# Is this inertia?

## Main Question or Discussion Point

I have a bar of metal 1 light second long and if i push it at one end it can not move at the other end until at least 1 second has passed because otherwise it would move faster than the speed of light. A .1 light second long bar .1 seconds later before the other end may move, and smaller and smaller and smaller etc., until finally we are the size of an atom. Does the same principle apply here? At subatomic levels such as a proton? Quark? Is this the cause of inertia or is it some other force that is manifesting itself?

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Drakkith
Staff Emeritus
Inertia is resistance of any physical object to a change in its state of motion or rest. This is NOT the same effect as your example is giving. The reason that it takes time for the other end of the bar to move is that the force that you are applying to the bar on one end takes time to propagate through the bar to the other end.

Inertia means that it takes MORE force or work to move something heavy than it does to move something lighter.

Cleonis
Gold Member
I have a bar of metal 1 light second long and if i push it at one end it can not move at the other end until at least 1 second has passed because otherwise it would move faster than the speed of light. A .1 light second long bar .1 seconds later before the other end may move, and smaller and smaller and smaller etc., until finally we are the size of an atom. Does the same principle apply here? At subatomic levels such as a proton? Quark? Is this the cause of inertia or is it some other force that is manifesting itself?
What you are referring to is a phenomenon that is called 'Born rigidity'. (Presumably Max Born was the first to discuss it thoroughly, so that later physicists started to name it after him.) An object that is in continuous acceleration is compressed in a way that has no classical counterpart.

It would not surprise me if in fact some theoretical physicist has explored an idea where inertia is linked with the tension that is accociated with Born rigidity.

For instance, check out the following discussion about http://www.mathpages.com/home/kmath422/kmath422.htm" [Broken]. Of course, such musings are sheer speculation.

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Something else to keep in mind:

A pressure wave in a medium, during propagation, will effect loss.
This is usually presented by IR radiation as the medium molecules compress and decompress .
Thus, I highly doubt that a normal "shock wave" would even "get" to the end of such a long rod. As such, I doubt that the end of such rod would even move at all.

I would venture the same with considerations of a very slow moving pressure wave impact.

AlephZero
Homework Helper
The speed of sound in most metals at room temperature is of the order of a few km/sec.

Any force you applied at one end would propagate along the rod at the speed of sound, not at the speed of light.

Perhaps I didn't state it quite right. Theoretically if i were able to push one end of a rod toward the other end, the other end could not move instantly because in doing so it would have violated the speed of light, so some time must pass before the other end may begin to move. On a subatomic scale, a particle (if it is not in a wave state), if it has 3 dimensions, would also be limited here, in that any force applied to 'one side' of the particle could not instantly make the other 'side' of the particle begin moving the same direction at that same exact instant ... or can it? Is this a quantum mechanical issue? I am not sure where to place this effect (if it even exists).

Particles don't have sides, they are zero-dimensional points, and lack spatial extension.

Drakkith
Staff Emeritus