Who are you? ____________________________________
Chapter 6: WORK & ENERGY - Reading Guide
Please answer the following questions while reading Chapter 2 of
Physical Science Explorations . All of the questions are asked in the
order that they appear in the book.
INTRODUCTION
(pg 83) The author wields a sledge-hammer blow to fellow physics teacher
Pablo Robinson, bravely sandwiched between beds of nails. Why is
Pablo unharmed by this dramatic classroom demonstration?
This is a rhetorical question! You will find out the answer in this chapter!
VOCABULARY
The vocabulary for this chapter (pg 98 under “Key Terms and Matching Definitions”) is: 10 WORDS
__________________ ______________________ ______________________
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6.1 Work – Force x Distance
1. (pg 84) It takes energy to push something and make it move. How much energy depends on the
force exerted and how long it is exerted. If “how long” means time, then we’re talking about
impulse, as in the previous chapter. Recall that impulse equals a change in momentum. In this
chapter, by “how long,” we mean distance. The quantity “force × distance” is equal to the
________________________. We call this quantity ________.
Work = force × __________
2. (pg 84) We do work when we lift a load against Earth’s gravity. The heavier the load or the higher we
lift it, the more ____________ we do. The amount of work done on an object depends on (1)
how much _________ is applied and (2) how _________ the force causes the object to
move.
3. (pg 84) When a weight lifter raises a heavy barbell, he does work on the barbell. He gives energy to
the barbell. Interestingly, when a weight lifter simply holds a barbell overhead, he does
_____________ on it. He may get tired holding the barbell still, but if the barbell is not
moved by the force he exerts, he does ____________ on the barbell. Work may be done on
his muscles as they stretch and contract, which is force × distance on a biological scale. But
this work is not done on the barbell. ___________ the barbell is different than holding the
barbell.
4. (pg 84) The unit of work combines the unit of force (__) with the unit of ___________ (m), the
newton-meter (N · m). We call a newton-meter the ___________ (J) (rhymes with cool). One
joule of work is done when a force of 1 newton is exerted over a distance of 1 meter, as in
lifting an apple over your head. For larger values we speak of kilojoules (kJ), thousands of
joules, or megajoules (MJ), millions of joules. The weight lifter in Figure 6.3 does work in
kilojoules. The work done to vertically raise a heavily loaded truck can be in _____________.
5. (pg 84) Which situation is doing more work, A or B? (Both same height, A is ½ the weight)
Situation A
Situation B
6. (pg 85)
1. How much work is needed to lift an object that weighs 500 N to a height of 4 m?
2. How much work is needed to lift it twice as high?
3. How much work is needed to lift a 1000 N to a height of 8 m?
Is this guy doing any work? ___________
6.2 Power – How Quickly Work Gets Done
7. (pg 85) Lifting a load _________ is more difficult than lifting the same load slowly. If equal loads are
lifted to the same height, the forces and distances are equal, so the work is the same. What’s
different is the __________. Power is the rate at which energy is changed from one form to
another. Also, power is the rate at which work is done. It equals the amount of work done
divided by the time interval during which the work occurs.
8. (pg 85) A high-power auto engine does work ________. An engine that delivers twice the power of
another, however, does not necessarily go twice as fast. Twice the power means the engine
can do ____________________ in the same amount of time–or it can do the same amount of
work in half the time. A powerful engine can speed up a car more _____________.
The unit of power is the joule per second, called the watt. This is in honor of ___________,
the eighteenth-century developer of the steam engine. One watt (W) of power is used when
one joule of work is done in one second. One kilowatt (kW) equals __________ watts. One
megawatt (MW) equals one ____________ watts.
SKIP TO PAGE 92 NOW TO READ ABOUT MACHINES
6.8 Machines – Devices to Multiply Forces
9. (pg 92) A machine is a device for ______________ forces or simply changing the direction of forces.
All machines employ the conservation of energy. Consider one of the simplest machines, the
___________. A lever is shown in Figure 6.16. When you do work by pushing one end of the
lever down, work is done at the other end. The work done on the output side of the lever
raises a load. If heat from friction forces is small enough to neglect, the work input is equal to
the work output:
Work input = _____________.
