Turbulent flow over
1. Type: Main Idea
(C) In the last paragraph, we are told that ‘several hours of
60-knot storm winds will put more momentum into the water than a week
of 10-knot breezes.’ Now, in this question we are asked about
winds of 60 miles per hour. How do these winds compare? If we look
back to the first paragraph, we see that 20 knots is equal to 22 miles
per hour. Therefore, these are roughly equivalent measures of comparison,
and we can relate the information in the last paragraph to our question,
choosing C, that the high winds contribute more momentum to the ocean
than do the lighter winds over a longer period.
Choice A is not correct, since we are told in the fourth paragraph
that at 10 meters per second (equivalent to 22 miles per hour), the
surface force is slightly more that the force over glass. Since this
is a much greater wind, we would expect the force to be even greater,
so choice A is not correct. Choice B suggests that the roughness created
by the waves would actually decrease the force of the wind. According
to paragraph three, this is true, though only at lower wind speeds,
not the high speeds described in this question. Choice D compares
the relative forces of two winds. Though you might guess that a wind
that is six times faster might exert a force six times greater, we
are told in the passage that the force actually appears to increase
proportionally with the square of the wind speed. Thus, the force
is closer to thirty-six times greater for the higher wind.
2. Type: Definition of a word of phrase
(B) The main idea of this passage is the turbulent flow of
wind over water. The passage introduces the topic in the first paragraph,
by describing the basic physics of turbulent flow over surfaces. The
next four paragraphs then expand this concept to water, and illustrate
the complications in understanding the force of wind on the water
surface. Choices A and C are not correct, in that these concepts are
only briefly mentioned in passing. High winds, like in choice D, are
mentioned frequently, though not specifically for their effect on
the momentum of the ocean. Indeed, the whole passage is applicable
to water in general, not only the ocean. Choice E is not correct.
Although experiments in wind force are described here, it is only
to bolster the author’s argument, not as the main subject of
the passage.
3. Type: Inference
(B) The suggestion that the occasional storm will make a large contribution
to momentum relies on the main argument that high winds can contribute
more to the momentum of water than light winds (see paragraphs two
and four, specifically). High winds can increase the intensity of
waves, or increase the apparent roughness of the surface, which allows
the exertion of more force. With the increased roughness, the force
increases even more than the square of the wind speed (which is the
usual estimation of the force exerted by wind on a surface). Choices
A and D are not relevant to the statement, and thus would not weaken
it if they were true. Choice C, if true, would affect the transfer
of momentum from both high and low winds to water, so this is not
a good choice. Choice E actually is true, and does not weaken the
argument.
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