In science this week, the students explored a variety of centers around the topic of air pressure. Following are the explanations:
The Million Dollar Bet
Take an empty water or soda bottle and lay it down horizontally on a table. Carefully set a small wadded up ball of paper towel in the mouth of the bottle. (The ball should be about half the size of the opening.) I bet you a million dollars that you can't blow hard and get the paper to go into the bottle! Why is this so impossible? You're trying to force more air into the bottle, but there's no room for the air already inside to go except back out the mouth of the bottle, taking the paper ball with it.
Flying Papers
Hold a regular sheet of paper to your bottom lip (you may have to play a bit to find the exact location) and blow hard across the sheet. The sheet flies up! This is the same reason airplanes can fly. As you blow across the top of the sheet, you lower the air pressure (because the air is moving faster), and thus the pressure on the underside of the sheet is now higher, and higher air pressure pushes the sheet upwards.
Ping Pong Funnel
Insert a ping pong ball into a funnel and blow hard. (You can tilt your head back so that the ball end points to the ceiling. Can you blow hard enough so when you invert the funnel, the ball stays inside? Can you pick up a ball from the table? As you blow into the funnel, the air where the ball sits in the funnel moves faster and generates lower air pressure than the rest of the air surrounding the ball. This means that the pressure under the ball is lower than the surrounding air which is, by comparison, a higher pressure. This higher pressure pushes the ball back into the funnel… no matter how hard you blow or which way you hold the funnel.
Magic Water Glass Trick
Fill a glass one-third with water. Cover the mouth with an index card and invert (holding the card in place) over a sink. Remove your hand from the card. Voila! The card stays in place because air is heavier than water, and the card experiences about 15 pounds of force pushing upward by the air and only about one pound of force pushing downward from the water - hence the card stays in place. (Try this trick over someone's head when you get good at it.)
Keep It Dry
The air inside the glass takes up space and pushes. The paper stays dry because the water can only get into the glass by squashing the air inside it. Air can be squashed, or compressed, a little, but then it pushes back and prevents the water from reaching the paper.
Glove in Jar
When you pull the glove out of the jar, the air pressure is lowered inside the jar. The jar contains the same number of air molecules, but they have more space between them (they are less dense). Molecules collide with each other less frequently and slow down, causing the air temperature to go down.
When you press the rubber glove into the jar, you are increasing the air pressure. The air becomes more dense as the molecules are crowded together. This also causes the air to heat up as molecules collide with each other more often.
Book Bag
As you blow into the plastic bag, more and more air is forced into the same
small space inside the bag. In order for more air to fit into the bag, the air molecules have
to be closer together. There are more of them in a given volume and they hit the inside of
the bag more often, so the pressure of the bag increases. When the amount of air pressure
pushing up on the bag from the inside is as great as the pressure pushing down on the bag
from the atmosphere, plus the weight of the books, the books can be supported by the
compressed air.
Super Straw
When the straw is just sitting in the glass there is nothing separating the air in the atmosphere from the air in the straw. This means that the air in the atmosphere and the air in the straw are pushing down on the water in the glass with the same force. When you suck on the straw it makes the water move up the straw. If you put your thumb over the end it traps the water in the straw, and your thumb separates the water in the straw from the air pressure of the atmosphere. If you pull the straw out of the water and keep your thumb over the end, the water stays in the straw. This is because there is no air pushing the water down from the top of the straw where your thumb is, but the air in the atmosphere is still coming up the open end at the bottom of the straw and pushing up against the water to keep it in the straw. The force from the air in the atmosphere pushing up is stronger than gravity pulling down! If you remove your thumb from the end of the straw the water will flow back out. This is because without your thumb there, the air is pushing with the same force from both ends of the straw. These two pushes cancel each other out so that gravity can pull the water down to the Earth, just as it was trying to do all along!
Sources:
http://www.thehomeschoolmom.com/teacherslounge/articles/air_pressure_experiments.php
http://www.kids-science-experiments.com/paperplunge.html
http://eo.ucar.edu/webweather/cloudact2.html
http://www.deltasee.org/CTC/Activity%2013%20Air%20Pressure%20Power.pdf
http://www.sciencemadesimple.co.uk/page72g.html
More on Air Pressure
http://www.csupomona.edu/~hcmireles/Courses/Sci210/Activities2005/UHa2.htm
