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PH15720
Laboratory Techniques An Introduction to MATHCAD
Lecture 2
Introduction
Review of Last Week
 Variables
 Names for Things
 Using Units
 Text areas & comments

Review of Last Session
What, Why & Wherefore ?
 Starting MathCAD
 Simple calculations
 Saving work to M: drive
 Emailing worksheets

Variables #1
Storage for numbers in MathCAD
 Write formulae without using ‘magic
numbers’
 Do not re-type intermediate results
 Simplifies changing parameters

Use of variables - Example

Compare the 2 solutions to a problem below


Which is clearer ?
Which is easier to change parameters ?
mP  vP mA vA 0 s olve vA
mP
80 kg
mA
60 kg
vP
m
vA
0.3
s
vP
mP 
mA
vA  0.4
mP 
vP
mA
Mass of panel
Mass of astronaut
velocity of panel
Calculate
velocity of astronaut
m
s
Display Result
0.3 
( 80  kg) 
m
s
60  kg
 0.4
m
s
Variables #2
Variable set by using
<name> := <expression>
 Assignment operator (:=) created by
colon (:) key


Declares the variable and sets
its value to <expression>
Variables #3
Variables displayed by = sign
 Press = key

 If
variable already defined, MathCAD will print
value =
 Otherwise MathCAD will create assignment
operator & placeholder for you to define value
:=
Names for Things
Very Important step in problem solving
 Applicable to all problems
 Applicable to all solution methods
 Develop convention & stick to it

Naming convention #1
Make names descriptive
 Use standard physics nomenclature
 UseCamelCapitalisation
 Examples:

 MRocket0
: initial mass
 VRocketFinal : final velocity

Problem should suggest names
Naming Convention #2
Keep names short
 Use comments to explain names
 Don’t try & put spaces in names
 Avoid using names already used for
mathematical/physical constants (unless
you mean it !)

 g,c,e,q,p
 MathCAD
will warn you
Naming Convention #3
Good examples:
 MBallA := 1.7 kg



vBallA0 := 23 m/s


Mass of Ball A
Velocity of Ball A at t=0
VResA0 := 12 volt

Voltage across Resistor A = 12V at t=0
Naming Convention #4


Bad examples:
A := 7 m/s




vTheBallThrownByJohn := 17 m/s


No clue as to physical quantity
Probably too short (unless diagram)
Conflicts with A = Ampere
Way too long
g := 19

Redefined acceleration due to gravity
Greek Letters

Enter into variables etc by:
 Select
from Greek toolbar
 Type corresponding roman character + <ctrlg>
 <ctrl-shift-p> = p

Enter into text regions (comments) by
changing to symbol font
Using UNITS #1
All variables in MathCAD may have units
attached to them
 Units propagate through calculations
 MathCAD checks to ensure ‘sanity’
 Results may be manipulated to desired
units

Using UNITS #2

To enter units:
 Assign
a variable (Name + :)
 Press * to introduce multiplication
 Either:
Enter units if you know them
 Press <ctrl-u> or units button
to bring up selection dialog

Using units #3
Units displayed on result
 Speed
Many
possible
 85 conversions
mph
9

nm  1  10
ReactionTime 0.1 s
m
6
m  1  10
m
3
mm  1  10
Dis tance SpeedReactionTime
cm  0.01 m
Dis tance 3.8 m
m  1m
Dis tance 12.467 ft
km  1  10 m
Dis tance 4.156 yd
m
in  0.025 m
ft  0.305 m
yd  0.914 m
furlong 201.168 m
3
3
mi  1.609  10 m
 10
Angs trom 1  10
m
 11
bohr  5.292  10
cubit  0.457 m
m
Using Units #4

To convert units of result
 Select
placeholder after result
 Type/select desired units for result
 MathCAD will scale result to new units.
 Will add extra terms if dimensions wrong
3.8 smph
m ft
Distance 12.467
Distance
8.5
Placeholder
Text Regions #1

Very Important to:
 Introduce
problem
 Introduce & explain variables
 Explain solution method
 Introduce & Explain answer
Text Regions #2

Text regions added by:
 Insert|Text
Region
“
key
 Typing a comment. MathCAD will convert the
text to a text region when the first space is
typed.

Text regions moved/resized with mouse –
watch cursor change