Since I’m only teaching this as a summer class and we only have two classes left, I went ahead and skipped all the lessons on simple machines and went straight to air pressure. I figure most of the kids have been playing with legos and already understand levers, pulleys and I wanted to cover some of the more interesting topics. So today we did a bunch of little demos/labs about air.
I started by asking the kids, ‘What is air?’ and they all has answers, molecules, oxygen, carbon dioxide, nitrogen, etc. So not a new topic for them. I did a slightly different version of the Real Science Odyssey Physics Air Lab #1, instead of using a meter stick , I went ahead and used a balance, like the one pictured. We placed two empty balloons on either side of the balance and saw that they have the same mass. Then one balloon was inflated and the kids were asked if it would weigh more/less or the same as before it was inflated. For the most part they realized we had just ‘stuffed’ the balloon full of molecules so it would probably weigh more, and indeed when placed on the balance its pan was a bit lower than the side with the empty balloon. Its not a huge effect, but its enough to say the inflated balloon has more mass.
We also did Air Lab #3 Hot Air is Cool! I prepared a 2 liter plastic bottle the day before because I wanted to test this lab out. I put a slighly inflated balloon over the lid of the bottle and put electrical tape over the rim to help seal it and keep the balloon from being ripped off. I asked the kids what would happen to the balloon if we put the whole thing in the freezer for a few minutes and they all guessed it would shrink as the air got colder, and the balloon would get bigger if we heated the air. Then we did the experiment. The effect of being in the freezer is pretty dramatic, one balloon even got sucked into the bottle a bit, and they will start to inflate as soon as you bring it out of the freezer. Putting the bottle into a pan full of warm bottle causes the balloon to inflate a bit more, but its not as dramatic as the shrinking. Perhaps I needed hotter water.
We also did Air Lab #4 Taking Hot Air for a Spin, where the kid cut out a spiral printed on card stock to make an ‘air current detector’. We attached a short string to the center of the spiral and when held over a hot surface its spin due to the rising hot air.
Before we moved on to Air Lab #5, I had to do my favorite air pressure demo – crushing soda cans. For this demo, you need empty soda cans (one can will do but trust me you will want to do more than one), we found Hansen soda cans work great, and put just a tablespoon or so of water in the bottom of the can. Place the can on a hot plate on high until you see steam coming out of the top of the can. Then, very quickly take the can with a pair of tongs and flip it upside into a large bowl of ice water. When you do this the hot steam inside the can condenses leaving the inside of the can with very low air pressure and the high air pressure outside the can has no problem crushing the can almost instantaneously. We got some great slow motion videos of this demonstration.
The Mythbusters did a similar experiment with a large oil tanker, but instead of using temperature to create the low pressure, they used a vacuum pump to pump out the air out of the oil tanker. We watched their video in class:
We also watched this video on the physics of air balloons by Its OK to be Smart.
We did one last lab, Air Lab #5 Are You Stronger than a Piece of Paper? I had never seen this demo and was pretty impressed with it. You put a ruler so its hanging half way off a table and tap the end… no surprise the ruler tumbles off the table. The lab asks the students to put the ruler back on the table so half of its hanging out into space but then place a regular piece of paper on top of the ruler, covering the part on the table. Now do you think it will be harder, easier, or the same to knock the ruler off the table? I was amazed at how hard it is to knock the ruler off. Its harder because the paper has a larger surface area and thus a LOT more air pressure pushing down on the paper. One of my complaints with this curriculum is that it talked about the air above our head in miles, and I think its important to get kids thinking in the metric system so I was a bit annoyed by their saying we have roughly 70 miles of air weighing down on us (70 mi is approximately 112km). The other bit they said in the chapter on air was that as the air heats up it has a lower mass and this is just wrong. The density decreases because the air takes up more space as it heats up but the mass of air does not change unless molecules are escaping or being added to your system.