Friday April 7, 2006
Quiz #3 was returned in class today.
The Expt. 4 reports and some Expt. 3 reports (from people who picked up
materials late) were collected today.
An in-class optional assignment was distributed in class today.
To earn full credit on the assignment it had to be turned in at the end
of class. You can turn in the assignment next Monday or Wednesday for
partial credit. You'll find a scanned copy of the assignment at
the end of these notes.
We be
covering a little bit of the material in Chapter 6 today and next
Monday.
We'll learn why winds spin counterclockwise (CCW) around Low pressure
and clockwise (CW) around High pressure in the northern
hemisphere. We'll see why the spinning winds reverse
direction in the southern hemisphere. You may already have been
to the southern hemisphere or you may go there one day. You'll
probably hear about how the Coriolis force or the Coriolis effect
causes water draining out of sinks and toilet bowls to spin in the
opposite direction in the southern hemisphere (it's not true).
That's another reason for covering the Coriolis effect in NATS
101. The "see question #11" in the figure above refers to one of
the questions on the In-class Optional Assignment.
This figure shows the winds spinning around middle latitude storms
(extratropical cyclones) and hurricanes (tropical cyclones) in both the
northern and southern hemispheres. The term cyclone refers to
winds blowing around low pressure. Note how the directions of the
spinning winds change as you move from one hemisphere to the other.
Note how middle latitude storms tend to move from west to east in both
hemispheres. Hurricanes, which are found in the subtropics, move
from east to west in both hemispheres. We will learn more about
why this occurs on Wednesday or Friday next week.
After studying Newton's laws of motion you will appreciate why an
inwardly directed force is need to keep winds spinning in a
circle. In the case of the rapid winds in a tornado, a very
strong force is needed (it turns out to be the pressure gradient force
(PGF) or pressure difference force).
Now on to Newton's 1st and 2nd laws of motion and some graphical
illustrations of what they mean.
The 1st law really has two parts: one that deals with stationary
objects and another that treats moving objects.
A stationary object is shown in all three figures above. In
the left example there aren't any forces at all being exerted on the
object, there is no reason for it to suddenly start to move. In
the middle and right examples there are two forces present but they are
of equal strength and point in opposite directions. They cancel
each other out and the net or total force is zero. Again the
stationary object won't suddenly begin to move.
In all of these examples (on p. 121 in the photocopied notes) the
next force is zero (the two forces present in examples #2, #3, #4, and
#5 cancel each other out). The object will continue to move in a
straight line at constant speed (the thin arrows show the direction of
motion, the length of the arrow provides an idea of speed.
Next we look at cases where this is a net force.
If there were no net force at the point indicated, Newton's 1st law of
motion would say the object would travel in a straight line at constant
speed (the light brown arrows). But the object turns to the
right. A force acting perpendiculary and to the right of the
object's direction of motion are neede.
Gravity provides the constant inward pointing force needed to keep a
satellite in orbit around the earth. Upper level winds blowing
through ridges and troughs need net forces that point sometimes to the
right of the wind and sometimes to the left of the wind to keep the
winds blowing parallel to the contour lines.
The 2nd law of motion really just say that if you exert a net force on
an object it will acceleration (i.e. start moving, speed up or slow
down, start moving in a different direction).
In the first example unequal forces (2 and 5) are applied to equal
masses (5 and 5). You can calculate the acceleration
by dividing force by mass. This gives you the acceleration, the
lower object will speed up ten times faster than the top object which
has a weak force exerted on it.
In the bottom example equal forces (5 and 5) are applied to two
different masses (2 and 10). Mass can be thought of
inertia. An object with a large mass is resistant to a change of
direction or speed. A large object is harder to start moving than
a small object (imagine pushing a stalled Volkswagen and a stalled
Cadillac out of an intersection)
Pressure differences create a force that points from high pressure to
low pressure. On a weather map the pressure gradient force (PGF)
always points perpendicular to the contours on the map and from high
toward low. The PGF force is stronger when the contours are
closely spaced and weak when the contours are further apart.
Some examples of PGF force directions and relative strengths are shown
at the bottom of the figure. Note the analogy between weather
maps and geographical features like hills and valleys.
AFter studying this information for a while, see if you can answer
Question #7 on the In-class optional assignment.
The PGF can cause stationary air to begin to move. In the top
example a stationary volume of air is placed in a center of low
pressure. The PGF will cause the air to begin to move toward low
pressure in the center of the picture. The dotted line shows the
direction of initial motion. This like placing a ball on the side
wall of a valley. The ball will roll downhill.
In the second example, a center of high pressure, the PGF causes a
stationary volume of air to again begin to move toward low pressure
which is outward and away from high pressure. In the analogy a
ball placed on the side of a hill will roll downhill and away from the
summit.
So given a pressure pattern you should be able to determine the
direction of initial motion (see Question #8). Or as in Question
#10 you should be able to determine whether the motion is being caused
by a center of high or low pressure.
We rushed through the origins of the Coriolis force at the end of class
and will come back to it again on Monday. For the time being have
a look at the rules for direction and strength of the Coriolis force
below and use them to complete the Optional Assignment by answering
Questions #9 and #12.
click here to see a scanned copy of the optional
assignment