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1
Physical Quantity and Physical Relation
Functional Form q = f (x1, x2, …)
There are Two Ways to Determine The Numerical Value of A Physical
Quantity q
Direct Measurement of q Measured Quantity
Determination of q from A Physical Relation for q Derived Quantity
Many Different Physical Principles (for an experiment)
Measured Quantity VS Derived Quantity
Data Reduction Diagram (DRD) for A Physical Quantity q, DRD-q
2145-391 Aerospace Engineering Laboratory I
2
Defining An Experiment with
Design of An Experiment Using DRDs
);;( cpxfy Dependent Variable
Independent Variables
Variable Parameters
Constant Parameters
Independent Variables
Parameters
C
x (unit x)
y (unit y) p = p1
p = p2
p = p3
4
Physical QuantityDescribing A Physical Quantity
In an experiment, we want to determine the numerical values of various physical
quantities.
Physical quantity
A quantifiable/measurable attribute we assign to a particular characteristic of
nature that we observe.
)(][,2.1.3.2
LengthLlmlDimensionQmeasureofunitQunitwrtvaluenumerical
][q
Q
Q
Describing a physical quantity q
1. Dimension
2. Numerical value with respect to the unit of measure
3. Unit of measure
5
Physical Relations/Principles
Physical Relation
A relation among physical quantities.
A (valid) physical relation obeys the principle of dimensional homogeneity.
There are many types of physical relations:
Definition [Equality is by definition, := ]
Physical laws/relations [Equality is by law/theory, = ]
Geometric relation (L): sine law, etc.
Kinematic relation (Lt):
Dynamic relation (MLt):
V
MVM :),(
amamF
),(
etc.,,dy
duamF yx
etc.,2
1 2atuts
6
Physical Relation
2
2
1),,( atutatufs
Physical relation: ,...),( 21 xxfq
In a physical relation
q is a function of x1 , x2 , …
q depends on x1 , x2 , …
,...),( 21 xxfq
In order to determine the numerical value of q
1. the physical relation f must be known, and
2. the numerical values of all variables and constants x1 , x2 , … must be known
7
Functional Form
Physical relation: ,...),( 21 xxfq
q is a function of x1 , x2 , …
q depends on x1 , x2 , …
the numerical value of q is found from
1. known f , and
2. known values of all x1 , x2 , …
We use parenthesized list of independent variables x1 , x2 , … after q
to indicate that
Functional
form
),...,( 21 xxq
‘sources’ of the numerical value of q
8
There are Two Ways to Determine The Numerical Value of A Physical Quantity q
Direct Measurement of q Measured Quantity
Determination of q from A Physical Relation for q Derived Quantity
Many Different Physical Principles (for any one experiment)
Some are based on Measurement and Measured Quantity
Some are based on Physical Relation and Derived Quantity
Example 1
9
Example 1: Free Falling ExperimentDetermine the distance the ball travels to ground
S = ?
Class Discussion
10
Principle 1: Measurement and Measured Quantity
We can measure s with a measuring tape
Instrument: Measuring tape
The numerical value of s is determined by
measurement with a measuring instrument
s is a measured quantity
(in the current experiment)
11
Principle 2: Physical Relation and Derived Quantity
Instrument: 1) stop watch (t)
t : stop watch
g: look up in a reference
We can calculated/derived the numerical value of s from
2
2
1),( gttgs
The numerical value of S is determined from
1. the known physical relation f : , and
2. the known numerical values of all other variables (t and
g) in the relation f
s is a derived quantity
(in the current experiment)
2
2
1),( gttgs
12
The Determination of The Numerical Value of A Physical Quantity qMeasured Quantity VS Derived Quantity
The Determination of The Numerical Value of A Physical Quantity q
In a physical phenomenon / current experiment, the numerical value of a physical quantity q can be
(and must be) determined either through
measurement with a measuring instrument Measured Quantity
or
derived through a physical relation Derived Quantity
s is a measured quantity
Its numerical value is determined via
measurement with a measuring
instrument.
s: measuring tape
Instrument: 1) measuring tape (s)
s is a derived quantity
Its numerical value is determined through
1. known physical relation f, and
2. known numerical values of all other variables in f
Instrument: 1) stop watch (t)
2
2
1),( gttgs
t : stop watch
g: look up in a reference
Physical relation
13
Measured Quantity VS Derived Quantity
The Determination of The Numerical Value of A Physical Quantity q
must be either through
Measurement with an instrument Measured quantity
or
Derived through a physical relation Derived quantity(and by no other means)
Because of existing physical relations/laws, we don’t want anybody to make up any
number for a physical quantity.
