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Reading Guide
Chapter 8: Energy KEY
Sections 8.1 – 8.9
Answer in complete sentences. Include the question in your answer, so that the reader
does not have to know the question in order to understand your answer.
Section 8.1
1. What two things must true for work to be done?
A force must be applied and motion must be produced
2. How much work is done when if you hold a 10 N bucket still for 10 minutes?
No work is done because no motion is produced.
3. At the end of the chapter, on p. 119, there are questions related to this section.
Answer question 2:
Three times as much work will be done if a barbell is lifted three times
as high. This is because work is directly proportional to distance
traveled.
4. At the end of the chapter, on p. 120, there are questions related to this section.
Answer question 21:
F = 20 N
d = 3.5 m
W?
W = F • d = (20 N) • (3.5 m) = 70 J
Section 7.2
5. Copy the power formula.
P = W/t
6. A new motor is bought to lift the curtain in a theater. If the motor has twice the
power and the curtains weight is the same as it was previously, how much faster can
it lift the curtain?
t = W/P more power means less time. Time is inversely proportional to
the power. This means that a motor twice as powerful will lift the same
curtain twice as fast (in half the time)
7. At the end of the chapter, starting on p. 119, there are questions related to this
section.
Answer question 5:
W = 100 J
t = 0.5 s
P?
P = W/t = (100 J)/(0.5 s) = 200 W
If t = 1 s then P = 100 W
Section 8.3
8. At the end of the chapter, on p. 119, there are questions related to this section.
Answer question 6:
The two main forms of mechanical energy are kinetic energy (KE) and
potential energy (PE)
Section 8.4
9. Define Potential Energy.
The energy that is stored and is ready to be used is called potential
energy.
10. At the end of the chapter, starting on p. 119, there are questions related to this
section.
Answer question 7:
a) If you do 100 J to lift a bucket of water then it had acquired 100 J
of potential energy at the top. This is because the work done to lift an
object to a certain height is equal to the gravitational potential energy
the object acquires at the top.
b) The gravitational potential energy would be twice as much, 200 J, is
the bucket is raised to twice the height.
Answer question 22:
F = 500 N
d = 2.2 m
W?
W = F • d = (500 N) • (2.2 m) = 1100 J
The potential energy of the barbell at the top is 1100J
Section 8.5
11. Is kinetic energy a vector or a scalar quantity?
Kinetic energy is a scalar quantity, it does require direction to be
defined.
12. If you do work on something, what does it gain?
When you do work on something it gains energy. Energy is the ability to
do work.
13. At the end of the chapter, on p. 119, there are questions related to this section.
Answer question 8:
Because of energy conservation we know the mechanical energy of the
boulder is constant. If it has 200 J at the top (when there is no kinetic
energy) then it must gain 200 J of kinetic energy at the bottom (where
there is no potential energy)
Answer question 9:
Kinetic energy is directly proportional to the square of the velocity.
Twice the speed, means four times the energy, 8000 J. If the speed is
three times as fast, the kinetic energy is 9 times as much, 18000 J.
Answer question 27:
a)
m = 3 kg
v = 4 m/s
W?
KE = ½ • m • v2 = ½ • (3 kg) • (4 m/s)2
KE = (1.5 kg) • 16 m2/s2
KE = 24 J
b) KE is directly proportional to the square of the velocity. Twice the
speed means four times the kinetic energy. KE = 96 J
Section 8.6
14. Write the law of conservation of energy in your own words.
Energy is never lost, only transferred.
15. At the end of the chapter, on p. 121, there are questions related to this section.
Answer question 38:
Yes, driving a car with air conditioning on increases fuel consumption.
And so does driving with the lights on or playing the radio. This is
because using air conditioning and turning on the light both require
energy to work; that energy has to come from somewhere, the only
input of energy (other than the battery) comes from the motor that
runs by fuel.
Section 8.7
Skim the section.
Section 8.8
16. Why is efficiency an important concept?
Efficiency is important because it tells you how much of your (or a
machine’s) effort is actually turn into something you can use, and not
just heat (which is lost to the environment and cannot be used)
Section 8.9
17. At the end of the chapter, on p. 121, there are questions related to this section.
Answer question 41:
When a person’s combined work and heat output is less than the energy
consumed they gain weight.
When a person’s combined work and heat output is greater than the
energy consumed they loose weight.
The energy input of living organisms is in the form of food. Food is
then broken down by the body: some of it is used right away and some
of it is stored. If the stored energy is not used, it stays in your body.
An undernourished person could perform extra work without extra
food but he or she will become even more malnourished. He or she will
continue to use the stored fat or even protein to keep up the with
energy request.
In Conclusion:
18. Write a question you have about this reading, OR tell me about something that you
know or have experienced that connects with the reading.