# Help with simple unit problem

1. Oct 19, 2006

### Dazed

I did this question, and apparently my teacher has terrible english and I have no idea what she means by this response. Does anybody understand what she's talking about and possibly tell me how to change my answer so that it conforms to whatever "1 s digits rule" she's talking about? Thanks for any help in advance.

v = fλ
f = v / λ
f = 300 x 10^6 m / 0.08m
f = 375 x 10^7 hz(Hz)

She marked this wrong and gave me this comment:

"( here is where the rules for number of s digits are used, wavelenghtj has 1 s digits, therefore the answer for the question should have 1 s digit.)"

2. Oct 19, 2006

### alfredbester

she means significant figures I guess so your answer should be
400 x 10^7 Hz.

3. Oct 19, 2006

### Hootenanny

Staff Emeritus
I think your teacher may be talking in terms of significant figures. i.e. the wavelength is quoted to 1sf and therefore the highest degree of accuracy you should quote is one significant figure. That's my take on it.

Edit: Guess I should learn to type faster.

4. Oct 19, 2006

### physics girl phd

The "s digits" likely means "significant digits", or "significant figures." The frequency answer should have the same number of significant digits as the number that your initial given wavelength had. This is common material in an introductory chem or physics class (I taught it in high school chem), so you should be able to look up this topic and figure out what is going wrong. Some people are sticklers for significant digits, some aren't (as long as you don't truncate the answer or extend it to extremes)... apparently your teacher is a stickler for these. It's good thing to know, as it eventually relates a bit to knowing your error in calculated quantities.

We all need to type faster! (say less)

Last edited: Oct 19, 2006
5. Oct 19, 2006

### Dazed

Okay, that makes sense if by "s digit" she means significant digit, so thats fine. Alfred Bester posted this answer "400 x 10^7 Hz" could you please tell me what exactly you did to arrive at that answer?

Im very new to physics and I find some of these things confusing. I'm guessing you just rounded 375 to 400 so it would have 1 significant digit, (the 4)?

6. Oct 19, 2006

### Hootenanny

Staff Emeritus
Correct, significant digits are the first non-zero digits in a number. For example; 0.08 has 1sf; 0.008001 has four significant digits.

7. Oct 19, 2006

### Dazed

Thanks a ton guys. Apparently I'm not good at retaining these key little details.

8. Oct 19, 2006

### leright

Usually my profs don't care much about significant figures, unless it is a lab report (many chem profs are picky with sig figs too). On tests and hw, 3 sig figs for all answers usually works.

But, in the professional world, significant figures and propagation uncertainty is very very important, which is why it is emphasized in labs. The precision to which you are able to make measurements is just as important as the number itself. There is a big difference between 0.50 m and 0.5000 m!