15
Example 2: Experiment – Determine the density of gas
Experiment: Determine the density of gas in a closed container.
Class Discussion(on the principles that we can use to conduct an experiment)
How can we find out the density of gas in this
closed container?
Is there just one way or are there many ways?
16
Physical Principles for An Experiment
Experiment: Determine the density of gas in a closed container.
There can be many physical principles (more than one) that we can use to conduct an
experiment and determine the numerical value of the desired physical quantity q.
V
MVM :),(
Principle 2: Use the perfect gas law
(Thermodynamic definition/relation for density under
specialized condition)
Instruments:
1. Pressure gage to measure pressure (p) in the unit of
pressure, pa
2. Thermometer to measure temperature (T ) in the unit of
temperature oC
Need to know gas to determine the gas constant R.
RT
pTRp ),,(
Pressure gage (p)
Thermometer (T)
Principle 1: Use mechanical definition of density
Instruments:
1. Scale to measure masses (M ) in the unit of
mass, kg
2. Measuring tape to determine volume (V)
V
MVM :),(
18
Measured Quantity VS Derived Quantity
The Determination of The Numerical Value of A Physical Quantity q
must be either through
Measurement with an instrument Measured quantity
or
Derived through a physical relation Derived quantity(and by no other means)
Because of existing physical relations/laws, we don’t want anybody to make up any
number for a physical quantity.
19
Measured QuantityIs it a measured quantity or a derived quantity? (in the current experiment)
A measured quantity q
is the quantity whose numerical value is read from the instrument in the
unit of q directly.
Principle 1: Use mechanical definition of density
Instruments:
1. Scale to measure masses (M ) in the unit of
mass, kg
2. Measuring tape to determine volume (V)
V
MVM :),(
M is a measured quantity
its numerical value is read from the instrument
(scale) in the unit of mass (kg) directly
is a derived quantity
its numerical value is derived from the physical
relation = M/V.
V ?
, M, V: Are they measured or derived?
20
, M, V : Are they measured or derived?
M is a measured quantity
its numerical value is read from the instrument (scale) in the unit of mass (kg) directly
}S/N....Scale,:{ kgM q { measured unit: Measuring instrument identity }
source of the numerical value of q is in braces.
is a derived quantity
its numerical value is derived from
1) the physical relation = M/V, and
2) known values of M and V.
]/[:),( 3mkgV
MVM
}S/N....Scale,:{ kgM ][.... 3mV
]t[,...),(,..., 2121 )( uniderived relationexplicit
righttheonconstantsandvariablesalloflist
xxfxxq
source of the numerical value of q is in parentheses.
21
V ? Do we really measure volume using an instrument from which the
numerical value of volume is read directly in the unit of volume (e.g., m3)?
Method 1: Use mechanical definition of density
Instruments:
1. Scale to measure masses (M ) in
the unit of mass, kg
2. Measuring tape to determine volume (V)
V
MVM :),(
}...#tapeMeasuring:{mh}....#tapeMeasuring:{md
][4
),( 32 mhdhdV
V is a derived quantity
its numerical value is derived from
1) the physical relation , and
2) known values of measured quantities d and h.
hdhdV 2
4),(
22
Measured QuantityIs it a measured quantity or a derived quantity, really? (in the current experiment)
Look at the unit of the instrument!
If you don’t read its unit from the measuring instrument, it
is not a measured quantity.
A measured quantity q
is the quantity whose numerical value is read from the instrument in the
unit of q directly.
23
In Summary: Measured Quantity VS Derived Quantity
Measured Quantity q: The numerical value of a measured quantity is
determined directly by measurement with a measuring
instrument, which reads out in the unit of q directly.
Derived quantity q :
The numerical value of a derived quantity is
determined
1. through a known physical relation f,
and 2. known values of all variables and constants
(Without knowing both 1 and 2 completely, we cannot find the numerical value of a derived
quantity q.)
,...),( 21 xxfq
,...),( 21 xxf
,..., 21 xx
24
Data Reduction Diagram (DRD)
for A Physical Quantity q, DRD-q
(for any one physical quantity in an experiment)
25
KEY IDEA for A DRD-q
A diagram that we can trace clearly, specifically, and systematically
1. the sources of the numerical values that enter our experiment at the source level
[source-level / bottommost-level boxes],
and
2. the transformations of numerical values [derived-box / data-analysis boxes]
from the source-level values,
through various physical / derived relations in the current experiment,
to the final value of the desired variable q.
