Re: A Physics Lesson for the Contributors to this NG



On Dec 29, 4:18 pm, HW@....(Henri Wilson) wrote:
On 29 Dec 2006 12:58:37 -0800, "PD" <TheDraperFam...@xxxxxxxxx> wrote:

Yes, it is obvious in a case where there is solid matter all the way to
the point of application of the force, as in the example you just gave.
However, in both the example of the hollow ball or the cup, where the
center of gravity is NOT within the solid material of the ball or the
cup, you still claim that the force is transmitted from the center of
gravity. Through which solid matter is the force transmitted from the
center of gravity, Hank? Are you so dense that you don't understand the
sketch?
Why are you introducing irrelevancies to this discussion?

Just accept the ball acts through its centre of gravity and the spring
connection is effectively at that point.

Because the spring connection is NOT at that point, not effectively,
not in reality. If it were, then rotating the ball about its center and
letting it go would have no effect, when in fact it really does, as you
can test by experiment. There is no reason to accept something that is
patently wrong, Henri. The point of application of the force of the
ball upon the spring is where the ball is connected to the spring.

You disagree with this, Henry?

Here's another one. There is a table with four legs. The table top is
square with thickness 1/2" and 36" on a side. The center of gravity
lies 1.5" below the center of the top surface of the table and
therefore does not reside in the solid material of the table. (This is
due to the contribution of the heavy legs of the table.) You rest a cup
of coffee of weight 2.5N at a point 4" from the edge of the table. The
cup of coffee exerts a force downward on the table at that point with
magnitude 2.5N. Now, Henry, the table exerts a force on the coffee cup
as well, probably 2.5N upwards. Where is the point of application of
that force, Henry? Is it from the center of gravity of the table? If it
is, why doesn't this result in a torque on the cup that would cause it
to tip over?


You *are* dense. It is a *hollow* ball. There is no solid material at
or in the immediate vicinity of the center of gravity of the ball.
There is no material through which to transmit the force from the
center of gravity.

It doesn't matter if it is a hollow ball. You can assume that if it is
homogeneous, it still behaves as though all its mass is concentrated at its
centre of gravity.

No, it doesn't, certainly not in the way that it interacts with the
spring. See my statement above about what happens if you rotate the
ball 30 degrees about its center, with the spring attached, and let go.
It in no way acts as though it were exerting a force at its center
(which has little to do with whether it acts as though its mass is
concentrated at the center.

Let me illustrate with another example that you can look up in your
favorite textbook. A ladder is leaning against a wall and therefore
makes contact with the wall and the floor. Certainly neither the floor
nor the wall exert their forces on the ladder as though they were
attached at the center of the ladder, and the forces of the ladder on
the wall and on the floor are not as though they were acting at the
center of the ladder.

Only because there is a gravitaional force gradient along the ladder's length.

Eh? It's a uniform ladder. Why would there be a gradient, Henri?


The forces act at the points of contact between
the ladder and the wall, and between the ladder and the floor, and
worked problems in your textbook will point this out clearly.

tHis is a different situation entirely.

That obeys the same rules of physics, the same laws of motion.


Actually, in rotating systems, one must use the moment of inertia rather than
the CofG..but in this thought experiment the latter will do nicely.

Anyway, for this exercise, you can consider the ball to be a point.

....this is all so terribly simple really.

Then draw it.
I have explained it quite clearly...

By positing ghosts that act from the center of gravity, even though
there is no solid material there.

If the Earth was hollow, Draper, you still wouldn't fall off...

That's right, Henri, because the integral of all the attractions from
the little bits of mass that make up the hollow Earth would still point
toward the center. This does NOT mean that the Earth transmits its
force from the center. The Earth, even the hollow Earth, transmits its
force from where there is gravitating mass, and there is none of that
in the center.
That is not correct.
Force and energy equations use the centre as the origin, not the surface.

That's because the *integral* produces that result, as any decent
textbook will work out. You'll note the same textbooks will include a
worked example calculating the force due to a charged ring on a point
charge on the ring's axis. The force on the point charge is directly
away from the center of the ring, even though there is manifestly no
charge at the center of the ring. This results, of course, from a
detailed calculation of where the force *really* comes from, which is
from the ring; the calculation shows that the components transverse to
the axis cancel and the components along the axis add, which results in
the *apparent* result that the force originates from the center of the
ring, even though it is obvious that in reality it does not. Likewise,
from a hollow sphere, a detailed calculation shows that the
gravitational attraction can be calculated as the sum of attractions of
rings (that the hollow sphere can be broken up into), with a similar
result.

Your dim, shallow understanding of physics and where these results
*come from* is thoroughly apparent, Henri. If you studied a bit more,
you might in fact have that degree you perhaps aspired to.


You'll note that this is a case where the force is *gravitational*,
which does not need an intervening medium to transmit the force. In the
case you mentioned, the force between a ball and an attached spring,
the force is not of this nature. It has everything to do with contact
forces in that case, and the force is applied at the point of CONTACT.

A force can be applied at the point of contact or at the end of a rigid rod
that is fastened to that point. It will be transmitted through the rod.

There is no rigid rod from the center of gravity of a hollow ball to
the point of contact of the spring, Henri.


In a rotating system of two masses connected by a string, the centrifugal force
exerted by one mass on one end is transmitted through the spring to become the
CENTRIPETAL force at the other end.
That would be obvious to any physicist...

Now please try to tell me what forces keep the spring extended.

I already told you that. It depends on the method of attachment of the
spring to the ball. It is quite likely that, through a mechanical
connection anyway, that the root origin of the force is
electromagnetic, that being the force that gives solids their
structural integrity.

You really have no idea what I'm talking about, have you?
You just want to argue...

Not at all. I've GIVEN you the answer. You just don't like it.

You are not addressing the question I asked

I most certainly am, and multiple times. Are you dense?


I asked you to refer to whatever text you like, including one you
studied from, to define physical equilibrium. It appears that you say
that because you studied physics, you can define equilibrium any way
you want.

Equilibrium means that a steady state has been reached....even a constant
acceleration can represent an equilibrium state if that is what one is
discussing.

That is NOT a physical definition of equilibrium, Henri, and I defy you
to find ANY physics book that classifies a constant, nonzero
acceleration as an equilibrium state. Now, as I said earlier, if you
want to define equilibrium to be something else, then be my guest, but
be clear that it's your own definition, and that in doing so you are no
longer offering a PHYSICS lesson, but a lesson on Wilsonesque thinking.

i'm not wasting time telling you what an equilibrium situation is Draper.
I suggest you take a course in physics. Why don't you join geesey repeating
first year for the third time...

As I said, Henri, I am quite open to reading ANY physics text you would
offer that defines a constant acceleration as an equilibrium state. If
you can't do that, then the course you recommend I take is not really a
physics course.

PD

.

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