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Problem Solving using the Conservation and Accounting
Framework
Now that the basic concepts of the conservation and accounting
framework have been formulated, we can suggest a problem solving
approach that is based on a set of generic questions. As students
approach a new physical situation focusing on a recurring set of
questions helps students focus on the important issues involved
in formulating models of the physics situation and finding the desired
quantities.
What is the system? Choose the boundary
This is almost always the starting point for the analysis of every
physical situation. In each engineering science, the question of
choosing the system is often phrased differently and the focus is
on a node, a free-body diagram, a closed system, or a control volume.
In fluid mechanics and thermodynamics this is usually accomplished
when students are usually asked to sketch the control volume and
in dynamics when students are asked to sketch the free-body diagram.
It is important for the students to learn that more than one system
may be required for the solution of a particular problem. Although
terminology differs, the goal is always the same — a clear description
of exactly what you intend to analyze. Implicit in this question
is the need to clearly identify the boundary and the surroundings.
The conservation and accounting framework recognizes the interconnectedness
of everything. To avoid becoming overwhelmed by the knowledge that
almost everything is impacted by almost everything else, answering
the first question forces the students to define the system, recognize
the surroundings and consider the interactions between the two.
During the process students are required to clearly define a system
and its boundary. Then, they can apply the accounting principle
by watching the boundary for things entering and leaving. They
can also watch the interior of the boundary to determine what changes
inside. The boundary can be something physical like a rigid container,
or it can be something imaginary. A useful example for introducing
concepts about the conservation and accounting frame is a checking
account at a bank where the boundary is an imaginary boundary.
The bank does not have small areas where they keep money. An account
is simply a convenient way to think about your money. A safe deposit
box is a physical boundary in a bank and you could use it to apply
conservation principles to money you put inside. The method requires
students to carefully identify boundaries and interactions between
the system and its surroundings.
What should we count?Choose what to count
Next, in order to apply the accounting principle students should
determine what to count. There are five quantities that are commonly
counted in engineering problems. These quantities are mass, momentum,
angular momentum, energy, and entropy. An advantage of the framework
is it focuses attention on physical properties and helps students
to think about physical processes in terms of these properties.
Further, the question focuses attention on what is actually happening
in the problem in terms of the extensive properties. Which of the
extensive properties — mass, charge, linear momentum, angular momentum,
energy, or entropy — should we be interested in? Which of these
properties are changing?
What is the time interval of interest?
This question focuses student attention on the process. What type
of process has occurred or will be occurring? This question is basically
asking the students to identify whether the rate form of the basic
principles or the finite time form is most appropriate.
What are the important interactions?
This question is intimately related to the previous question. For
example if linear momentum is to be counted then the student should
be on the look out for interactions that transport momentum: external
forces and mass flow. Or if a student believes that forces acting
on a system are important, then linear and possibly angular momentum
must be counted. Although the mechanisms and names vary from property
to property, the underlying idea of an exchange of something with
the surroundings is a common feature of any engineering system.
Know how to count
The last thing that you need to be able to do is determine how
much of a quantity is inside your boundary. For example, if you
are counting energy to solve a problem then you will have to determine
the amount of energy inside your boundary. The majority of the
time spent in the class deals with this concept therefore it is
impossible to enumerate all the intricacies here. Essentially these
concepts involve relating temperature to internal energy, speed
to kinetic energy and vertical height to gravity potential.
Tools for Insight in Analysis
Two additional concepts that are sometimes useful in the analysis
and design of a system are the degrees of freedom (dof) and order.
Order is the number of independent storage elements inside the
system boundary. The easiest operational definition for independent
storage is one that is not dependent on another. Two storage elements
are dependent if knowing the quantity stored in one implies the
storage in the other is also known. For example, a mass moving in
a single direction has the ability to store kinetic energy and linear
momentum. If the linear momentum is known, the velocity of the mass
is known, once the velocity is known, the kinetic energy is also
known therefore the mass has one order. If the mass can move in
two directions, the order is two since knowing kinetic energy will
not completely specify the two momentums but knowing energy and
one momentum will fix the other momentum. Students often find the
exercise of determining order helpful in identifying what conservation
equations to write, knowing how many equations to expect when they
are finished, and for helping to define a proper set of variables
to use in the formulation.
Determining the Degree of Freedom (dof)
is an exercise in identifying different types of variables. One
type of variable is a flow or motion. For example, velocity and
current are motion variables. The dof is the minimum number of independent
motion variables required for describing the conservation equations.
By determining the dof, students are forced to think about the problem
formulation before they begin to write equations. In addition, the
dof will indicate when extra constraint equations are required.
For example, suppose a mass moves in a plane such that it remains
a constant distance from a point of rotation. The mass has one dof
because a single angle and its derivatives are sufficient to express
all the motion related quantities in the conservation equations.
If the conservation equations are expressed in terms of two variables
(say horizontal and vertical positions) then the conservation equations
will have more variables than can be uniquely determined. What is
required is a kinematic constraint equation that relates the motion
variables together. By counting the dof and the number of motion
variables in the conservation equations, a student can determine
if constraint equations are required.
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References
- Grinter, L.E. (Chair), Report on Evaluation of Engineering
Education, American Society for Engineering Education, Washington, DC,
1955.
- Harris,
Eugene M. DeLoatch, William R. Grogan, Irene C. Peden, and John R. Whinnery,
"Journal of Engineering Education Round Table: Reflections on the
Grinter Report," Journal of Engineering Education, Vol.