A DRD for A Physical Quantity q
26
Data Reduction Diagram (DRD)
Principle 1: Use mechanical definition of density
Instruments:
1. Scale to measure masses (M )
2. Measuring tape to determine
volume (V)
V
MVM :),(
Experiment: Determine the density r of gas in a closed container.
Bottommost level = Braced Boxes / Measured
quantities only
]/[:),( 3mkgV
MVM
}...S/NScale,:{ kgM }#...tapeMeasuring:{mh
}#...tapeMeasuring:{md
][4
),( 32 mhdhdV
DRD -
27
Example 3: DRD
Class Discussion
Construct a DRD for (the determination
of the numerical value of) the density
V
MVM :),(
Principle 2: Use the perfect gas law
(Thermodynamic definition/relation for density under
specialized condition)
Instruments:
1. Pressure gage to measure pressure (p) in the unit of
pressure, pa
2. Thermocouple to measure temperature (T ) in the unit of
temperature oC
Need to know gas to determine the gas constant R.
RT
pTRp ),,(
Pressure gage (p)
Thermocouple (T)
28
Instruments:
1. Pressure gage to measure pressure (p) in the unit of
pressure, pa
2. Thermometer to measure temperature (T ) in the unit of
temperature oC
Need to know gas to determine the gas constant R.
]/[),,( 3mkgRT
pRTp
Pressure gage (p)
Thermocouple (T)
???R}S/N...gage,Pressure:{pap }S/N...r,Thermomete:{ CT o
][15.273)())(( KCTCTT oo Unit conversion
Because there is a transformation of a numerical value through a relation, we consider
unit conversion as one of the data analysis step.
This is a derived box (parenthesized box).
What kind of quantity is R, measured or derived?
29
Referenced Quantity
For some quantities, we may not be able to measure or derive it directly in
the current experiment.
We take their numerical value from some reference source.
We refer to this kind of quantities in the current experiment as
Reference Quantities
Regardless, being a physical quantity, the numerical value of a reference
quantity must be either
measured, or
derived
by the original author of the value.
30
Derived-Referenced Quantity VS Stated-Referenced Quantity
Derived-Referenced Quantity
Example: Determination of density from
1. thermodynamic table, and
2. known values of p and T (and type of gas,
tg)
2007,...}Boles,andCengel4,-ATable
:{kg/mTablemicThermodyna)tg,,( 3Tp
}S/N...gage,pressure:{ pap
][15.273)())(( KCTCTT oo Unit conversion
}S/N...r,Thermomete:{ CT o
)gasoftypetg(
}......{
tg
31
Although the physical relation is not stated explicitly as an equation,
table,
chart,
etc.,
have an underlying physical relation.
We need to know the numerical values of p and T first before we can look up the
table to get .
The numerical value of depends on the numerical values of p and T.
{.....}TablemicThermodyna)tg,,( Tp
Use functional form and parentheses for a derived quantity.
32
Derived-Referenced Quantity VS Stated-Referenced Quantity
Stated-Referenced Quantity
Example:
710} p. D/2, Table York, New Wiley,Edition,Fourth
Dynamics :Mechanics gEngineerin 1998, Kraig,andMeriam:/{ 2 smg
In this case, g is not a derived-referenced quantity.
Its numerical value is looked up directly, without the knowledge of the numerical
values of other quantities.
33
In this case, g is a derived-referenced quantity since we take that it depends on
the elevation h.
710. p. D/2, Table York, New Wiley,Edition,Fourth
]2
[m/sDynamics :Mechanics gEngineerin 1998, Kraig,andMeriam)( hg
....}:{ mh
34
Back to Example 3 Pressure gage (p)
Thermocouple (T)
]/[),,( 3mkgRT
pRTp
}S/N...gage,Pressure:{pap }S/N...r,Thermomete:{ CT o
][15.273)())(( KCTCTT oo Unit conversion
p.910}1,-ATable,2007,.....Boles,andCengel:/{ KkgJR
DRD -
Source / Bottommost Level - Braced Boxes only
This is where numerical values first enter our experiment
35
Summary of Types and Boxes of Quantities in DRDConvention for Boxes of Various Types of Quantities in DRD
1. Measured Quantity q
q { measured unit: Measuring instrument identity }
Measured unit is the unit that is read directly from the instrument, no unit conversion.
}#4gagepressureLab:{ pap
[Braced box, source-level box.
Use braces on the LHS.]
source of the numerical value of q
2. Derived Quantity q
]t[,...),(),...,( 2121 uniderived relationexplicit
righttheonconstantsandvariablesalloflist
xxfxxq
]p[),,( aRTRTp
Derived unit is the unit that is a result of the physical relation f and the actual units that correspond to the
numerical values of x1, x2, … that are input into the relation f, no unit conversion.
