Monday, Sept. 11, 2006

"Storm Clouds" on the horizon (i.e. various things that are due or coming due in the not too distant future)

The 1S1P reports were collected today.  It will take some time to grade all these reports.  You should expect to start seeing some of the reports being returned next week.
The Optional Assignment is due Friday (extra copies still available)
The Experiment #1 reports are due next Monday.  You should complete the experiment and return
your materials this week.
Quiz #1 is Wednesday next week (Sept. 20).  The Quiz #1 Study Guide should appear about mid-week.

Now don't get discouraged and think that there are only black clouds headed our way.  There are some good things coming this week also.  Here is one of them, here is another.
The following wasn't covered in class.



The gravitational attraction between two objects depends first of all on the distance separating the objects.  The  gravitational force becomes weaker the further away the two objects are from each other.  In the bottom picture above and the top figure below we see that the attractive force also depends on the masses of the two objects.



 The complete formula is shown in the middle of the page above.  G is a constant.  On the surface of the earth G, M, and R don't change.  The gravitational acceleration, g, is just G times Mearth divided by ( Rearth )2 .
Down at the bottom of the page are the Metric and English units of mass and weight.

You'll find the following material discussed on p. 29 in the photocopied notes.  In 3 simple steps you can understand how a barometer works.  It is probably worth mentioning that barometers are used to measure pressure (atmospheric pressure).  It is not coincidence that the word bar that appears in barometer is the same bar that appears in millibars.  We will be learning about weather maps next week and will come across isobars, contours of pressure.




A manometer can be used to measure pressure difference.  The manometer is just a u-shaped tube usually made of glass so that you can see the liquid that is inside.  The liquid can slosh back and forth just like the pans on a balance can move up and down.  A manometer really behaves just like a pan balance.

In this picture the fact that the liquid levels are the same in the right and left tubes means P1 and P2 are the same (note you really don't know what P1 and P2 are, just that they are equal).



Now the situation is a little different, the liquid levels are no longer equal.  The orange shaded portion of the liquid is the balance that we had in the previous picture.  The pressures at the levels of the two blue arrows are equal (the red shaded fluid is the balance).  P2 is not able by itself to balance P1, P2 is lower than P1.  P1 plus the pressure produced by the column of extra liquid on the right balances P1.  The height of the column of extra liquid provides a measure of the difference between P1 and P2.



We have changed the manometer by lengthening the right tube and sealing it off at the top.  Air pressure can't get into the right tube any more.  The balance is again shaded in orange at the bottom of the barometer.  Pressures at the two blue arrows indicated are equal.  Pair is equal to the pressure produced by the column h inches tall on the right.  If Pair changes, h will change.  You now a way of measuring and monitoring the atmospheric pressure.

Barometers like this are usually filled with mercury.  Mercury is a liquid.  You need a liquid that can slosh back and forth in response to changes in air pressure.  Mercury is also dense which means the barometer won't need to be as tall as if you used something like water.  A water barometer would need to be over 30 feet tall.  With mercury you will need only a 30 inch tall column to balance the weight of the atmosphere at sea level under normal conditions (remember the 30 inches of mercury pressure units mentioned in class a week or so ago).  Mercury also has a low rate of evaporation so you don't have much mercury gas at the top of the right tube.



Finally here is a more conventional barometer design.  The bowl of mercury is usually covered in such a way that it can sense changes in pressure but not evaporate and fill the room with poisonous mercury vapor.




Under normal conditions sea level pressure is about 1000 mb (about 30 inches of mercury).  It can be higher and lower than this but usually falls in the range 950 mb to 1050 mb.  Record high and low sea level pressure values are shown in the chart.  Note the record low values have all be observed in hurricanes.

Hurricane Wilma set a new low sea level pressure reading (882 mb) last year for the Atlantic.  At the time the winds were 185 MPH.

Air pressure is a force that pushes downward, upward, and sideways.  The bottom person in the people pyramid below must push upward with enough force to support the other people.  The air pressure in the four tires on your automobile push down on the road (that's something you would feel if the car ran over your foot) and push upward with enough force to keep the 1000 or 2000 pound vehicle off the road.



"People pyramid"


Three layers of air in the atmosphere are shown above (each layer contains the same amount of air, 10% of the air in the atmosphere).  This picture reminds you that air pressure decreases with increasing altitude.

The layer at the ground and at the bottom of the atmosphere is "squished" by the weight of the air above.  Squeezing all of this air into a thin layer or small volume increases the air's density.  The highest air density is found at the bottom of the atmosphere.

The next layer up is also squished but not as much as the bottom layer.  The density of the air in the second layer is lower than in the bottom layer.  The air in the 3rd layer has even lower density.  It is fairly easy to understand that air density decreases with increasing altitude. 

Finally if you look closely at the figure you can see that pressure decreases most rapidly with increasing altitude in the dense air at the bottom of the atmosphere.
Next we'll do a little demonstration (the demonstration doesn't involve dropping water balloons as shown below).

All of the forces acting on the water balloon are shown in the next figure.


The figure at left shows air pressure (red arrows) pushing on all the sides of the balloon.  Because pressure decreases with increasing altitude, the pressure pushing downward on the top of the balloon is a little weaker than the pressure pushing upward at the bottom of the balloon.  The two sideways forces cancel each other out.  The total effect of the pressure is a weak upward force (shown on the right figure, you might have heard this called a bouyant force).  Gravity exerts a downward force on the water balloon.  In the figure at right you can see that the gravity force is stronger than the upward pressure difference force.  The balloon falls as a result.


In the demonstration a wine glass is filled with water.  A small plastic lid is used to cover the wine glass.  You can then turn the glass upside down without the water falling out.


Now we'll look again at all of the forces and see how this is possible.

All the same forces are shown again in the left most figure.  In the right two figures we separate this into two parts.  First the water inside the glass isn't feeling the downward and sideways pressure forces (because they're pushing on the glass).  Gravity still pulls downward on the water but the upward pressure force is able to overcome the downward pull of gravity.

The demonstration was repeated using a 4 Liter flash (more than a gallon of water, more than 8 pounds of water).  The upward pressure force was still able to keep the water in the flask.