Class notes by RE-SEED Leader - Alex Vanderburgh

Pressure - Not to be confused with "Force" but often is. It's another pesky ratio: "Force per unit Area". P = F/A or F = PA
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Forces have a direction as well as a size. Pressure has no direction. Or perhaps you can say it is the same in all directions. [Force is a "vector". Pressure is a "scalar".] It is a complex idea, and the words "vector and scalar" will be new to most middle school students. When I squeeze a balloon the pressure inside changes everywhere, not only between my hands. [Pressure is what you get if squeezed on all sides, plus the top and bottom.]

How does an hydraulic jack work? Consider a container full of hydraulic fluid. On one wall of the container, there are two round holes - i.e. cylinders with pistons - a large one, and a small one. At a given pressure, the force needed to keep the pistons from moving is much higher for the large one. [In fact, it depends on the relative areas - force equals pressure times area.] If I push the small piston in, the pressure goes up, and it takes much more force to hold the large one. The pressure on the two pistons is the same. The areas are not, and the forces are not. Note that the large piston won't move as far as the small one does. So put the car on the large one, and use the the small one to pump in more fluid. [Well, there are a few minor design details, such as valves etc..]

Demo 2 Clay and a 500g "weight".

If you put the weight on the clay, the force is spread over a large area and the clay is not damaged. If we loop a string or a wire over the clay, we can cut the clay easily with the same 500g weight! [Note: One member of the group suggested that you can cut a block of Ice this way without cutting it - since it refreezes as you cut through it!]

If you spread out a force over a large area no damage is done. A ladder is often used to rescue victims who fall through broken ice. It is laid out on the ice to spread out the weight of the rescuer! By definition, pressure is the ratio of Force to Area. Pressure will increase if the area is reduced, as with a sharp point, or edge, and decrease if the area is spread out as with the ladder on the ice.

Pascal's Law:

Pressure in a fluid is caused by gravity, and is the product of weight density and depth. It is a new idea for us. How can a "Pressure" exist at a "point"? [Especially a point that has no area!] Fortunately, we do not have to worry about it, since we will always be concerned with the pressure on an object, or the pressure on the walls or base of a container. We will always have a finite area. The idea of a "point" has an interesting history, but we stay away from it, for it leads to many non-intuitive paradoxes, and the literature resorts to calculus to explain them.]

Is water pressure different on the Moon? Does it go to zero in free fall?

Demo 3 & 4
Water and holes in a plastic bottle. [A heated ice pick makes nice smooth holes. It may be harder to make them all the same size however.] Try dropping the bottle with water spraying out the holes. (You will find that the water stays inside, as long as the bottle
is falling.)

Demo 5
It turns out we live under pressure - 14 lbs/square foot - a.k.a. one "Atmosphere". We can easily make steam in a tin can by boiling a few cc's of water therein. When we condense the steam by inverting the can in a pan of water, atmospheric pressure is enough to collapse the can. [Note that the density of steam is .000596 g/cc. (See Appendix III in the RESEED book.) If you fill a 12 oz can with steam, (about 355cc), when you condense the steam, you get about 2/10 of a gram of water. Most of your 355 cc can is empty!]

A similar demo, but perhaps not appropriate for students, is to blow up a balloon with steam. The small erlenmayer flask in the kit works well. You may need somebody with gloves to hold the hot flask when you stretch the balloon over the top. It will inflate fast. When you take it off the fire, the balloon will disappear into the flask! Heating it again will pop it out! [What will happen when you tie it off and remove it from the bottle?

Bernoulli's Principle: Moving air lowers the pressure.

Demo 7
Home made atomizer. No need to be fancy. Just use two clear straws and colored water. One straw is vertical in the water. As you blow through the other straw across the top, you can easily see the water rise. When it gets to the top it comes out and becomes a cloud of mist. It may take some practice to make it work easily.
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Homework:

1. Consider an cylinder immersed vertically in a liquid. Show that the difference in the force down due to the pressure in the liquid at the top of the object and the force up due to the pressure in the liquid at the bottom of the object is equivalent to the weight of the displaced liquid.

2. Is a balloon "attracted" by a stream of water? [or by a stream of air?] Is "pressure" part of this scene?

3. Demo 3 has a two liter plastic jug with 3 holes at different heights. Try it over a bathtub: a. With the cover off. b. With the cover on, but with a light grip - just enough to make it squirt.
c. With the cover on, and use a two handed high pressure grip.
d. With the cover off, and water coming out, drop it!

4. If you blow up a balloon and take it down to the bottom of the ocean will the pressure flatten it out into a disk, or into a smaller sphere?

5. If you attach a weight that it can carry at the surface, will it go back up when you pull it down to the bottom and let it go?

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Errata:
Page 44 line 1 Change "Thus the pressure is 5N etc..." to
"Thus the pressure is 50 gramforce per square centimeter."

Page 50 line 31 Change "off the ground" to "above the table".
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No doubt you have been wondering about the units that are used for
"Pressure".

pounds per square inch - (Naturally comes to mind.)

Newtons per square meter - (looks reasonable.)

Pascals - One Newton / square meter (The Pascal is the official
SI unit.)

Atmospheres - 14.7 pounds per square inch (760 Torrs)

Centimeters of Mercury - One Atmosphere is 76.1 of these.

Inches of Mercury - One atmosphere is 29.92 of these.

Torrs - millimeters of Mercury - [Used for Blood Pressure and
vacuums]
(Named in honor of Evangelisti Torricelli.)

Bar - 100,000 Pascals - 1.013 Atmospheres -(Bar-o-meters?)

(Not to be confused with the Barye, which is 1 dyne/square cm or
1/10 of a Pascal.)

Compare psi with Torr - The psi is much bigger - Torrs are used to
measure vaccuums. A millitorr is about the lowest you can get
easily. Vaccuum experts have acheived vaccuums about a billion
times lower.

(Incidentally, psi is pounds per square inch. Tires take about 32.
That and the area of the foot-print will give you an estimate of the
weight of the car.)

Why do people deflate the tires of sand-dune buggies?
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