Did my tutor give an incorect equation?

  • Thread starter Salerk
  • Start date
  • #1
Salerk
9
0

Homework Statement



An FM radio station generates radio waves that have a frequency of 95.5 MHz. The frequency of the waves from a competing station have a frequency of 102.7 MHz. What is the difference in wavelength between the waves emitted from the two stations?

f1= 95.5
f2=102.7

Homework Equations



From what I know

Speed = Frequency x wavelength (v=fλ)

I need to find out wavelength 2 - wavelength 1. This i understand no problem thing that's got me is the equation the tutor gave to work out the equation is...

λ2-λ1=v/f2 - v/f2 <-- Why would I be subtracting the same from the same the answer would be = to zero, would the equation not be...

λ2-λ1=v/f2 - v/f1

I love my physics classes but sadly our tutor 80% of the time runs out of time before he finishes the class but still tests us on the things he missed out (this being one of them) So, I am a little stuck from this point on since I am not sure what the speed would be, since we have not been told what it is nor how to find it out.

I tryed to look up the speed online and what I found (hopfully corect) is 343 m/s,


The Attempt at a Solution



My attempt.

λ1-λ2=v/f1 - v/f2

343/95.5 = 3.59(2dp) - 343/102.7 = 3.34 (2dp)

= 3.59-3.34

=0.25m

Now, I am not sure if this is corect, since its actually a multi choice on the answer and the nearest answer is actually 0.22m (interactive test so told me that the right answer when expermented)

my main problem is where did I go wrong to get 0.25 where he got 0.22.

Anyone able to give me some help/advice on this?

thanks

Ritchie.

p.s. really sorry on spelling, can't attach my disleaxer form on here (smiles)
 

Answers and Replies

  • #2
Dick
Science Advisor
Homework Helper
26,263
621
You put in the speed of sound for the waves and dropped the M in MHz. Radio waves travel at the speed of light. Your tutors equation certainly does have a typo. Guess he has disleaxer...
 
  • #3
Panda
194
0
1. Your take on the equation is correct.

2. Your Velocity is wrong. 343m/s looks like the spped of sound but radio waves travel at the speed of light 'C', find a value for 'C'.

3. Looking at your calculations you have applied the frequency incorrectly, this may have been because you were struggling with the wrong velocity and trying to get it to fit. What does the M represent in MHz?

p.s. Dyslexia didn't stop me once I left school, modern "speil chockers" are a god send.
 
  • #4
neutrino
2,091
2
f2 in both terms must've been a typo. It is v/f1 - v/f2, if you want the difference to be positive. Also, you're dealing with electromagnetic waves here, not sound. These are radio waves, which have the same speed as light.
 
  • #5
antonantal
243
20
λ2-λ1=v/f2 - v/f2 <-- Why would I be subtracting the same from the same the answer would be = to zero, would the equation not be...

λ2-λ1=v/f2 - v/f1

You're right.

I tryed to look up the speed online and what I found (hopfully corect) is 343 m/s

The radio waves, like any other electromagnetic waves travel at the speed of light.
The speed of light in the air is approximately the speed of light in the vacuum, which is 299,792,458 m/s

EDIT: crossposted with neutrino, Panda and Dick
 
Last edited:
  • #6
Salerk
9
0
My brain must be taking a small vacation on this one.

ok, the speed of light I knew, just did not realize they used same speed, should have most dence of me lol.

Anyways I've tryed to put the numbers in and its spiting out the wrong answer to what I am looking for, i account this to the M i droped as was mentioned, M = mega MHz megahertz (right?)

and that's em ?x10^6 I beleave. but I am not sure how to take this and add it into the equation to help if I am even right on this.

as for spell check, love it also, just the old computer I am useing at the moment does not have one working at the moment.


EDIT:

ok think i got it figured.

I see now where i was forgeting the M and have that figured and if I am right, and according to my calc I am only off by 0.01 and that I put down to useing 2.9x10^8 not 3.0x10^8.

Calculation:

2.9x10^8 / 95.5x10^6(to turn into that wonderfull M)
=3.0366

2.9x10^8 / 102.7x10^6
=2.8238

3.0366-2.8238

= 0.21m

Thanks for all the help!
 
Last edited:
  • #7
Panda
194
0
Speed of light 299,792,458 approximates closer to 3.0x10^8 than 2.9x10^8. If you take out the rounding error you should get 0.22m
 

Suggested for: Did my tutor give an incorect equation?

  • Last Post
Replies
5
Views
279
Replies
25
Views
357
Replies
25
Views
103
Replies
4
Views
405
Replies
3
Views
325
  • Last Post
Replies
4
Views
395
Replies
1
Views
195
Replies
7
Views
333
Top