Re: normal force

kup...@xxxxxxxxxxxxxx wrote:

"I also understand that things on hard surfaces are in equilibrium
because the pull of gravity just happens to be balanced out by the e-m
repulsion. But why do they reach equilibrium?"

In reply to this assumption that most systems reach equilibrium, the
following comments address the nature of equilibrium with regards
normal forces-those due to an object's responses to external stress
and strain:

Situations don't necessarily reach equilibium and, in general, many
that look like they're in equilibrium are gradually changing over long
time scales. For example, my large CRT computer monitor appears to be
in equilibium while sitting on my desk. But I can inform you that after
a few years of this situation, the desk is permanently bent in the
middle--even if the monitor is removed. Has the desk stopped bending in
the middle or will it worsen? So things that are in equilibium in one
time scale (hours) may still be evolving in another (years).

Similarly, a situation may be in equilibium at one measurement scale
(meters) may be in motion at a much smaller scale (nanometers). To use
the above example again, if I were to look at the computer-desk border
at an atomic level, atoms and molecules would be vibrating actively:
thermal vibration, emission and absorbtion of photons, etc.

Equilibrium is usually discussed because a) it's a simpler, stable
situation that can be studied at length and b) at our human time and
distance scales many things appear to be in macroscopic equilibirium.
But it would be difficult to conclude that things in general are
actually in equilibium. It's more likely that things are changing
gradually over time: wood bends, tires deflate, stairs wear away,
floors lose integrity. Even window glass that appears solid is actually
a liquid at large time scales. If you pull a pane out of an old house,
it is noticeably thicker at the bottom.

Back to the normal force, gravity pushes object A into object B. This
creates stresses inside and at the interfaces of the objects. If the
stress response of B is strong enough, then A stops moving or--if you
like--oscillates around some fixed value. If B's stress response is
insufficient--such as placing a bowling ball on top of a loaf of
bread--then object B will see its internal structure irreversibly
changed: it will crush. There are intermediate cases where something
changes shape but isn't completely crushed.

Just as a piece of furniture gradually sinks into the carpet, the
equilibrium position observed may drift over large time scales and thus
the two forces (gravity, normal force) may not be identically equal as
we are taught to expect. And even if they are equal, at an atomic level
things will still be dancing around the equilibrium position.

A follow-up question to the one you pose: if I place object A on top of
object B, what does its macroscopic stress response (both internal and
surface) look like? That's a fascinating question and one that material
scientists, structural engineers, and solid-state physicists (among
others) take seriously.

-M

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