This was the last class for this year so we headed to the park with the All About Air Classroom Kit from Steve Spangler Science. I had used the windbags in the high school class to demonstrate Bernouille’s principle – you can inflate a 2 meter long bag with one breath by taking advantage of this principle. We started out with the windbags and I asked the students to put three big breaths in the windbags and then estimate how many breaths it would take to fill them. Then I demonstrated how to do it with just one breath, hold the opening of the bag wide open about 10 inches from your mouth and blow a steady stream of air into the bag. Increasing the speed of the air around the mouth of the bag lowers the pressure causing the air around the mouth of the bag to rush into the lower pressure area filling the bag!
The solar bag is basically a black garbage bag that is 15 meters (50 ft) long! We took it to the middle of the park and filled it with air, using the wind instead of blowing and then tied off both ends. As the bag is warmed by the sun, the air inside will move at faster and faster speeds causing the bag to expand, giving the air a greater volume and therefore a smaller density than the surrounding air and the bag will float!
This kit worked pretty well. My only complaint is that the string provided in the kit to tether the solar bag (required!) is really cheap thin cotton string and it snapped on us a couple of times. Luckily, we tethered both ends so we did not lose the bag. If I had noticed the quality of the string before arriving at the park I would have used my own. We did end up with a small hole in the solar bag at one point, but I always have duct tape in my backpack and quickly patched up the hole. You have to do this in the morning when its still cool and it has to be a sunny day of course. We had trouble getting the bag to go very high by 11 am because it was already getting too warm outside. I also don’t think we put quite enough air inside the bag before closing it, but that kind of kept it more manageable.