Wednesday Sep. 7, 2011
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Three songs from a new artist that I just stumbled upon, Eilen
Jewell. You heard "Everywhere I Go",
"Hooked", and "Too Hot to Sleep."
We
had
time
for
"Mess
Around" in the T Th class (seeing her live in a small bar or cafe
in Brussels would be fun).
Today was most likely the last call for Expt. #1,
there are a few sets of materials still available. Reports are
due Sept. 19 one week from next Monday. Try
to
wrap
up the experiment early next week so that
you can return the materials and pick up the supplementary information
handout.
Also the 1st 1S1P assignment
is
now
available. What that means is this is your first opportunity to
start on your semester long quest of earning 45 1S1P pts. In this
current assignment you have a choice of three topics. Two of
these will count as part of the 4 report total you are allowed to
submit during the semester, the 3rd is a Bonus Assignment. You
can write as many Bonus Assignment reports as you'd like. I
would
encourage you to turn in at least one report so that you can
begin to get an feeling for how the reports will be graded.
We spent part of the beginning of the period covering a little new
material. If this were a real quiz (the first real quiz is two
weeks from today) you would have been given the entire period.
I'll bring this steel bar to class on Friday and pass it
around.
I suspected correctly that some people might be able to guess how much
it weighs now without ever seeing
it or lifting it. The 1 inch x 1 inch crossection of the bar
might remind you of the column of air that mentioned in class last
Friday. Under normal conditions a 1" by 1" column of air
stretching from sea level to the top of the atmosphere would weigh 14.7
pounds (the pressure at the base of the air column would be 14.7 pounds
per square inch (psi)). The iron bar also weighs 14.7 pounds.
A 1" x 1" steel bar 52 inches long weighs 14.7 pounds, the same
as a 1" x 1" column of air that extends from sea level to the top of
the atmosphere 100 or 200 miles (or more) high.
I wanted to add a little clarification about how weight depends on mass
and gravity. Here's what we said last Friday.
Let's make that a little more precise.
Weight is actually mass times the gravitational acceleration, g.
On the earth the value of g never changes. That's why two objects
with the same mass will always have the same weight (even though they
might be made of different materials and have different densities and
volumes).
If you leave the earth and go to the moon or another planet, g will
change. Here are some more details
about how the value of g (the gravitational acceleration) is determined.
Normal sea level atmospheric pressure is about 14.7
psi. These aren't the units that meteorologists would normally
use.
Typical sea level
pressure is 14.7 psi or about 1000 millibars
(the
units used by meterologists and the units that we will use in this
class most of the time) or about 30 inches of mercury (refers to
the reading on a mercury barometer, we'll cover mercury barometers next
week, they're used to measure pressure). Milli means
1/1000 th. So 1000 millibars is the same as 1 bar. You
sometimes see typical sea level pressure written as 1 atmosphere.
I'm also going to bring a bottle of mercury to class on
Friday. Mercury is very dense and I want you to feel how heavy it
is. Mercury is denser than steel. Density is mass (grams)
divided
by volume (cm3
) The density of mercury is 13.6 grams/cm3. The
density of steel is about 7.9 grams/cm3.
A 30 tall column of mercury (1" x 1" crossection) would weigh 14.7
pounds and would produce a pressure at its base of 14.7 psi.
You never know whether something you learn in NATS 101 (or ATMO
170A1 as it's now called) will turn up. I lived and worked for a
short time in France (a very enjoyable and interesting period in my
life). Here's a picture of a car I owned when I was there (this
one is in mint condition, mine was in far worse
shape)
It's a Peugeot
404. After buying it I took it to the
service station to fill it with gas and to check the air pressure in
the tires. I was a little confused by the air
compressor though, the scale only ran from 0 to 3. I'm used to
putting 30 psi or so in my car tires (about 90 psi in my bike
tires). After staring at the scale for a while I finally realized
the numbers were pressures in "bars" not "psi". Since 14.7 psi is
equivalent to 1 bar, 30 psi would be about 2 bars. So I filled up
all the tires and carefully
drove off (one thing I learned you have to
watch out for in France is the "Priority to
the right" rule).
One last thing, and something that I didn't mention in
class.
The word bar is going
to show up in at least terms we'll be using in this class. Bar
means pressure
