Archive for February 2011
This is year 5 teaching physics. The first year I completely ignored significant figures (aka sig-figs), there was too much other stuff to do. I think that was the right call back then. Now, as the course becomes more defined and I focus on what the students really need to learn and understand, I’m becoming less tolerant of sig-fig abuses.
In our recent lab, the kids had to measure the height of the toy popper. The poppers I have are 3/4″ and 1.5″ in diameter. When students measure these using a mm scale, I routinely get heights of 0.01 meters and 0.02 meters. The next step they calculate the acceleration and give me results to four significant figures. I need to get a few stamps made up for grading. One will say SIG FIGs, another will say SHOW UR WORK, and a third will say FORMULAS? since the kids seem to think I know what formula they are planning on misusing and abusing. Maybe I can boil all of my grading down to a series of stamps.
The problem here is that I tell them repeatedly to measure to at least two sig-figs. You can comfortably measure the popper to 10.0 +/- .5 mm on the scale. I have to take most of the blame here, since I don’t do enough in the beginning of the year teaching them to measure. I need to put together a lab that teaches them how to measure, weigh, and calculate. Then I need to hold them to those standards for the entire year. I don’t understand how students get to the 11th and 12th grade and can’t successfully use a ruler.
I often wish I could go back in time and see if I was as oblivious in 12th grade. Was I this lost? Every time we use the meter stick I instruct them that we only use the metric system in our class, and still a few will measure in inches. (I’ve been saying for 5 years that I will spray paint over the inch scale soon.) We did the Egg Crash Lab yesterday (big success), and more than a few of them couldn’t find “half a meter” on the meter stick. If this was the first time they’ve used a meter stick, I would understand, but we use them pretty much every other week and we are six months into the course. Maybe I’m just expecting too much.
What is interesting is as I’m becoming more aware of the problems with sig-fig abuses, I’m finding errors in other places. The software that maintains our student records keeps the grades to 2 decimal places. (This is essentially 4 sig-figs, although you can technically have a 100.00 grade.) The system rounds the grade to a whole number for the report card, but it also rounds them before calculating a semester and final grade. It should be averaging the entire grade, then rounding. In addition, the system stores the grade point average to 4 decimal places which is essentially 6 sig-figs. I noticed the problem when I recently calculated semester grades and many were off by a point from the system calculated grades. To the student that is struggling to learn physics, that one point can mean a B instead of an A, or an F instead of a D.
To quote Zack from The Big Bang Theory, “that’s the beauty of science, there’s no one right answer.” Bazinga!
First, let me be completely up front. I borrowed this activity from my pal Deborah Carder. You can find her link in my blogroll, she does great hands-on activities and labs. I met her at NSTA Philly last year, she is the Energizer Bunny of science teachers, I don’t know how she does it.
Anyway, as I had mentioned in earlier posts on momentum, I wanted to do the egg drop competition, but I’m in a one-story building. This year we are doing the “Egg Crash” competition. The basic concept is that teams get 10 sheets of computer paper, 1 meter of masking tape, a pair of scissors, and 20 minutes to construct a free-standing object to safely catch the egg. They drop their own egg from a height starting at 0.5 meters above the top of their structure. The egg is inspected before and after each drop, the higher they go, the more points they win. The surviving eggs are dropped from 0.5 meters higher each round until they all finally break.
I usually allot 25 points for a lab, I will probably go 50 for this one. Deb said she does 100 points, but that’s a test grade and I don’t think a one-day lab should be worth that much. I’m still working out the scoring, but I think I will assign a grade to each height. If they fail at 0.5 m above their structure, they get 30/50, but they also get a single start-over with a new egg. Each 0.5 m interval earns 5 more points. That means surviving 2.0 m earns 50/50. I’m willing to give 5 or 10 extra credit points if they can survive a drop from 2.5 or 3.o meters over the top. I was going to do direct competition for the points, but what if everyone fails at 1.0 meters? With my grading system, they all get 70% since nobody really earned the A or B.
I handed out the Egg Crash Description and Rules on the day we started learning about Momentum. I told them it will be about a week before we do the competition; I wanted them thinking about the problem and their design as we learn about momentum and impulse. This week I will show a great video called “Car Crash Tech.” The video discusses the state of the art auto safety systems and the effects of air bags and other innovations.
I’m hoping for some creative solutions from the kids. Maybe I’ll have a picture or two to post here in a couple of weeks.
- Many kids don’t do their homework
- Many kids copy their homework from the kid that does his/her homework
- The kid that does his/her homework tends to do well in the class
- The kid that copies his/her homework tends to not do well in class
WebAssign allows me to give them an assignment they can’t copy since each student gets a different set of numbers in the problem. Oh sure, some still don’t do their homework, but the zero in the grade book is indisputable when we can pull up the WebAssign grade for Mom and Dad. The kids have a week to do an online assignment. No, the dog did not eat your computer. No, you had a week to do it. No, I don’t take late work.
My homework template was set up to allow up to five entries on an assignment. At first, there was no penalty for getting it wrong. I did an experiment in my calculus class and penalized them 10% for each incorrect attempt after the first two attempts. The kids really resented that. I think they felt like they were willing to keep trying, they didn’t want to be penalized for not quitting. Lesson learned, next experiment.
Recently I tried a new approach (ah, the scientific method at work). I started giving 10%-20% extra credit for completing the assignment 48 hours before it is due. With the assignment due Mondays at 8am (the start of school), I was getting a bunch of “help me” emails on Sunday evening. The extra credit was an attempt to reduce these emails. What I found is that more of the students were completing the work and doing it ahead of time. Yahoo! They will do their work if they feel they can boost their grade with extra credit. The reality is most of them don’t earn the extra credit, but they get the points for doing their work, and these online assignments are equivalent to a quiz grade every week.
But wait, this gets even better (I’m smiling as I write this). The kids think they outsmarted me. Ready for this … they are getting together to work on the problems. Oh, I don’t know, call it … a study group. AAAHHH!!