Energy
Chapter 8 – Hewitt
Review Questions
1. A force sets an object in motion.
When the force is multiplied by the
time of its application, we call the
quantity impulse, which changes
the momentum of the object.
What do we call the quantity force
x distance, and what quantity can
it change? (8.1)
 Work
 It
changes the object’s energy.
2. Work is required to lift a barbell.
How many times more work is
required to lift the barbell three
times as high? (8.1)
 Three
 Work
is equal to force multiplied by
distance
 SO three times the distance (three times
as high) requires three times as much
work.
3. Which requires more work,
lifting a 10-kg load a vertical
distance of 2 m or lifting a 5-kg
load a vertical distance of 4 m?
(8.1)
 BOTH
the SAME.
 F x d = 10kg(9.8-m/s2) x 2m = 196 J
 F x d = 5kg(9.8-m/s2) x 4m = 196 J
4. How many joules of work are
done on an object when a force
of 10-N pushes it a distance of
10-m?







Given
Find
F= 10 N
W
d = 10 m
Basic Equation(s)
W= F x d
Working Equation
W= F x d
Key Idea
Solution
Answer
10 N x 10 m= 100 J
5.(a)
How much power is
required to do 100-J of work on
an object in a time of 0.5-s?
 Given
W
Find
P
Key Idea
= 100 J
 t = 0.5 s
 Basic Equation(s)
Solution Answer
 P = W/t
100 J / 0.5 s = 200 W
 Working
Equation
5.(b)
How much power is
required to do 100-J of work on
an object in a time of 1.0-s?
(8.2)


Given
Find
Key Idea
W = 100 J
P
t = 1.0 s
Basic Equation(s)
Solution
Answer
P = W/t
100 J / 1.0 s = 100 W

Working Equation



6. What are the two main forms of
mechanical energy? (8.3)
 Potential
Energy and Kinetic Energy.
 Potential Energy is the energy of position
 Kinetic Energy is the energy of motion
7.a.
If you do 100-J of work
to elevate a bucket of water,
what is its gravitational
potential energy relative to
its starting position? (8.4)
 100-J
because the work you do goes into
increasing the gravitational potential
energy.
 Work-Energy Theorem
 W = Δ E, work done equals the change in
7.b. What would the gravitational
potential energy be if the bucket
were raised twice as high? (8.4)
 PE
h
 Or
d
= mg , so twice the height means
twice the energy.
W = F , so twice the distance means
twice the work means twice the energy.
200-J
8. A boulder is raised above the
ground so that its potential
energy relative to the ground is
200-J. Then it is dropped.
What is its kinetic energy just
before it hits the ground? (8.5)

Conservation of energy say the total energy
must remain the same. Since the boulder will
lose 200-J of gravitational potential energy as it
falls it will then gain 200-J of kinetic energy.
9.
Suppose an automobile has 2000-J of
kinetic energy. When it moves at
twice the speed, what will be its
kinetic energy? (8.5)
= ½ mv2
 If v = 2 v, then (2 v)2 = 4v
 Then KEfinal = 4 KEinitial
 What if the velocity is 3 times the original?
 KE
10. What will be the kinetic energy
of an arrow having a potential
energy of 50-J after it is shot
from a bow? (8.6)
 50-J
 Conservation
of energy, energy is
transferred not lost or gained.
 SO, the 50 J of potential energy stored in
the arrow as it is pulled back (position), is
immediately transferred to kinetic (motion)
when the arrow is released.
11. What does it mean to say that
in any system the “total energy
score” stays the same? (8.6)
 It
means Energy is Conserved.
12. In what sense is energy from
coal actually solar energy? (8.6)
 Material
that forms coal was produced by
the sun’s energy.
 Law of Conservation of Energy
 Coal is formed from plant matter that
accumulates at the bottom of a water
source, sediment piles on top, pressure
and time.
13. How does the amount of work
done on an automobile by its
engine relate to the energy
content of the gasoline? (8.6)
 The
work done is less than the energy in
the gasoline.
 The difference is in the form of heat
energy. Work put into the system is
transformed into thermal energy (heat)
14. In what two ways can a
machine alter an input force?
A
machine can change the magnitude or
direction of a force.
 A machine cannot alter the amount of
energy in a system.
15. In what way is a machine subject to the law of
energy conservation? Is it possible for a machine
to multiply energy or work input?
 No,
a machine cannot multiply energy or
work input.
 Energy in a machine is not gained or lost,
only transferred or transformed.
16. What does it mean to say that a
machine has a certain mechanical
advantage?
 This means that a machine has the ability
to multiply a force by a certain amount.
17. In which type of lever is the output force
smaller than the input?
 Type
3 lever, always
 Type 1 lever sometimes
 Type 2 lever, never
 THIS QUESTION WILL NOT BE ON
YOUR TEST!
18. What is the efficiency of a machine that
requires 100 J of input energy to do 35 J of
work?
 35%
 Efficiency = useful work output divided by
total work input
19. Distinguish between the theoretical mechanical
advantage (tma) and actual mechanical advantage (ama).
How would these compare if a machine were 100%
efficient?
 TMA-no
friction
 AMA with friction
 The values would be the same, because
the machine is 100% efficient.
 THIS QUESTION IS NOT ON YOUR
TEST!
20. What is the efficiency of her body when a cyclist expends 1000
Watts of power to deliver mechanical energy to the bicycle at the rate of
100 Watts?
 10%
 Efficiency
= actual mechanical advantage
divided by the theoretical mechanical
advantage.
 Efficiency = work output divided by work
input