Archive for January 2011
Okay, I’ve been a bad boy. It was more than a year ago I said I would update the post with the instructions. You guys didn’t call me on it, so I forgot. The original post is here along with a pdf of the plans I had printed for myself:
The original plans make a balloon that is 60 inches high. That is perfect for the classroom, it takes three pieces of tissue paper per panel and you need six panels. You will find I went big on this one and made a 90 inch version for myself. It is rather unwieldy and difficult to handle, but bigger balloon means bigger lift. At this point, I can put one of these 60″ balloon together in about an hour, where it takes the kids at least 2 hours.
One of the challenges we ran into was finding the aluminum wire we used to keep the mouth open. You want this for two important reasons; first, to add ballast; but more importantly, without it, the kids can’t catch the balloons on their heads. I found the wire at Dick Blick, an art supply store. They list it under sculpture wire, we use the 14 gauge wire. It costs $16 for a 350 foot spool. Each balloon uses about 3 feet. That’s a lot of hot air balloons.
When it is too windy outside, we tend to launch these in the gym. I have two heat sources. The indoor one is a heat gun I purchased from Harbor Freight on sale for $10. It works great, I make sure I handle it so nobody gets burned. The outside source is a plumbers propane torch. Nobody handles that but me, I don’t need to write up any (more) accident reports. If you are using the torch, make sure the kids keep the balloon opened up, if you aren’t careful you can catch the balloon on fire. Oops, only did that once, very cool though. If you do that, light it from the bottom, it goes up into the air and disintegrates … poof. Bad science teacher.
Some day I may make one of those cool chimney launchers, but then I’ll have to store it in my room. It’s already looking like Sanford and Sons in my classroom, I don’t know how much more junk I can store.
I don’t remember seeing these before. I just got an email that the link to the Java Applet for acceleration had failed. It didn’t take me long to find the gentleman’s page and I realized that he has an extensive list of these physics applets. Here is the main page in English:
There are some really useful tools here for demonstrating mechanics to your students. Best of all, he has a download button, you can just put them all on your computer and not worry about his link changing. I haven’t been able to make the downloaded applets work, if someone can post some help, it would be appreciated. I tried just clicking on them, they open up Firefox, but they all seem to fail.
If you have access to computers in your classroom, you can design lab experiments where the students input conditions into these applets and read the results from the screen. Not as much fun as hands-on, but a lot better than lecture. If you do create an applet lab, please forward it to me, I’d love to attach it to this post.
We finished the lab today. I gave the kids two days to do it. Most of them figured out the initial velocity by the end of the first day. The start of the second day, I put two hints on the board. For question 2, I put up t=d/Vagv. For question 3, I told them they needed to calculate the acceleration of the popper.
I decided to be only somewhat helpful. At the start of day 2, I told them the initial velocity should be in the range of 5 m/s. I told them I would not answer questions about their numbers if the formulas were not there and units were not shown. I generally only told them they were either on the right track or wrong, nothing more. Most of them had a tough time making the leap to the distance in part 2 was how far the inverted popper moved from rest to the calculated initial velocity. Once they got that, they were well on their way to solving the problem.
I did an interesting experiment while they worked. I set up a LabQuest to sample at 1 ms intervals. I build a tiny tray from cardboard and string and attached it to the force sensor. I set the meter to trigger at a force greater than 2.5 N, zeroed the sensor, and let it rip. It showed a nice impulse function that took 23 ms and a peak force of close to 7 N.
I could use some help with my interpretation of the graph. I believe the integral of the Force v. Time curve gives me the impulse (the LabQuest gave me a value of 47 N*ms). If I divide that value by the mass of the popper (9.1 g), I get a delta v of 5.16 m/s. This is in agreement with the numbers the kids got in the experiment.
Now if I divide the delta v by the time, I should have the acceleration. The LabQuest samples every millisecond and there are 23 points, so I think the time is either 22 ms or 23 ms. The acceleration works out to be 235 m/s^2. Doing this, I only get a force of 2.1 N, but the graph shows close to 7 N. The students calculated forces in the 6-7 N range. I think the discrepancy has to do with using the integral (which should be more accurate) and getting a peak force compared to an average force. Can someone either confirm this or correct it for me please?