Lecture
F1
Mud:
Intro
Concepts
&
Definitions
(40
respondents)
1.
Explain
PRS
#1
(1
student)
For
a
ﬂuids
situation
to
be
Hydrodynamic
or
Aerodynamic
in
nature,
the
ﬂuid
must
be
moving
relative
to
an
object.
In
the
PRS
question,
the
water
does
not
move
past
the
buoy.
So
that’s
an
example
of
Hydrostatics.
2.
Why
is
p
=
lim
�
F
n
/
�
A
,
and
not
just
F/A
?
(1
student)
This
is
just
notation
convention.
The
definition
is
similar
to
that
of
a
derivative,
where
�
A
becomes
dA
in
the
limit.
Later
in
the
course
we
will
write
dF
n
=
p
dA
when
integrating
all
the
pressure
forces
on
a
wing.
3.
If
the
observer
frame
changes,
wouldn’t
p
(
x,
y,
z,
t
)
be
different
functions?
(6
students)
Very
good
question.
I
think
I
was
unclear
on
what
I
meant
by
“same”
or
“different”,
both
on
the
PRS
question
and
in
the
discussion.
Yes,
the
two
observers
will
see
different
p
(
t
)
time
traces
at
each
observer’s
origin
(
x,
y,
z
)
=
(0
,
0
,
0),
for
example.
But
at
the
instant
when
the
two
origins
cooincide
(obesrvers
are
momentarily
on
top
of
each
other),
the
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 Fall '05
 MarkDrela
 Fluid Dynamics, observer, Anthony Howard, PRS Question, observer frame changes

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