1. Mar 8, 2005

### Ba

I just got done taking a test, and one question really was bothering me. My teacher had one problem where there was 20 m/s^2, this is one signifigant digit? Right? Yet last time I gave my answer in one signifigant figure, I got a lecture and told he'ld let me get away with it this time. I ended up not doing it on this test but it really is bothering me. Should I have just done it anyways? I can justify it with the textbook we use and probally somebodies notes of him as well. :grumpy: As it is I circled it, wrote a note, and underlined it darkly several times.

2. Mar 8, 2005

### Chi Meson

A techincality here. Was the quantity written as "20 m/s^2" or "20. m/s^2" (Note the decimal in the second example). The first quantity is ambiguous (could be one or two sigs). The second example has definately two sigs.

If it was the first way, then your teacher is in error for insisting that there are two sigs. He should have either put the decimal ofter the zero, OR put a line over the zero OR written it as 2 x 10^1 m/s^2.

3. Mar 8, 2005

### DaveC426913

Hm. No that's 2 sigdigs.

1 sigdig would be $$2*10^1$$.

It's the left-side zeros that don't count towards sig digs.

4. Mar 8, 2005

### Ba

No, the zeros behind the decimal places are what doesn't count. Take some long time say 500 million years, this is one signifigant figure.

And yes I know it's a technicality, but it really does bother me. There was no decimal point behind the twenty, I checked. The last time when I got the lecture it was 100 kg, once again no decimal point or indication that all figures were signifigant.

5. Mar 8, 2005

### Gza

I find the whole obsession with sig figs in introductory physics classes to be pretty stupid, IMHO.

6. Mar 8, 2005

### franznietzsche

That's not correct. 500.00 is 5 sig figs. Behind the decimal place counts. Leading zeros do not count. 500 is technically one sig fig, because there is no decimal.

And yes, the whole idea is idiocy incarnate.

7. Mar 8, 2005

### mathlete

I couldn't agree more. For about 5 years straight we'd start every year off with something on sig-figs. I'm just glad i'm way past that point now. At least I didn't have any professors who were obsessive with it, because I know a lot of them would mark poitns off if your answers were the wrong number of sig-figs which is pretty stupid imo.

8. Mar 8, 2005

### JasonRox

Tell the prof to go **** himself.

9. Mar 8, 2005

### cepheid

Staff Emeritus

10. Mar 8, 2005

### JasonRox

That's what I would do.

I'm really close to saying it right now. I'm actually really pissed off at the school right now.

11. Mar 9, 2005

### cepheid

Staff Emeritus
That doesn't mean you should go swear at your prof...cool it man.

12. Mar 9, 2005

### BLUE_CHIP

You think thats obsessive,

My applied teacher requires:

Non-recycled paper
Lined paper
Blue margin
Only two holes punched in the margin
No double banking in your answers (i.e. only 1 ' = ' on each line)
Must be in fountain pen with blue ink
No ink eradicator or erasing fluid

If one of the criteria isnt met then you get ZERO

a wee bit obsessive

(P.S. all answers should be given to 3 significant figures too :tongue2:)

13. Mar 9, 2005

### ZapperZ

Staff Emeritus
It actually isn't THAT stupid if you consider that students tend to cite the whole 10 digit (or more) number that they read off their calculators. The whole pedagogical reason for emphasizing significant figures is to make sure the students are aware of the scale of things and how accurate things are. I have seen even college kids still rattling off a 12-digit number they read off their calculators. This shows a complete lack of "perspective" in knowing the accuracy of things and to what extent certain values can be accepted with confidence.

So yes, these things can appear to be very tedious and rudimentary at this level. But the implication of not emphasizing such things can be hysterical especially when it transcends just physics and into real life applications.

Zz.

14. Mar 9, 2005

### Moonbear

Staff Emeritus
:uhh: Please tell me that's a joke Blue_Chip.

