<< [...]
>>>DNAunion: First, I will start off with a quick coverage of an
intuitive,hypothetical,everyday example.
At a local gym, a weightlifter lies down on a flat bench which has
twobvertical, sturdy bars that support at nearly full-arms reach a barbell
weighing 300 lbs. He forcefully contracts his chest (and other) muscles and
lifts the barbell, followed by locking out his elbows. At this point (and
indeed, before and after), because of the attractive force imposed on the
barbell by gravity, the tendency of the barbell is to take up a position as
physically close to the Earth's center of mass as possible (i.e., its
tendency is to move towards the floor and to remain there once it achieves
this). As the man unlocks his elbows and slightly relaxes his muscles, the
barbell begins a slow descent. Why? Is the man *pulling* the barbell down
towards the ground? No. He is simply allowing the barbell to move according
to its natural tendency. Once the barbell is just above his chest, the man
tenses his muscles again, applying an upward-directed force: the barbell
slowly rises. But wait a minute. Why must the man strain so har
d if the
barbell doesn't have a tendency to move downward towards the ground?
Obviously, the barbell still DOES have that tendency - it opposes the man's
attempts to separate it farther and farther from the Earth's center of mass
at every second. But how can that be - if the barbell has the tendency to be
moving downward, imposed on it by gravity, then how could it possibly be
moving upward? Because its *behavior* can be different from its *tendency* if
a sufficient opposing force is applied (note that the force of muscular
contraction applies a tendency to the barbell: as mentioned below, tension in
muscles *tends* to cause objects to move). So it is one tendency/force
opposing another: the behavior of the system (man and barbell) depends upon
which force overcomes the other. The tendency for the barbell to take up a
position as physically close to the Earth's center of mass exists before the
man lies down on the bench, while the man first hoists it of the rack, while
the man allows it approach his chest, and even while the man strains to lift
it away from his chest. The tendency of the barbell itself remained
constant throughout the process, even though the behavior of the barbell
(actually, the barbell-human system) changed.
>>>Richard Wein: Unfortunately, this example is no different in principle
from your teeter-totter example.
When the man exerts an upward force on the barbell which is greater than the
force of gravity, its tendency/behaviour is to rise. When he exerts a
smaller force than the force of gravity, its tendency/behaviour is to fall.
***********************
DNAunion: I disagree. When the man exerts an upward force on the barbell
that is greater than the force of gravity, the barbell's *behavior* is to
rise up away from the ground, but the barbell's *tendency* is still to move
towards the ground. That is why the man has to strain - he is opposing, and
must overcome, the constant tendency of the barbell to "fall". If the
*barbell* in fact had a tendency to rise, the man could let go of it and it
would keep moving higher and higher.
***********************
>>>Richard Wein: You are arguing that the effect of gravity is a tendency,
but the effect of muscles is not. Why? This is an arbitrary distinction.
************************
DNAunion: And one that I did not make. In fact, I explicitly stated, "(note
that the force of muscular contraction applies a tendency to the barbell: as
mentioned below, tension in muscles *tends* to cause objects to move). So it
is one tendency/force opposing another".
In this analogy, the barbell has an ever-present and inescapable tendency to
take up a position as close to the Earth's center of mass as possible. The
barbell itself does not have any tendency to rise. That comes from a
separate, secondary, temporary force that is applied to it.
************************
>>>Richard Wein: Now, you might argue that the barbell has many tendencies
at the same time:it has a tendency to fall (due to gravity), a tendency to
rise (due to
muscles), a tendency to stay still (due to inertia), a tendency to bend (due
to the weights on the ends of the bar), a tendency not to bend (due to the
rigidity of the bar), etc. The actual behaviour of the barbell is then the
net result of all these tendencies. If you wish to take this position, then I
accept your use of the word tendency. But, in this case, you can't
characterise the tendency to fall as *the* tendency of the barbell; it's just
one of many tendencies.
****************
DNAunion: Okay, we're back to the fact that this is an analogy. That the
tube/bar connecting the two sides with weights can bend is irrelevant to the
analogy's point. The analogy shows in an easy-to-understand manner that an
object's tendency and behavior can be considered as two separate things, and
that since they are not one and the same, that they can differ: the tendency
can remain constant even though the behavior changes. Any tendencies other
than those that directly impact the behavior and/or tendencies of the
barbells I mentioned (whether the barbell "rises or falls") are superfluous.
And yes, in an everyday gym, here on Earth, without supermassive black holes
entering the picture, the natural tendency of the barbell is to take up a
position as close to the Earth's center of mass as possible, due to gravity.
And this tendency exists before, during, and after the weightlifter touches
the bar. In this example, that *is* its' primary tendency. ****************
My timer on AOL just gave me a warning. I have to either finish up now and
start another post, or save this to MS Word and edit it, then post it back
here (in which case all the quotes and ellipses and dashes get converted to
funky series of alphanumeric characters). More later.
This archive was generated by hypermail 2b29 : Thu Nov 16 2000 - 17:57:16 EST