SC 130 Physical science activity six: Under pressure
At present this is a outline of notes without explanation. As Fermat said in the margin, the notes are elsewhere. In this instance, in my head.
Gear
Albezia sticks
balloon balance
balloons
blower/air cleaner
folder
glass
newspaper
paper
ping pong ball
string
tape
water jug
Newton's third law
For any force a counterforce occurs: balloon rocket. Note that the surface of the balloon provides pressure that drives the air out the back of the primitive rocket. Use this as a lead in to the air pressure activities below.
Pressure
How many sheets to hold down a stick?
Obtain some Albezia lebbeck sticks or other snappable wood.
Place a stack of newspaper on the table.
Ask the class how many sheets of newspaper are sufficient to hold down the stick so that it can be broken in half with a chop of the hand.
Demonstrate that a single sheet is sufficient.
Demonstrate what happens without the sheet. Wear safety glasses and ensure that no one is within a 45° angle down strike. The wood will likely launch up and out like a one way boomerang.
Discuss. Do not simply explain. Discuss why. To the extent possible remain in a Socratic dialog mode.
Albezia, being round and rough, does not work as well as a thin pine yardstick. Increasing the surface area of the Albezia by using a forked stick or pre-halving the stick may help.
Pressure equals the force per unit area. For an equal application of force, a smaller area exerts a higher pressure due to the inverse relationship for the area.
Solving for the force indicates that larger areas produce more force for a constant pressure.
Defying gravity with water: pressure is all around us
Cut out a piece of manila folder or other suitable material.
Fill a drinking glass with water.
Place the paper over the open end of the glass.
While holding the paper, invert the glass.
Release the paper.
Discuss. Do not simply explain. Discuss why. Note that this might only work in Kitti.
Occasionally a student will postulate that air in the glass would cause the demonstration to fail. Having some air in the glass does not cause the demonstration to fail. Demonstrate this as necessary. This works with really tall containers. For a bigger mental impact use a larger container.
The arc of a water spout
Find a container into which three holes can be placed. Bigger is better, more fun, and messier. A five gallon water container that has already developed a leak is useful. Ask at the local water company. Patch any hole that interferes with the demonstration.
Put three holes into the side of the container vertically. If need be, cover the holes with tape. Keep a "tape tab" so the tape can be easily removed.
Ask the class to sketch or otherwise describe the path the water will make.
Fill with water.
Remove the tape tabs.
Discuss. Do not simply explain. Discuss why. What is the shape of the water arcs? How does pressure vary with depth?
Balloon balance
Connect two balloons via a needle valve.
Inflate one balloon and attach to one side of the valve with the valve closed.
Under inflate a second balloon and attach to the other side.
Ask the class to predict what will happen when the valve is opened.
Open the valve.
Discuss. Do not simply explain. Discuss why.
Bernoulli
Why planes fly
Cut out a long strip of paper.
Place under your lip.
Blow.
Discuss. Do not simply explain. Discuss why.
Magic floating balloon
Use a blower that blows up.
Put a balloon in the air column.
Discuss. Do not simply explain. Discuss why.
Curve ball
A curved plastic toy ball slinger.
A ping pong ball.
Throw. By now the students are used to ducking for cover. Best done outdoors!
