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Posts Tagged ‘**significant figures**’

### I’m Done With Sig Figs

Posted August 10, 2012

on: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.

The next step is trying to explain uncertainty and significance of our data. I came up with an activity I think will work:

- 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?

### Advice with Significant Figures

Posted January 18, 2012

on:Teachers – how are you handling significant figures? I’m a bit at odds with my textbook and I’m wondering what the rest of the world is doing. I’ve discussed my issues with our chemistry teacher, he tends to agree with me, but it’s just the two of us. Let me explain.

I teach from Holt Physics. The book treats sig figs mostly okay. When they provide numbers for problems, they are always precise. Usually the numbers are in scientific notation, so you know where you stand with your given information. The book does state that 1500 could be 2 or 3 or 4 significant figures because we don’t know about the two trailing zeroes. I tell the students to err towards caution in those cases and treat that number as though there are 4 sig figs. The book correctly states that the answers are rounded to the least significant number of figures. You all know what I mean.

Here is where we part ways. I teach my students to carry an extra place while doing calculations. For example, if I’m dividing 35 by 62, my working answer is 0.565. If this is my answer to the problem, I would round this to 0.57. If I’m using this number in another calculation, I would use all three digits. The textbook rounds this here along the way even when it is used later. I’ve even seen problems where they have rounded more than once in the same problem. (There is no way I can remember the actual problem right now.) The results are often an error of about 10% difference between my answer key and my calculations.

While we are at this, I have a question I’m stuck on. Suppose you read a meter stick and you get a reading of 8.65 cm. That is three significant figures. Now you move a little ways up the ruler and read 22.40 cm. The accuracy of the ruler hasn’t changed, but I’m now working with 4 significant figure versus 3 before. I wouldn’t round the second number, it is as accurate as the device, but the first number isn’t 8.600. How do you account for this when you are dealing with the significant figures of a problem?

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