Since work equals force multiplied by distance, we can say that input force × input distance =
output force × output distance.
(Force × distance)_________ = (force × distance)___________
Fill these in for the formula!
support or point of support on which a lever rotates is called a ___________. When the
fulcrum is close to the load, a ________ output force is produced by a small input force. This is
because the input force is exerted through a large distance and the load is moved over a short
distance. In this way, a lever can multiply force. But no lever or machine has been found that
can multiply __________OR ____________. Our understanding of energy suggests that none
ever will be found. We are so confident of this that we say energy is
___________________________________________________________________________
10. (pg 92) The
11. (pg 92) Which situation is it easier to lift the load, A or B?
Situation A
Situation B
12. (pg 93)Another simple machine is a ________. Can you see that it is a lever “in disguise”? When
used as in Figure 6.19, it changes only the direction of the force. But when used as in Figure
6.20, the output force is doubled. Force is increased and distance moved is decreased.
13. (pg 93) Any machine that multiplies force does so at the expense of __________. Likewise, any
machine that multiplies distance does so at the expense of force. No machine or device can
put out more ___________ than is put into it. No machine can create energy; it can only
transfer it or transform it from one form to another.
6.9 Efficiency – A Measure of Work done for Energy Spent
1. (pg 95) Given the same energy input, some machines can do more work than others. The machines
that can do more work are said to be more ________________.
Efficiency can be expressed by the ratio:
Efficiency = _____________
Even a lever converts a small fraction of input energy into heat when it rotates about its fulcrum. We may
do 100 joules of work but get out ______ joules. The lever is then ______ percent efficient, and we
waste 2 joules of work input on heat. In a pulley system, a larger fraction of input energy goes into heat.
If we do 100 joules of work, the forces of friction acting through the distances through which the pulleys
turn and rub about their axles may dissipate 60 joules of energy as heat. So the work output is only
_____ joules, and the pulley system has an efficiency of _______ percent. The lower the efficiency of a
machine, the greater the amount of energy _________ as heat.
1.5 What would be the efficiency of a machine that you put 100J into but only got 50J of work out?
1.6. What would be the efficiency of a machine that you put 200J into but only got 1500J of work out?
2. (pg 95) According to the caption of Figure 6.22 (below) The graveyard of kinetic energy is
______________________________.
3. (pg 95)Explain why “the graveyard of kinetic energy is thermal energy after looking at the pictures
above.
4. (pg 96) An automobile engine is a machine that transforms chemical energy stored in gasoline into
_________________energy. But only a fraction of the energy in the gas is used by the car to
move forward. Some of the fuel energy in the gas goes out in the hot exhaust gases and is
wasted. Also, nearly half of the energy stored in the gas is wasted in the __________ of the
moving engine parts. In addition to these inefficiencies, some of the gas doesn’t even burn
completely. So the energy in the unburned gasoline also goes unused.
After reading the above paragraph, do you think a car is an efficient machine or an inefficient
machine?
6.10 – Sources of Energy
5. (pg 96) Except for nuclear power, the source of practically all our energy is _________________.
Even the energy we obtain from petroleum, coal, natural gas, and wood comes from _____________.
That’s because these fuels are created by photosynthesis, the process by which plants trap solar energy
and store it as plant tissue.
____________ is also directly transformed into electricity by photo-voltaic cells, like those found in
solar-powered calculators. We use the energy in sunlight to generate electricity indirectly as well.
Sunlight ________________ water, which later falls as rain; rainwater flows into rivers and turns water
wheels, or it flows into modern generator turbines as it returns to the sea.
_________, caused by unequal warming of the Earth’s surface, is another form of _______ power.
The energy of wind can be used to turn generator turbines within specially equipped windmills. Because
wind power is not reliable, concentrated energy sources such as fossil and nuclear fuels are the choice
contenders for large-scale power production.
