no explanation for why objects have inertia. Its existence is an observation that is then taken as a law.
In the standard model of particle physics (which uses the framework of quantum field theory where mass is a measure of part of the energy content of a particle), mass for fundamental particles can be obtained via particular interactions among quantum fields...massless particles propagating in a non-trivial vacuum can give these particles an apparent mass-energy. There are other mechanisms for the appearance of what we ultimately interpret as mass in theories beyond the standard model of particle physics.
Suffice it to say that the issue of the fundamental nature of inertia has become a technical one, which can no longer be argued based on "first principles" mechanics. Physicists have wrangled a lot of the unexplained postulates of classical mechanics into a fewer number of unexplained postulates in our more fundamental theories.

6. Apr 23, 2004