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Unit Ten
ENERGY
READING PASSAGE
Friction, Internal energy, and Heat
Toss your keys along the floor, they’ll skitter along for a bit as friction does negative
work on them, reducing their kinetic. Quickly, the keys come to rest, and the kinetic
energy you gave them is gone. But in this case they can’t turn around (as they did when
you picked them up), gather speed from the same frictional force that acted to slow them,
and jump back into your hand with the same kinetic energy. Once the motion stops, the
friction stops, and it can not restore the kinetic energy of the keys as gravity is able to do.
Frictional forces don’t store energy in the form of potential energy. But all the kinetic
energy that you gave the keys when you tossed them doesn’t just disappear.
The keys skid across the surface of the floor, scraping and catching. The affected surface
molecules of the keys and the floor are pushed through some tiny distance, giving them extra
kinetic energy. Molecules throughout any solid object bounce around in all directions even
though they are held in place by their bonds with the molecules around them. When
molecules of the keys and the floor strike each other, however, they bounce around even
faster. The affected molecules slam into the nearest neighbors (in all directions) and these,
too, move a bit faster. All the organized kinetic energy the keys had just before they hit the
floor disappears, and most of it scatters aimlessly among the molecules. That chaotic energy
now is part of the energy stored in the matter. We call that energy the internal energy of the
keys and the floor. The internal energy in matter can be thought of as the sum of the kinetic
and potential energies of all of its molecules including, as we see, energy called chemical,
gravitational, and nuclear, and energy associated with the presence of mass itself.
When the molecules of matter move faster, the matter becomes warmer, that is, its
temperature increases. But all the keys’ original kinetic energy doesn’t just go to raise the
temperature of the keys and the floor. A portion of that energy goes into work that deforms,
or scratches, the keys and the floor, and some of the energy even goes into making sound. But
the important point here is that whenever frictional forces do work, some of the work goes

(Adapted from Physics, an introduction by Jay Bolemon, 1989)
READING

COMPREHENSION

Exercise 1: Answer the following questions by referring to the reading text.
1. What’s the property of molecules in a solid object?
…………………………………………………………………………………………
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2. What’s internal energy of a matter?
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3. What happens when frictional forces do work?
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4. In your own words, define the efficiency of a machine?

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…………………………………………………………………………………………
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5. What is heat?
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Exercise 2: Contextual reference (Dealing with words in bold type)
1. “it” in line 6 refers to
a. the motion
b. thee friction
2. “Them” in line 12 refers to
a. the affected surface molecules of the keys
b. the affected surface molecules of the keys and the floor

7. Much of the energy released from the …………………………….of gasoline or
diesel fuel in a car’s engine does no useful work.
8. ……………..………………of a car’s engine is defined by the percent of the energy
released by the fuel that goes into work on the car.
GRAMMAR IN USE
Present participle with some special functions
A present participle phrase is the one of which the central element is a present participle
formed as an –ing form of verb
Example: The phosphor gas rose up into the air, making specks of light.
You have learnt the use of present participle in replacing relative clause and clause of
reason with active meaning. The following will present some others commonly applied in
science writing.
1. Present participle phrase (also known as an –ing clause) is used to give an
explanation
Example:
The molten iron, having been in contact with the coke in the lower part of the furnace,
contains several percent of dissolved carbon.
In the above example, the participle phrase is used to give an explanation for the action
mentioned in the main clause.
2.
Present participle phrase is used to mention something as a part of the action
mentioned in the main clause: that can be either an addition or a result or
consequence of that action.

Example:

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a. Toss your keys along the floor, they’ll skitter along for a bit as friction does negative
work on them, reducing their kinetic. (consequence)
b. The keys skid across the surface of the floor, scraping and catching. (addition)

form. Normally, this can be done with the sentence in which the subject in the time clause is

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the same as that in the main clause. However, in many cases, especially in science writing,
these are different. Thus, we have to retain the subject regardless of the difference.
Therefore, the above sentences can be rewritten in this way:
a. Once the motion stopping, the friction stops, and it can not restore the kinetic energy.
b. All the kinetic energy that you gave them when tossing them doesn’t just appear.
c. When striking each other, the molecules of the keys and the floor bounce around even
faster.
d. When the molecules of matter moving faster, the matter becomes warmer.
e. The important point here is that whenever frictional force doing work, some of the
work goes into increasing the internal energy of that matter.
f. Almost every time anything moving through some distance, friction from some source
does work and transforms some kinetic energy into internal energy.
Note: that you may have an impression that the clause is not at all shortened (reduced) in
length (or in number of words). However, the word ‘shorten’ or ‘reduce’ just implies the
reduction in grammatical aspect, i.e. we reduce a clause into a phrase. That’s why the use of
the word ‘replace’ is quite reasonable for the cases.
To emphasize the completion of an action with respect to another, we use the perfect
participle:
having done
Example:
Having carefully prepared, he successfully detected the questionable element in the
compound.
In such a case, we have more than one way to express the relationship between two
actions (one is conducted before the other).
You can write the sentence in these ways:
a. After he had carefully prepared, he successfully detected the questionable element in
the compound.

Thus, the length is in those equations implicitly.
…………………………………………………………………………………………
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5. In the British system, the unit of power is the foot-pound per second. Often the horse
power is used.
…………………………………………………………………………………………
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6. At low speeds, the two formulas merge. They yield the same result.
…………………………………………………………………………………………
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7. We apply Newton’s laws of mechanics only in inertia reference frames. These frames
move at constant velocity.
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