83, No. 1, pp. 69-94 (1994) (includes as an Appendix the Grinter Report,
issued in September, 1955).
- Glover, Charles, J., and Carl A. Erdman, "Overview
of the Texas A&M/NSF Engineering Core Curriculum Development,"
Proceedings, 1992 Frontiers in Education Conference, Nashville,
Tennessee, 11-14 November 1992, pp. 363-367
- Glover, Charles J.,
K. M. Lunsford, and John A. Fleming, “TAMU/NSF
Engineering Core Curriculum Course 1: Conservation Principles in Engineering,”
Proceedings, 1992 Frontiers in Education Conference, Nashville,
Tennessee, 11-14 November 1992, pp. 603-608
- Glover, Charles J., K. M. Lunsford, and John A. Fleming,
Conservation Principles and the Structure of Engineering, 3rd
edition, New York: McGraw-Hill College Custom Series, 1992
- Pollock, Thomas C., “TAMU/NSF
Engineering Core Curriculum Course 2: Properties of Matter,” Proceedings,
1992 Frontiers in Education Conference, Nashville, Tennessee, 11-14
November 1992, pp. 609-613
- Pollock, Thomas C., Properties of Matter, 3rd edition,
New York: McGraw-Hill College Custom Series, 1992
- Everett, Louis J., “TAMU/NSF
Engineering Core Curriculum Course 3: Understanding Engineering via
Conservation,” Proceedings, 1992 Frontiers in Education Conference,
Nashville, Tennessee, 11-14 November 1992, pp. 614-619
- Everett, Louis J., Understanding Engineering Systems
via Conservation, 2nd edition, New York: McGraw-Hill College Custom
Series, 1992
- Glover, Charles J. and H. L. Jones, “TAMU/NSF
Engineering Core Curriculum Course 4: Conservation Principles for Continuous
Media,” Proceedings, 1992 Frontiers in Education Conference,
Nashville, Tennessee, 11-14 November 1992 Conference, pp. 620-624
- Glover, C. J. and H. L. Jones, Conservation Principles
for Continuous Media, 2nd edition, New York: McGraw-Hill College
Custom Series, 1992
- Erdman, Carl A., Charles J. Glover, and V. L. Willson,
“Curriculum Change: Acceptance and Dissemination,” Proceedings, 1992
Frontiers in Education Conference, Nashville, Tennessee, 11-14 November
1992, pp. 368-372
- B. A. Black, “From Conservation to Kirchoff: Getting
Started in Circuits with Conservation and Accounting,” Proceedings of
the 1996 Frontiers in Education Conference, Salt Lake City, Utah, 6-9
November 1996
- Griffin, Richard B., Louis J. Everett, P. Keating,
Dimitris C. Lagoudas, E. Tebeaux, D. Parker, William Bassichis, and
David Barrow, "Planning the Texas A&M University College of
Engineering Sophomore Year Integrated Curriculum," Fourth World
Conference on Engineering Education, St. Paul, Minnesota, October 1995,
vol. 1, pp. 228-232.
- Everett, Louis J., "Experiences in the Integrated
Sophomore Year of the Foundation Coalition at Texas A&M," Proceedings,
1996 ASEE National Conference, Washington, DC, June 1996
- Richards, Donald E., Gloria J. Rogers, "A New
Sophomore Engineering Curriculum -- The First Year Experience,"
Proceedings, 1996 Frontiers in Education Conference, Salt Lake
City, Utah, 6-9 November 1996
- Heenan, William and Robert McLaughlan, "Development
of an Integrated Sophomore Year Curriculum,” Proceedings
of the 1996 Frontiers in Education Conference, Salt Lake City, Utah,
6-9 November 1996
- Mashburn, Brent, Barry Monk, Robert Smith, Tan-Yu
Lee, and Jon Bredeson, "Experiences with
a New Engineering Sophomore Year,” Proceedings of the 1996 Frontiers
in Education Conference, Salt Lake City, Utah, 6-9 November 1996
- Everett, Louis J., "Dynamics as a Process,
Helping Undergraduates Understand Design and Analysis of Dynamics Systems,"
Proceedings, 1997 ASEE National Conference,
- Doering, E., “Electronics Lab Bench in a Laptop:
Using Electronics Workbench to Enhance Learning in an Introductory Circuits
Course,” Proceedings of the 1997 Frontiers in Education Conference,
November 1997
- Cornwell, P., and J. Fine, “Mechanics in the Rose-Hulman
Foundation Coalition Sophomore Curriculum,” Proceedings of the Workshop
on Reform of Undergraduate Mechanics Education, Penn State, 16-18 August
1998
- Cornwell, P., and J. Fine, “Mechanics in the Rose-Hulman
Foundation Coalition Sophomore Curriculum,” to appear in the International
Journal of Engineering Education
- Cornwell, P. and J. Fine, “Integrating
Dynamics throughout the Sophomore Year,” Proceeedings, 1999 ASEE
Annual Conference, Charlotte, North Carolina, 20-23 June 1999
- Burkhardt, H. "System physics: A uniform approach
to the branches of classical physics." Am. J. Phys. 55 (4),
April 1987, pp. 344–350.
- Fuchs, Hans U. Dynamics of Heat. Springer-Verlag,
New York, 1996.
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2001 Foundation Coalition. All rights reserved. Last modified
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