Students were asked to watch the following videos and/or read 5.1 Balloons in How Things Work: The Physics of Everyday Life.
And lastly Smarter Every Day, does a cool experiment with a balloon in his car.
While we waited for everyone to arrive for class I gave students large syringes with mini marshmallows inside. By plugging the syringe they then had a contained volume of gas. By compressing the plunger and decreasing the volume in the syringe, they increased the pressure and the marshmallow gets smaller (kind of looks like a raisen). As they pull out the plunger, increasing the volume of the gas in the syringe, decreases the pressure on the marshmallow and it expands. Make sure students keep a finger over the plug to keep it from shooting across the room.
I also crushed a few soda cans with air pressure – you can see a description and video in my previous post on buoyancy and pressure. For the main activity we tried to measure the buoyant force on a floating object. I had two large plastic containers with overflow spouts in them, and we filled them up until water started pouring out the spouts. Once the water stopped coming out, students placed their objects in the water, which caused the water level to rise and overflow. The overflow was captured in a container and students either measured the mass of the water that was displaced or they measured the volume and calculated the mass from the density. They then compared the weight of the object floating in the bath to the weight of the water displaced. They should have been the same and were in a few cases but many were off by 30-50 grams. It was pretty evident that surface tension was a problem in our overflow spout, so cutting down the pipe we were using, or putting a little bit of dish soap in the bath would help with that. having a smaller bath – smaller area on top might have made it more accurate as well. One group tried placing a 45 gram boat in the water and no overflow occurred, so we definitely had a problem with the set up. I may have to try again with just a regular bowl that I can fill all the way up to the top and collect the overflow in a basin and see if I get better results.