[johnson039 (OP)]

surely, all of us learnt the term inertia in high school physics mechanics,
but what is its nature actually?
i mean, why an object will reluctant to change their state of motion??

is there any friction causing the inertia??
 []

[Madidus_Scientia]

An object is quite happy to change its state of motion, as long as you can provide the energy for it. If you are huffing and puffing trying to move a dirty great boulder you would definitely say its reluctant to move, but that's just because you haven't provided enough energy. It would roll over for a bulldozer.

[lyner]

I certainly wasn't taught the word "inertia" at School because there are no equations which involve it..
I was taught Mass: F = Ma
Momentum: P = mv
Kinetic Energy: mv²/2

Is anything else needed for describing moving or stationary objects?

[lightarrow]

surely, all of us learnt the term inertia in high school physics mechanics,
but what is its nature actually?
i mean, why an object will reluctant to change their state of motion??

is there any friction causing the inertia??
 []

First, by inertia we intend inertial mass. Second, the fact that an object which have (inertial) mass is reluctant to change its state of motion is a principle of physics (F = ma), which can be derived from space omogeneity, assuming Hamilton's principle of stationary action, or from Mach's principle:
http://en.wikipedia.org/wiki/Mach's_principle

or postulating the existence of a force that an accelerated object makes on other massive objects. This effect was put in a semiquantitative form by Dennis Sciama: postulating the existence of a gravitational force which, in analogy with the EM force produced by accelerated charges, is generated by accelerated masses and depends on distance as 1/r:

F = (GMm/c2)*a/r

F = force made on an object of mass m from an object of mass M and acceleration a
r = distance between the two objects

he integrates on all the visible universe's volume, using the present values for Hubble's constant H (the integration upper limit is taken as c/H) and the average density of matter, and he computes, approximately, the inertial force experienced by an object in an accelerated frame of reference, and the result is of the order of magnitude of the real value.

[yor_on]

Inertia is the unwillingness of an object of 'any kind of mass' (I think?) to accelerate.
It's not 'the larger you are the harder you fall'.
More like 'the larger you are the more you will resist' (that fall).

(Hmm:)..

(So momentum is constructed by velocity and mass, right.
And if inertia is the reaction to that momentum building.
Could you call it inversed momentum?
Nah:)

Btw: isn't inertia resting on the concept of 'invariant mass'?
That 'amount' that are seen to be invariant regardless of reference frames.
Or is it valid for all kinds of mass, relativistic, etc etc??
Awhhh.

[lyner]

I certainly wasn't taught the word "inertia" at School because there are no equations which involve it..
I was taught Mass: F = Ma
Momentum: P = mv
Kinetic Energy: mv²/2

Is anything else needed for describing moving or stationary objects?

For people at School, that is.!!!!
And I think it is that to which the original post was referring.

[johnson039 (OP)]

if we are standing in a bus, at rest originally. then the bus start accelerating, at that time we 'll have a feeling that we want to fall back right?
so what is its principle?

[lyner]

Our feet move forward because they are stuck to the floor and there is a large (sideways) force on them. There is not so much of a force on our bodies (just what is transmitted via our legs so we accelerate forwards much less. We 'stay behind', compared with our feet because our feet accelerate more than our body.
Our brain decides that we are falling backwards because we evolved without the 'bus experience' and it tries to make sense of what is happening.

[LeeE]

I think the most interesting aspect of inertia is it's relationship with acceleration - without inertia there is no acceleration.  An object without inertia, such as a photon, doesn't accelerate to 'c' but immediately assumes that velocity, in zero time.  This doesn't stop it from having momentum though.

Zero acceleration isn't the same as infinite acceleration, however, as the change of velocity occurs over a non-zero period of time, no matter how small.

[lyner]

So how do you distinguish between Mass and your 'inertia' word?

[LeeE]

So how do you distinguish between Mass and your 'inertia' word?

Are you asking me?  I'm not sure what you mean.

[Bikerman]

So how do you distinguish between Mass and your 'inertia' word?

Well, there are two ways of looking at mass - gravitational mass and inertial mass.
Gravitational mass describes how a body behaves in a gravitational field. The smaller the mass, the less the gravitational force that acts on that mass.
Inertial mass is the resistance to movement (more specifically the resistance to a change in movement).
Now, the interesting thing is that there is no a-priori reason why the two should be the same, but we find, by experiment, that they ARE the same.
Einstein's General Relativity actually makes the assumption that the two are the same (called the 'equivalence' principle) which leads to the conclusion that acceleration and gravity are equivalent.

[lyner]

So how do you distinguish between Mass and your 'inertia' word?

Are you asking me?  I'm not sure what you mean.

Nothing personal was intended - you are just in the line of fire  [].

Whenever I hear the word 'inertia' used in non-relativistic discussions, it is not defined. It is used as a general 'catch all' term for some feeling about the way a stationary or moving mass will behave. Its use is, invariably 'poetic and subjective', at a point in an argument where Maths should be used to prove a point and the argument falls on its face. Not surprising if the terms aren't defined.
Problems involving express trains, bullets, seatbelts and the like are very adequately solved without recourse to the term 'inertia' because, as I said at the top of this thread, the quantities of mass, momentum and Kinetic energy are well defined and they are sufficient.
Call me a boring old pedant and I will own up to being old.

[lightarrow]

I think the most interesting aspect of inertia is it's relationship with acceleration - without inertia there is no acceleration.  An object without inertia, such as a photon, doesn't accelerate to 'c' but immediately assumes that velocity, in zero time.  This doesn't stop it from having momentum though.

Zero acceleration isn't the same as infinite acceleration, however, as the change of velocity occurs over a non-zero period of time, no matter how small.

But here you are postulating the existence of photons with speed ≠ c in the void and that it's forbidden.

[LeeE]

I think the most interesting aspect of inertia is it's relationship with acceleration - without inertia there is no acceleration.  An object without inertia, such as a photon, doesn't accelerate to 'c' but immediately assumes that velocity, in zero time.  This doesn't stop it from having momentum though.

Zero acceleration isn't the same as infinite acceleration, however, as the change of velocity occurs over a non-zero period of time, no matter how small.

But here you are postulating the existence of photons with speed ≠ c in the void and that it's forbidden.

Hmm...  I can't see where I'm postulating anything there.

FWIW, I think of (rest) mass as the quantity of matter and inertia as a property of matter.  They seem related, to me, in a similar way to numbers and operators; numbers are quantities, and can have a range of values whereas addition, subtraction, multiplication and division are valueless.

I'm still a bit confused  []

[yor_on]

Anyone seen this one?
http://www.physicsforums.com/showthread.php?t=3708

[Soul Surfer]

Inertia is where the recently much publicised Higgs boson comes in. the quantum mechanical vacuum is a seething field of all the possibilities that coud happen at all times including the exostence of all sorts of exotic particles  (as well as well known ones) including the higgs boson.  The Higgs process suggests that a particles interection with the potential higgs boson creates its mass ie its inertia.