Chapter 11. Work

Transcription

Chapter 11. Work

Chapter 11. Work
In this chapter we explore
• How many kinds of
energy there are;
• Under what
conditions energy is
conserved;
• How a system gains
or loses energy.
Chapter Goal: To develop
a more complete understanding
of energy and its conservation.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley.
Chapter 11. Work
Topics:
• The Basic Energy Model
• Work and Kinetic Energy
• Calculating and Using Work
• The Work Done by a Variable Force
• Force, Work, and Potential Energy
• Finding Force from Potential Energy
• Thermal Energy
• Conservation of Energy
• Power
The Basic Energy Model
W > 0: The environment does work on the system and the
system’s energy increases.
W < 0: The system does work on the environment and the
system’s energy decreases.
Work and Kinetic Energy
Consider a force acting on a particle as the particle moves
along the s-axis from si to sf. The force component Fs
parallel to the s-axis causes the particle to speed up or slow
down, thus transferring energy to or from the particle. We
say that the force does work on the particle.
The unit of work is J. As the particle is moved by this
single force, its kinetic energy changes as follows:
Work and Kinetic Energy
Work Done by a Constant Force
Consider a particle which experiences a constant force
which makes an angle θ with respect to the particle’s
displacement. The work done is
Both F and θ are constant, so they can be taken outside the
integral. Thus
You should recognize this as the dot product of the force
vector and the displacement vector:
EXAMPLE 11.1 Pulling a suitcase
QUESTION:
Tactics: Calculating the work done by a
constant force
constant force
constant force
EXAMPLE 11.6 Calculating work using the
dot product
QUESTION:
dot product
dot product
dot product
The Work Done by a Variable Force
To calculate the work done on an object by a force that
either changes in magnitude or direction as the object
moves, we use the following:
We must evaluate the integral either geometrically, by
finding the area under the cure, or by actually doing the
integration.
EXAMPLE 11.7 Using work to find the speed
of a car
QUESTION:
of a car
of a car
of a car
The Work-Kinetic Energy Theorem when
Nonconservative Forces Are Involved
A force for which the work is not independent of the path
is called a nonconservative force. It is not possible to
define a potential energy for a nonconservative force.
If Wc is the work done by all conservative forces, and Wnc
is the work done by all nonconservative forces, then
But the work done by the conservative forces is the
negative of the change in potential energy, so the workkinetic energy theorem becomes
EXAMPLE 11.9 Using work and potential
energy together
QUESTION:
energy together
energy together
energy together
energy together
energy together
Conservation of Energy
Energy Bar Charts
We may express the conservation of energy concept as an
energy equation.
We may also represent this equation graphically with an
energy par chart.
EXAMPLE 11.11 Energy bar chart I
EXAMPLE 11.11 Energy bar chart I
EXAMPLE 11.12 Energy bar chart II
EXAMPLE 11.12 Energy bar chart II
Problem-Solving Strategy: Solving Energy Problems
Power
The rate at which energy is transferred or transformed
is called the power, P, and it is defined as
The unit of power is the watt, which is defined
as 1 watt = 1 W = 1 J/s.
EXAMPLE 11.15 Choosing a motor
QUESTION:
EXAMPLE 11.15 Choosing a motor
Chapter 11. Summary Slides
General Principles
General Principles
General Principles
Important Concepts
Important Concepts
Important Concepts
Important Concepts
Applications
Applications
Chapter 11.
11. Questions
A child slides down a playground slide
at constant speed. The energy
transformation is
A.
B.
C.
D.
E. There is no transformation because energy
is conserved.
A child slides down a playground slide
at constant speed. The energy
transformation is
A.
B.
C.
D.
E. There is no transformation because energy
is conserved.
A particle moving along the x-axis experiences the
force shown in the graph. If the particle has 2.0 J
of kinetic energy as it passes x = 0 m, what is its
kinetic energy when it reaches x = 4 m?
A. 0.0 J
B. 2.0 J
C. 6.0 J
D. 4.0 J
E. −2.0 J
A particle moving along the x-axis experiences the
force shown in the graph. If the particle has 2.0 J
of kinetic energy as it passes x = 0 m, what is its
kinetic energy when it reaches x = 4 m?
A. 0.0 J
B. 2.0 J
C. 6.0 J
D. 4.0 J
E. −2.0 J
A crane lowers a steel girder into place at a
construction site. The girder moves with
constant speed. Consider the work Wg done
by gravity and the work WT done by the
tension in the cable. Which of the following is
correct?
A.
B.
C.
D.
E.
Wg and WT are both zero.
Wg is negative and WT is negative.
Wg is negative and WT is positive.
Wg is positive and WT is positive.
Wg is positive and WT is negative.
A crane lowers a steel girder into place at a
construction site. The girder moves with
constant speed. Consider the work Wg done
by gravity and the work WT done by the
tension in the cable. Which of the following is
correct?
A.
B.
C.
D.
E.
Wg and WT are both zero.
Wg is negative and WT is negative.
Wg is negative and WT is positive.
Wg is positive and WT is positive.
Wg is positive and WT is negative.
Which force does the most work?
A. the 10 N force
B. the 8 N force
C. the 6 N force
D. They all do the
same amount of work.
Which force does the most work?
A. the 10 N force
B. the 8 N force
C. the 6 N force
D. They all do the
same amount of work.
A particle moves along the x-axis with
the potential energy shown. The force on
the particle when it is at x = 4 m is
A. –1 N.
B. –2 N.
C. 1 N.
D. 2 N.
E. 4 N.
A particle moves along the x-axis with
the potential energy shown. The force on
the particle when it is at x = 4 m is
A. –1 N.
B. –2 N.
C. 1 N.
D. 2 N.
E. 4 N.
A child at the playground slides down a
pole at constant speed. This is a situation
in which
A. U → Eth. Emech is conserved.
B. U → Eth. Emech is not conserved but Esys is.
C. U → Wext. Neither Emech nor Esys is conserved.
D. U → K. Emech is not conserved but Esys is.
E. K → Eth. Emech is not conserved but Esys is.
A child at the playground slides down a
pole at constant speed. This is a situation
in which
A. U → Eth. Emech is conserved.
B. U → Eth. Emech is not conserved but Esys is.
C. U → Wext. Neither Emech nor Esys is conserved.
D. U → K. Emech is not conserved but Esys is.
E. K → Eth. Emech is not conserved but Esys is.
Four students run up the stairs in the time shown.
Rank in order, from largest to smallest, their power
outputs Pa to Pd.
A.
B.
C.
D.
E.
Pd > Pb > Pa > Pc
Pd > Pa = Pb > Pc
Pb > Pa = Pc > Pd
Pc > Pb = Pa > Pd
Pb > Pa > Pc > Pd
Four students run up the stairs in the time shown.
Rank in order, from largest to smallest, their power
outputs Pa to Pd.
A.
B.
C.
D.
E.
Pd > Pb > Pa > Pc
Pd > Pa = Pb > Pc
Pb > Pa = Pc > Pd
Pc > Pb = Pa > Pd
Pb > Pa > Pc > Pd

Chapter 11. Work

Transcription

Similar documents

DIGITAL SYSTEMS

14. Państwo

Why Study Money, Banking, and Financial Markets?

Using Pearson SuccessNet

BackPack Level 1 Little Books

Galileo Planetary Motion Tycho Brahe`s Observations Kepler`s Laws

Pearson Scott Foresman

History of Computing - UBC Department of Computer Science

Money, Interest Rates, and Exchange Rates

Final http://www.lbpsb.qc.ca/eng/asdn/index.asp The Third Annual