Archive for January 18th, 2012
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?