The most concentrated form of usable energy is stored in _________ and plutonium, which are
nuclear fuels. Public fear about nuclear processes prevents the growth of nuclear power. But it is
interesting to note that the ___________________ is kept hot because of nuclear power, which has
been with us since time zero.
A by-product of nuclear power in the Earth’s interior is ________________. Geothermal energy is
held in underground reservoirs of hot water. Geothermal energy is predominantly limited to areas of
volcanic activity, such as Iceland, New Zealand, Japan, and Hawaii. In these places, heated water near
the Earth’s surface is tapped to provide steam for running turbo-generators.
In locations where heat from volcanic activity is near the ground surface and ground water is absent,
another method holds promise for _____________________________. That’s dry-rock geothermal
power (Figure 6.24). With this method, water is put into the cavities in deep, dry, hot rock. When the
water turns to steam it is piped to a turbine at the surface. After turning the turbine, it is returned to the
cavity for re-use. In this way electricity is produced cheaply and cleanly.
Right now, out loud say “Thank You Mr. Sun, for providing all of the energy
that I used every day”.
So except for what power? ___________________ All of our energy comes
from the sun!
Except for geothermal power, methods for obtaining energy have serious _________________________
consequences. Although nuclear power doesn’t pollute the atmosphere, it is controversial because of the
nuclear _____________ generated. The combustion of fossil fuels, on the other hand, leads to increased
atmospheric concentrations of carbon dioxide, sulfur dioxide, and other pollutants. Methods of using
solar energy are limited in that they require proper atmospheric conditions.
As the world population _________________, so does our need for energy. Common sense dictates
that as new sources are being developed, we should continue to optimize present sources and use what
we consume ____________ and ______________.
Figure 6.24
6. (pg 97) How deep is the hole dug in the picture above? ____________________
7. (pg 97) Why is the second hole dug? ________________________________________________
8. (pg 97) What happens to the water that is sent down the hole? ___________________________
9. (pg 97) Why are is this type of power plant exciting for the future?
6.11 - Energy is Needed for Life
Your body is a _____________–a wonderful machine. It is made up of
smaller machines, the living _________. Like any machine, a living cell
needs a source of ____________. Most living organisms on this planet
feed on molecules called hydrocarbons. Hydrocarbon compounds release
energy when they react with oxygen. Like gasoline burned in an auto
engine, there is more potential energy in the hydrocarbons than there is
in
the molecules that result after the hydrocarbons are “burned” for food. The energy difference is what
sustains life.
_________________, or burning, takes place when certain molecules combine with oxygen in air and
release ___________________ molecules plus large amounts of _______________. Combustion occurs
when food is processed, or metabolized, in your body’s digestive system. The combustion in your body is
similar to the combustion that occurs in an ______________ when it burns gasoline. The main
difference is the rate of combustion. During metabolism, the reaction rate is much slower and energy is
released as needed by the body. Once the reaction starts, it is self-sustaining, similar to burning fossil
fuels.
The reverse process is more difficult. Only _____________ and certain one-celled organisms can make
carbon dioxide combine with water to produce hydrocarbons like sugar. This process is
_____________________. It requires an energy input, which normally comes from sunlight. Sugar is the
simplest food. All other foods, such as carbohydrates, proteins, and fats, are made up of combinations of
carbon, hydrogen, __________, and other elements. So green plants use the energy of sunlight to make
food that gives us and all other organisms _________________.
When you look at a tall tree, you might wonder where the atoms came from that compose it.
Interestingly, the atoms that make up a tree come mostly from the ________. Carbon dioxide molecules
in the air make their way into tiny pores in the leaves. By photosynthesis, the carbon is taken from the
molecules and incorporated into the tree. What happens to the __________? It’s expelled by the tree.
And that’s nice for us, for we need oxygen to ___________!
Right now, out loud, say “Thank you Mr. Tree for helping me to live”!
After reading the above few paragraphs, why is it concerning that the Amazon Rain Forests, or any
forests for that matter, are being cut down at a greater rate now than ever before in human history?