[Parenthesized-box, derived box.
Use parentheses on the LHS]
source of the numerical value of q
36
3.1. Derived-Referenced Quantity q
Reference}:{tablemicThermodyna),...,( 21 unitreference
relationderivedtheininvolvedconstants
andvariablesalloflist
xxq
916}p.2007,...,Boles,andCengel4,-ATable
:{kg/mTablemicThermodyna)tg,,( 3Tp
[Parenthesized-box, derived box.
Use parentheses on the LHS]
source of the numerical value of q
Reference unit is the unit that corresponds to the numerical value that is given in the reference, no unit
conversion.
3.2. Stated-Referenced Quantity q
Reference}:{ unitreferenceq
710} p. D/2, Table York, New Wiley,Edition,Fourth
Dynamics :Mechanics gEngineerin 1998, Kraig,andMeriam:/{ 2 smg
[Braced box, source-level box.
Use braces on the LHS.]
source of the numerical value of q
37
Summary of Rules and Guides for a DRD
1. Braced-Boxes / Source Level
At the bottommost/source level only, and nowhere else.
2. Parenthesized-Boxes / Derived Levels
Can never be at the bottommost/source level since they need
sources of numerical values from somewhere else.
q { …. }
q ( …. )
38
Summary of Rules and Guides for a DRD
3. Numerical Transformation
Every step of numerical transformation from the
bottommost/source/braced-box level to the DRD-variable (q) must be recorded in
the DRD [via a derived/parenthesized box].
Relations that result in corresponding numerical transformations are
definition,
physical relation (geometrical, kinematical, and dynamical relation),
calibration relation,
unit conversion,
etc.
39
Summary of Rules and Guides for a DRD
4. Unit
Every box in a DRD must have the corresponding unit stated.
Various types of units (terminology by convention)
Measured unit
Derived unit
Reference unit
A derived unit in a derived box in a DRD must be consistent with both
the units of the source variables of that box, and
the relation in that box.
41
Defining An Experiment with
);;( cpxfy Dependent Variable
Independent Variables
Variable Parameters
Constant Parameters
Independent Variables
Parameters
C
x (unit x)
y (unit y) p = p1
p = p2
p = p3
42
Defining An Experiment
Often in an experiment, the objective is not simply to find a single value
of a single physical quantity but
QUESTION:
‘whether and how y is related to x under the condition ( p , c ):
a physical relation:
);;( cpxfy
43
C
x (unit x)
y (unit y) p = p1
p = p2
p = p3
Experiment: );;( cpxfy
Dependent Variable
Independent Variables
Variable Parameters
Constant Parameters
Independent Variables
Parameters
44
Example
T (K)
p (pa) = 1
= 2
= 3
Fixed gas (R)
Isochoric process
p is dependent variable
T is independent variable
is variable parameter
R (tg) is constant parameter
QUESTION:
‘whether and how the pressure p is related to the
temperature T under the condition of various
density and constant gas type (R).
Physical relation: ))tg(;;( RTfp
);;( cpxfy
x (unit x)
y (unit y)p = p1
c
Line of constant p
p = p2
p = p3
45
Example
From Abbot, I. R. H. and von Doenhoff, A. E., 1959, Theory of Wing Sections: Including A Summary of Airfoil Data, Dover, pp. 496-497.
y = cl
x = (deg)
p = Re
p1
p1
47
Design of An Experiment Using DRD and Its Consequences
1. Question/Relation
Set the goal that we want to answer the question
‘whether and how y is related to x under the condition ( p , c ):
Experiment: y = f ( x ; p ; c )
2. Graphical Representation of Results
We then know that the graphical representation of the relation
should look like this:
C
x (unit x)
y (unit y) p = p1
p = p2
p = p3
y = f ( x ; p ; c )
48
3. Data Reduction Diagram (DRD)
Construct a data reduction diagram (DRD) for each of the final
variables: y, x, p, and c
DRD-y
DRD-x
DRD-p
DRD-c
49
4. From this set of DRDs for the whole experiment
1. All The Measured Quantities and Instruments
Measured Quantities: We know all of the measured quantities in this
experiment from the bottommost/source level
Instruments: We know all of the instruments we need
for this experiment
DCW: We can construct a data-collection
worksheet.
All The Derived Quantities and Physical Relations
Derived Quantities and Physical Relations: We know all of the
derived quantities and all of the corresponding physical relations.
DAW: We can construct a data-analysis worksheet.