Ba, hopefully your note on the test will be sufficient to show you understand the concept of significant figures. As others have pointed out, at best, the way the number was written leaves ambiguity as to the number of significant digits. If your teacher expects you to be careful of significant digits in your answer, then he/she must be careful in using them to write the question. The best way to eliminate ambiguity is to use scientific notation, as Chi Meson pointed out. That way, it is clear whether the number is meant to be 2 x 10^1 or 2.0 x 10^1.

There's no reason to swear at your profs. They will be more inclined to listen to you and be sympathetic if you talk to them calmly and explain your reasoning showing them the same respect you'd like them to show you. If students come to me to question the way an answer was graded, what I'm really determining is whether they honestly do understand the question and answer and can defend their answer, or if they are just quibbling for points any way they can get them.

15. Mar 9, 2005

### cronxeh

Sig figs are important.

20 m/s is 1 sig fig
20. m/s is 2 sig figs
20.0 is 3 sig figs
20.00 is 4 sig figs
and so on

Sig figs become significant when you start doing calculations
((20 + 30.2) / 4.1) + ((4.56 + 2.1)/5.124) = you'd think answer is 13.54 but it isnt

16. Mar 9, 2005

### BLUE_CHIP

Its about as funny as having to use 4 different colours minimum in a diagram (not felt-tip pens) also a wrong answer immediatly voids all method marks....... so by effect one can get the answer right and get the wrong method and get more marks than getting the method right and the answer wrong.........

17. Mar 9, 2005

### Moonbear

Staff Emeritus
I just want to lend my support to this explanation. I'm not going to give an example in physics, because I'd probably bungle it, though those experiments probably work more often close to tolerance limits where accounting for the degree of accuracy in your calculations can be the difference between success and disaster.

Instead, I'm going to give an example of why significant digits would be important in another application, so the students can see that it's not just important in physics.

Let's assume you're working in quality control for a pharmaceutical company. That company manufactures a pill that is supposed to contain 200 mg of a drug. Your job is to collect samples of each batch of pills off the assembly line and test them to make sure everything is working correctly and the pills do indeed contain 200 mg of the drug. (Yes, this is a real job that people do, and critical to complying with regulatory agencies to keep pharmaceuticals safe). What degree of accuracy would be acceptable to you in your measurements? If all of your instruments are only accurate to one significant figure and you determine that your batch of pills contains 200 mg of the drug, how confident are you in that dose of drug?

With accuracy to only one significant figure, you might have anywhere between 150 to 250 mg of drug in that pill. That's quite a dose difference. However, if your instruments are all accurate to 3 significant figures, you will have more confidence in that dose being closer to 200 mg (a range of 199.5 to 200.5 mg).

On the other end, as instruments increase in their degree of precision and accuracy, they get more expensive to buy, because they are harder to make. So, if a range of 199.5 to 200.5 mg is sufficiently within safety margins for the drug and you don't need to be accurate to 5 or 10 significant digits, knowing this allows you to purchase equipment that meets your needs without spending an excessive amount on equipment that far exceeds what you need in terms of accuracy.

18. Mar 9, 2005

### Norman

I know this is pedantic, but couldn't 20 be considered exact since there is no decimal place?

19. Mar 9, 2005

### JasonRox

My anger is not related to marks on a test. It is because I got dropped from a class (computer program) and might not even get back in because of some damn computer. They also told me that this happens every year, but yet they do **** about it.

Also, I took Calculus II, and so far it feels like Physics. If I wanted Phyiscs, I would have taken Phyiscs. Teach me math dammit.

I'm really angry at the school. Even more angry because they have been dumbing down the courses so bad it's not even funny. They used to teah Spivak's Calculus to first year math majors, but dropped that a few years ago. They used to have Calculus based Introductory Physics, but dropped it 2 years ago. They are currently make changes to the courses to this date, and I wouldn't be surprised if they dumbed it down even more.

Note: The chair knows I have thought about changing school's, and tried to talk to me out of it, but another prof offered a letter of recommendation to avoid doing classes over again (transfer credits).

Note: I have decided to stay for monetary reasons, but I wouldn't be surprised if I stirred up **** in a year or two. They need to clean up their act if they want to get this school somewhere academically.

20. Mar 9, 2005

### Moonbear

Staff Emeritus