Archive for the ‘Demonstrations’ Category
My father just sent me this TED Talk. He doesn’t read my blog and didn’t know about the other TED Talks I posted. This one is a little different, Ramesh Raskar from MIT has developed a camera that can slow motion down to the point of being able to see a pulse of light travel. You just have to see it to believe it.
And in case you aren’t seeing the embedded video:
This is not a new topic for me, it’s been a burr in my saddle for some time now. All of the introductory physics textbooks address significant figures in much the same way. The problem is – nobody in the “real world” uses sig figs. At the same time, introductory physics isn’t the time to introduce complex error analysis models.
I’m having this discussion with Andy Rundquist of Hamline University. I asked Andy how they handled this at the college level. He told me they don’t teach significant figures and pointed me to a very lengthy article discussing why significant figures are all wrong. The article suggests the use of Monte Carlo analysis its place. That may make sense on a lab, but not on classwork and homework problems. The uncertainty article did have a suggestion; use six significant figures for calculations and round the final answer to three sig figs. The article does a good job explaining the reasoning, and I’m fine with it. The three extra “guard digits” preserve the accuracy, and the rounding makes the answer more reasonable.
- I will project an archery target on the board.
- Students will move back about 20 feet and shoot a round of Nerf darts at the target. They will be far enough back that most of them will shoot a 6, or 7 and not a 9 or 10, at least at first. Each student will take a turn.
- We will plot the overall results. We should get something resembling a normal distribution curve, but I won’t tell them that.
- I will ask the kids to average the data and come up with a value of x.x +/- y.y and start a discussion on whether or not that represents the data.
- We will then put a ring or other object on an electronic scale and write the mass with the error in the same way.
- After some discussion, I will bring up slides of normal, rectangular, triangular, and maybe exponential distribution curves. I want them to discuss the fit of the models to the data.
- My goal is that they understand that error is probability.
- About a week later we will drop rulers and calculate individual reaction times. This would be a good time to bring back the distribution graphs and perhaps even input our data into a statistical analysis program to find the best fit.
I think this will work and go over well. I’d love some feedback. It’s a first pass, what did I miss?
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.
I was discussing Newton’s Laws and trying to explain how the tension increases in an elevator cable when it starts moving up. The kids get it that the tension when it’s not moving is equal to the weight of the load, but once it starts moving, they get wacky. Some seem to think the only weight at that point is the force from F=ma and the elevator is now weightless.
I put 1 kg mass on the large spring scale and showed how it pins the reading if I pull up. They saw it, but it didn’t click.
On a whim a took I put the 1 kg mass and lifted it with some string. My standard classroom string is macrame string from A. C. Moore. It costs about $3 for 1000 ft ball of string. I think I go through a ball of it every year. Anyway, the 10 N weight is nearing the limit of what the string can hold. I accelerated the string upward just like I did with the scale and the string snapped. Watch your toes, it fell to the floor and cracked a tile… oops.
I came up with this in my early years of teaching (2002) and I forgot about it until tonight. At the time, I was borrowing a friend’s physics classroom to do a graduate assignment on reading. I prepared an article on how fireworks produce light and color. The concept I was trying to get across was how electrons in the shell jump to a higher energy level, then give off light as they drop down to a ground state.
I chose as a prop a Zippo lighter without fuel, but in the little basket on top I placed some magician’s flash cotton. This stuff is great, you can purchase it at a local magic shop. It is stored wet so that it doesn’t ignite, you take out only as much as you think you will need ahead of time and let it air dry.
I used one of the students in the class, he was the electron. The idea was that as he got heated up by the thermal energy, he got elevated into an excited state. He then had to stand on one of the desk chairs to show the elevated state. Now the problem was that he needed to get down, but he couldn’t without releasing some energy. To do that, he had to flick the Zippo and step down. As he flicked the Zippo, there was an impressive flash of light – the photon being release from the electron in a high state.
I know this worked, I heard a bunch of involuntary utterances of “I get it” and “ooh.” I think that was the day I knew I belonged in the classroom.
Fire – good.
I admit I’ve been holding out on you. Let’s just say I thought this one was a bit too low level. I beefed it up and it’s perfect for my Conceptual Physics classes now. I just did it this week and I like the results.
See that rocket shaped balloon on the right… good luck finding them. I seem to only be able to find the regular party balloons. Okay, I didn’t look that hard, but if they were at the local dollar store, I would definitely buy them.
So what’s the big deal about this lab, it seems almost grade school level? First off, again, almost none of my students have ever built a balloon rocket. This never ceases to amaze me. Second, it’s deceptively challenging. Lastly, the kids actually get the principles because of this activity.
To make this work in the classroom, I use a ringstand on one end with several book on the base for weight. I tie a string and put it across the room, leaving the other end open so they can put straws on the string. Each team gets their own ringstand/string setup.
The lab is broken into two missions. The first mission is easy; first, make it go across the room. Then put the balloon at 45 degrees to the string and see what happens. Then repeat for 90 degrees. They sometimes guess that the balloon will spiral, most are surprised but figure out why it spirals.
Mission two is really difficult, I tell them they are being challenged, but they are not graded on success, they are graded on effort and documentation. The mission is to make the balloon go down and automatically come back. They are told they can use two balloons. Once they get into this mission, the kids tend to put two balloons facing opposite each other and let go at the same time. They seem to think that the balloons will know to take turns. They learn first hand that the opposing forces cancel each other out. That’s really the gem in this lesson. I don’t care about them making it come back.
What most do next is blow up one balloon bigger than the other thinking it will move the way they want and then come back because it lasts longer. Again, they usually figure out that the opposing forces cancel. They next try to delay the release of one of the balloons. I get a few creative ideas here using bent straws and twisted balloons, but so far no amazing designs.
I like to sit back and watch this one. You can see the lights go on when they figure out the opposing forces cancel. Below is the lab handout. If you get some good solutions to the return mission, I’d love for you to post the solution here.
I kind of made this one up, kind of adapted it from the electronic timer manual. The idea is that we use a ramp to accelerate a steel marble, have it pass through timer gates, measure the distance between the gates and calculate the velocity. Do that a couple of times for accuracy.
Now measure the height of the table using a meter stick. Use a fishing weight on a string to find the point directly under the edge of the table. We now have the horizontal velocity, the height of the table, we can calculate how long it will take to fall. Next we do the math and place a penny where the steel marble should land.
In the beginning, it would works sometimes, but not always. I determined that our heavy epoxy table tops caused the steel ball to bounce, losing some of its horizontal velocity. The bounce was easily dampened by placing two or three sheets of paper under the end of the ramp. We also had some issues with hitting the side of the photogate. Lining up the gates with the ramp was a minor issue, but an important one. After the speed was determine, we moved the photogate away used the already calculated speed.
Here is the Lab as I wrote it up, it needs some updates like the paper under the ramp. I realized later that I should have had the kids measure the starting height of the ramp. Then we could go back to it later when we do energy and analyze the results.
Next year I think I will have them hit the penny first time, then a dime, then a small washer, so each time it must be more accurate.
As always, comments and ideas are welcome.