Newton's first law Inertia

What really is INERTIA?

Is it a force.?
A vector or a scalar?
What can be known only by the defination?

Does it really have anything to do with physics?

russ_watters
Mentor
Inertia is probably the most fundamental concept in physics: it is resistance to acceleration. For the purpose of the first learning of Newton's laws (f=ma), mass and inertia are the same, but don't take that to mean that the term "inertia" is superfluous. It isn't. There are several types of inertia.

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INERTIA is a phenomenon (it is neither a vector nor a scalar).

Chi Meson
Homework Helper
Russ:
The high school teacher in me comes out; Inertia is the resistance to change in motion. I know you know that.

russ_watters
Mentor
Chi Meson said:
Russ:
The high school teacher in me comes out; Inertia is the resistance to change in motion. I know you know that.
Oh, is that what the "a" stands for in that equation? Yeah, I knew that - fixed now.

edit: perhaps I was thinking in terms of politics...? http://dictionary.reference.com/search?q=inertia

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gunblaze said:

What really is INERTIA?

Is it a force.?
A vector or a scalar?
What can be known only by the defination?

Does it really have anything to do with physics?
Don't know if this is what you're asking, but for me this was once like the question asked in another thread, "What are Field Lines?" If your question is Why is inertia? or, Why does matter resist changes in motion?, then I believe the answer, to date, is, "We don't know." - which is why clive referred to it (correctly) as a phenonemon.

cepheid
Staff Emeritus
Gold Member
Yeah, just to throw in my two cents towards the "phenomenon" camp, I've heard it described this way:

Inertia is a property of matter.

Mass (inertial mass) on the other hand, is a quantity, in fact it quantifies inertia in that it provides a means of physically measuring the effect i.e. we apply a force F to a body, we measure a, we determine what m is, the larger it is, the more difficult a time we had of accelerating the body. What do you guys think of that?

Andrew Mason
Homework Helper
gunblaze said:

What really is INERTIA?
in·er·tia [i núrsh?] noun
1.apathy: inability or unwillingness to move or act
2.resistance to change: the property of a body by which it remains at rest or continues moving in a straight line unless acted upon by a directional force

[Early 18th century. From Latin, “lack of skill, inactivity,” formed from iners (see inert

The inability or unwillingness of a person to move or act is only indirectly related to that person's mass.

For some people, inertia can be overcome with money. For others, one has to use brute force, in which case Newton's second law is a reasonable approximation.
A vector or a scalar?
A scalar.
Is it a force.?
No. Not even a force in the social sense.

AM

cepheid said:
Yeah, just to throw in my two cents towards the "phenomenon" camp, I've heard it described this way:

Inertia is a property of matter.

Mass (inertial mass) on the other hand, is a quantity, in fact it quantifies inertia in that it provides a means of physically measuring the effect i.e. we apply a force F to a body, we measure a, we determine what m is, the larger it is, the more difficult a time we had of accelerating the body. What do you guys think of that?
You raise a question that's been on my mind for some time.

It's interesting to me that, given the direct correlation between mass and its inherent inertia (simplistically speaking), we actually use inertia to quantify the mass of a given body. If that property did not exist (hypothetically, of course), could we quantify mass by any other means?

What I'm really getting at: Is the correlation between mass and inertia a known absolute? Is it possible that there exists matter that is more (or less) massive than its inertial properties suggest?

cepheid
Staff Emeritus
Gold Member
I was saying mass quantifies inertia, not the other way around. If it were not for this property, the quantity known not surprisingly as "inertial mass" would not have any meaning. It is literally defined by the m in Newton's 2nd law. So mass IS a measure of an object's resistance to a change in state of motion, ie of its inertia. That's how I understand it.

Gravitational mass as I understand it is quantity that crops up as another proportionality constant. One that relates the gravitational field strength to the force (weight) experienced by a body in that field. Also symbolized as "m" and called "mass". The fact that inertial mass and gravitational mass are equivalent is very important, as I recall.

I don't think "mass" has meaning outside these contexts (in classical physics). Corrections to these notions of mine are most welcome.

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in·er·tia [i núrsh?] noun
1.apathy: inability or unwillingness to move or act
2.resistance to change: the property of a body by which it remains at rest or continues moving in a straight line unless acted upon by a directional force

[Early 18th century. From Latin, “lack of skill, inactivity,” formed from iners (see inert

The inability or unwillingness of a person to move or act is only indirectly related to that person's mass.

For some people, inertia can be overcome with money. For others, one has to use brute force, in which case Newton's second law is a reasonable approximation.
A scalar.
No. Not even a force in the social sense.

AM
Hey all,

I'm a programmer and don't know too much about physics.

I'm currently working one a game with realistic physics and I'm use the physics library http://www.bulletphysics.com/" [Broken]. It simulates rigid bodies.

Andrew Mason says Inertia is a scalar but in Bullet it is represented as a vector. So I'm still very confused as to what Inertia is. I understand the phenomenon idea (mentioned above) but I'm having trouble understanding how to computers to a vector or a scalar.
I learned at school (along time ago) that f(force)=m(mass)a(acceleration) but this would be a rule there would be used in Bullet and not a number(vector or scalar).

Any idea on this would be nice. :)

Johnny

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Inertia is the property of bodies which always oppose the change of state.

If a body is in the state of rest because of inertia property it would like to continue its state of rest.

To change this state we need to apply force.

This gives foundation for newton's first law.

[...]
I'm currently working one a game with realistic physics and I'm use the physics library http://www.bulletphysics.com/" [Broken]. It simulates rigid bodies.

Andrew Mason says Inertia is a scalar but in Bullet it is represented as a vector. [...]
I learned at school (along time ago) that f(force)=m(mass)a(acceleration) but this would be a rule there would be used in Bullet and not a number(vector or scalar).

Any idea on this would be nice. :)
[...]
From a quick look through their forum it seems what you are looking for is the "http://en.wikipedia.org/wiki/Moment_of_inertia" [Broken]"
It is a 2d tensor (matrix) though and not a vector (1d tensor). If you want you can see it as an extension of the concept of mass for rotations.

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I pretty much agree with the post of d_vsuresh...

Basically, in my experience, inertia is a colloquial or intuitive sort of term. You never usually calculate "inertia" as a fundamental quantity, unlike mass or acceleration. Sometimes we have quantities known as "inertial forces", but that's basically a matter of nomenclature.

Essentially inertia is just the propensity of a body to continue in the same state of motion unless acted on by outside forces. If acted on by outside forces, the inertia of the body tends to resist those forces.