DNAunion: Basically, I have been using the term overcome in situations where
an object or process has two tendencies/forces operating in what amount to
opposite directions, with one of the tendencies/forces exerting itself enough
to, well, overcome the other.
College texts also use the term overcome in relation to the pitting of two
forces/tendencies against each other, with the “winner” being
said to overcome the “loser”. (Note that again, an individual
tendency can remain constant, despite the fact that the behavior of the
overall process changes).
“We can summarize the relationships between free energy, enthalpy,
and entropy as follows:
An exothermic reaction ([delta]H < 0) that increases entropy ([delta]S >
0) occurs spontaneously
([delta]G < 0).
An endothermic reaction ([delta]H > 0) will occur spontaneously if [delta]S
increases enough so
that the T[delta]S term can overcome the positive [delta]H.” (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)
Two separate changes can occur in reactions – changes in enthalpy and
changes in entropy. Reactions that involve increases in enthalpy tend not to
occur, while reactions that involved increases in entropy tend to occur. In
reactions in which these two tendencies are in opposition with each other,
then one has to overcome the other if the reaction is to proceed.
“Under weightless conditions, bones are not stressed by the weight of
the body, and muscles no longer work to oppose the force of gravity.”
(Frederic H. Martini, Fundamentals of Anatomy and Physiology: Fourth Edition,
Prentice Hall, 1998, p276)
The word overcome is not used in this quote, but notice that the statement
implies that muscles work to oppose the force of gravity: for muscles to lift
an object, they must overcome the force that opposes that action. This is
stated more explicitly in the next several quotes.
“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)
Now explicitly stated: “before movement can occur, the applied tension
must *overcome* the resistance of the object”. The natural tendency
for the stationary object is to remain stationary (Newtons’ First Law
of Motion) and a sufficient force is needed in order to overcome that natural
tendency.
“A skeletal muscle 1 cm^2 in cross-sectional area can develop roughly 4
kg of force in complete tetanus. If we hang a 2-kg weight from that muscle
and stimulate it, the muscle will shorten. Before the muscle can shorten,
the cross-bridges must produce enough tension to overcome the resistance
– in this case, the 2-kg weight. (Frederic H. Martini, Fundamentals of
Anatomy and Physiology: Fourth Edition, Prentice Hall, 1998, p295-296)
Here we see that for a weight suspended from a muscle to be raised, muscular
contraction must be strong enough to *overcome* the natural resistance to
upward motion the force of gravity imposes on the 2-kg weight.
“If all the muscle units are stimulated and the resistance is
relatively small, the muscle will shorten
very quickly. In contrast, if the muscle barely produces enough tension to
overcome the resistance, it will shorten very slowly.” (Frederic H.
Martini, Fundamentals of Anatomy and Physiology: Fourth Edition, Prentice
Hall, 1998, p276)
Ditto: muscular contractions must be strong enough to overcome the resistance
to upward motion an object has due to gravity in order for the object to be
raised.
“Figure 10-14c shows what happens if we attach a weight heavier than 4
kg to the experimental muscle and then stimulate the muscle. Although
cross-bridges form and tension rises to peak values, the muscle cannot
overcome the resistance of the weight, and so it cannot shorten.”
(Frederic H. Martini, Fundamentals of Anatomy and Physiology: Fourth Edition,
Prentice Hall,
1998, p297)
The flip side of the coin: muscular contractions that are not strong enough
to overcome the resistance to upward motion an object has due to gravity, the
object is not raised.
“To understand this relationship, attach a rubber band to one of the
rings in a three-ring notebook. Put a finger through the rubber band and use
it to pull the notebook across the table. Your finger represents the muscle
fiber; the rubber band, the attached tendon; and the notebook, a bone of the
skeleton. When you first apply tension, the rubber band stretches and
becomes stiffer. Over this period, external tension is rising. The notebook
starts to move when the rubber band becomes sufficiently taut – that
is, when the tension in the rubber band (the external tension) overcomes
friction and the weight of the book (resistance).” (Frederic H.
Martini, Fundamentals of Anatomy and Physiology: Fourth Edition, Prentice
Hall, 1998, p293)
Again, two tendencies pitted against each other with one overcoming the other.
Here’s another interesting point concerning this last quote. IT IS AN
ANALOGY! It is presented as a means of taking a concept or process that is
*not* intuitive, and explaining it by referencing concepts and processes that
*are* intuitive.
People who wanted to be difficult could nit-pick this analogy to death.
(1) What if the rubber band were imperfect, having a “weak link”
where it was much thinner than in other areas? It could snap and the
notebook would not move.
(2) What if the table were tilted away from the person? The notebook would
be influenced to a much greater degree by gravity and the resistance would
increase. Would the rubber band still be able to overcome the resistance
without first snapping?
(3) What if the notebook and part of the rubber band were in a chamber near
absolute zero, with the other part of the rubber band projecting out to room
temperature, and as the person began to pull, something fell in the
near-absolute zero chamber and the rubber band shattered?
(4) What if a supermassive black hole passed close enough to the system, on
the notebook’s side? Wouldn’t this pull the notebook away from
the person more strongly than the rubber band could pull in the opposite
direction?
(5) What if this was done on a table or tray in an airplane, and just as the
person began to pull, the airplane crashed into a mountain side? What would
happen to the notebook?
(6) There are many others (one could point out each of the multitude of
various differences between notebooks and bones of the skeleton, for example).
Of course these are all irrelevant to the analogy. An analogy is not meant
to take every single, possible – and absurd - set of conditions into
consideration. It is meant to make a hard-to-understand concept intuitive.
Did you understand what the author of the analogy was trying to explain? If
so, then move on: it's an analogy for goodness sakes, not a dissertation.
When reading analogies, readers should assume, unless it is explicitly stated
otherwise, that conditions are *as normal as possible* (not as weird as
possible).
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