DNAunion: I can believe I gave in and spent hours scanning through my college
texts for forms of the word "tendency" in order to counter my opponents'
claims that I am wrong in my usage: but I did.
To show that tendencies and behaviors can differ, I originally presented a
valid example. However, it wasn't very relevant to the topic. My example
dealt more with probabilities: men *tend* to be stronger than women, but
there are many women that are stronger than some men. Each individual man
does not himself tend to be stronger than women: some men are, some are not.
But taken as a whole, there are more men that are stronger than women than
there are women that are stronger than men. That is, men as a group are
*more likely* to be stronger than women than women as a group are to be
stronger than men. My example directly related "tend" with "are more likely
as a whole". This is not the type of tendency I needed to discuss.
Therefore, I will now present material that directly relates to the kind of
tendency involved in our discussions: ones in which a tendency affects all
objects/processes of the same type, but that can differ from the actual
behavior of the object/process if another force/tendency enters into
consideration.
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 two
vertical, 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 hard 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.
Now on to some quotes from college texts.
"In an endothermic reaction, the products contain more bond energy than the
reactant, heat is absorbed, and [delta]H is positive. Reactions tend to
proceed if they liberate [heat] energy (if [delta]H), but this is only one of
the two important parameters of free energy to consider; the other is
entropy." (Harvey Lodish, Arnold Berk, S. Lawrence Zipursky, Paul Matsudaira,
David Baltimore, & James Darnell, Molecular Cell Biology: Fourth Edition, W.
H. Freeman and Co., 2000, p37)
The *tendency* for a reaction that liberates heat is it occur. But its
*behavior* will be different than its tendency if the reaction would require
too large of a decrease in entropy. Generalizing the interplay between the
two, one tendency is in one direction (reactions that liberate heat tend to
occur) and the other tendency in the opposite direction (reactions that
decrease entropy tend not to occur): the behavior of the reaction depends on
which of the two tendencies overcomes the other.
"Passive transport does not require energy. It occurs because of the
tendency for dissolved molecules to move, or diffuse, from higher to lower
concentrations [they may still need assistance of a membrane protein if they
are large, polar, or ions]. … Whether unaided or facilitated by a membrane
protein, passive transport is always exergonic ([delta]G < 0). In contrast,
active transport always requires the input of energy because such processes
are endergonic ([delta]G > 0)
and would not be thermodynamically feasible in the absence of an energy
source." (Wayne M. Becker, Jane B. Reece, & Martin F. Poenie, The World of
the Cell: Third Edition, Benjamin/Cummings Publishing Co., 1996, p 199)
In passive transport the *tendency* and the *behavior* of molecules can
differ. Molecules tend to move from regions of higher concentration to lower
concentration, going down their concentration gradients, but they may not
actually do so if they are too large, polar, or are themselves ions (all of
these involve the actions of the electromagnetic force).
"When the muscle cells contract, they pull on those [collagen] fibers the way
a group of people might pull on a rope. The pull, called tension, is an
active force - energy must be expended to produce it. … Tension [in
muscles] tends to produce movement, but before movement can occur, the
applied tension must overcome the resistance of the object. Resistance is a
passive force that opposes movement. The amount of resistance can depend on
the weight of the object, its shape, friction, and other factors." (Frederic
H. Martini, Fundamentals of Anatomy and Physiology: Fourth Edition, Prentice
Hall, 1998, p277)
Tension in muscles *tends* to produce movement of an object, but it doesn't
have to (the behavior will differ from the tendency if the tension cannot
overcome the resistance).
"If you whirl a key at the end of a string, the key tends to fly off in a
straight line path, in accordance with the first law of motion. The fact
that the key continues to move in a circle must mean that a force is acting
on it that deflects it from its straight-line path. …The steady
outward-pulling force felt by the hand - the centrifugal force as transmitted
by the string - does not tend to pull the key away from the center of the
circle. In the strict sense, there is no such applied force. Because of its
inertia, the key is merely exhibiting its tendency to move in a straight
line." (Melvin Merken, Physical Science with Modern Applications: Fifth
Edition, Saunders College Publishing, 1993, p82-84)
The tendency of the key itself is to fly off in a straight path. But its
behavior differs from its tendency in this case because a taut string applies
a force that influences the key's motion.
"The Moon in motion tends to follow a straight-line path, according to
Newton's First Law, the law of inertia. The fact that it follows a curved
path meant to Newton that a force must be pulling it out of its straight-line
path, just as the key described in Section 5-7 is deflected from its
straight-line path. (Melvin Merken, Physical Science with Modern
Applications: Fifth Edition, Saunders College Publishing, 1993, p82-84)
Same as above. The tendency of the Moon itself is to fly off in a straight
path. But its behavior differs from its tendency because the gravitational
affects of the Earth influence the Moon's motion.
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