Sep. 18 In-class Optional Assignment Answers
If you completed and turned in the assignment in class you earned 0.2 extra credit points.
If you completed and turned in the assignemnt on Monday you will earn 0.1 points.

1.    Archimedes Law tells us that a 1 gallon bottle is immersed in water (in a swimming pool for example) will         experience an 8 pound upward bouyant force (because a 1 gallon bottle will displace 1 gallon of water and 1 gallon of water weighs 8 pounds).  Archimedes Law allows you to determine the strength of the upward buoyant force but doesn't tell you what causes it.  What does cause the upward bouyant force?

Pressure increases as you go deeper and deeper in the water (it increases very quickly because water is much denser than in air).  The pressure of the water pushing upward on the bottom of the bottle (a) is greater than the pressure of the water pushing down on the top of the bottle (b).  The pressures pushing sideways (c) are all of equal strength and cancel out.  This results in a net upward pressure difference force. I was pleasantly surprised to find that many students knew the answer to this question.

2.   Kilograms or grams are units of mass in the metric system.  In the metric system, weight has units of dynes or Newtons. 
2a.   What units are used for weight in the United States?
This is the easy part of the question.  Pounds are units of weight (so are ounces).
2b.   What units are used for mass in the United States? 
The answer is slugs.  I personally don't know anyone that actually uses these units. 

3a.   Does air pressure  INCREASE  DECREASE  or remain  CONSTANT  with increasing altitude?
3b.   Does air density  INCREASE  DECREASE  or remain  CONSTANT  with increasing altitude?

Both air pressure and air density decrease with increasing altitude.  Pressure at any level in the atmosphere is determined by the weight of the air overhead.  There is less and less air above as you move upward in the atmosphere, so pressure decreases.  An air layer at some level in the atmosphere is compressed by the weight or pressure of the air above.  Since there is less and less air left overhead as you move upward in the atmosphere, the weight or pressure of the air will decrease.  The layers of air are compressed less and the air density decreases.

4.   Air in the troposphere can be stable or unstable.  The stratosphere is always a very stable layer.  What is the difference between a stable atmosphere and an unstable atmosphere?  I.e. what can happen in an unstable atmosphere that doesn't happen in stable air?

Strong up and down air motions or vertical mixing of air are possible in an unstable atmosphere.  A thunderstorm is a good example of unstable conditions.  Up and down air motions are inhibited when the air is stable.

5.   How many feet are in 1 mile?

There are 5280 feet in a mile (5000 feet per mile is probably close enough in this class).  A "nautical" mile has 6076 feet and is thus slightly longer than a "statute" mile.  1 knot is 1 nautical mile per hour and is slightly faster than 1 MPH.  Most of the time in this class we will assume that knots and MPH are equivalent.