Ch 10.2 Quiz

1.) A car speeds away with 139.6 km/hrfrom a factory which emits a whistle sound at 1,318 Hz. What is the frequency registered by the driver of the car (see sheet 12,use 343 m/s for the speed of sound)? Indicate with a negative (positive) sign whether the frequency registered would be higher (lower) if the car moved toward the station

2.) You approach a wall while emitting sound at 326 Hz. The sound is reflected from the wall back at you and you register a reflected frequency of 551.5 Hz. What is your speed (see sheet 14,15,use 343 m/s for the speed of sound)?

3.) The length of a guitar string is 0.5817 m. What is the maximum wavelength of a standing wave on this string (see sheet 19) ? Indicate with a positive (negative) sign whether this is the only allowed wavelength (there are many other allowed wavelengths which are fractions of the wavelength you calculated).

4.) The traveling wave velocity on a string is 124.48 m/s and the longest wavelength of a standing wave on the string is 1.603 m. What is the frequency (see sheet 19) ? Indicate with a negative (positive) sign whether you calculated the minimum (maximum) frequency of a standing wave on this string.

Question 1:

Answer is NEGATIVE, because the closer you move to the station, the higher the frequency

You use the equation f' = ((v-v_o)/(v)) * f
There is a minus sign because the observer (v_o, the car) is receding and moving

Now with numbers

A car speeds away with 217.7 km/hrfrom a factory which emits a whistle sound at 1,401 Hz. What is the frequency registered by the driver of the car (see sheet 12,use 343 m/s for the speed of sound)?

FIRST, convert the km/hr to m/s by multiplying by 1000 m/km / 3600 s/hr
217.7 x (1000/3600) = 60.4722

f' = 343 - 60.4722 / 343 = .82369 * 1401 = 1153.998

wavelenght/2 = L so rearrange it and you get 0.xxxm*2 (0.xxxx is whatever your meters are)

The answer is NEGATIVE for question 3.

Anyone figure out how to do 2 or 4 yet?
I've been trying to figure out 2 for some time now but can't seem to get it right.

I figured the equation would be something like f' = (v) / (v+vs) * f
Where vs is the source (you) and there is a + because you are approaching

I thought it might be
frequency registered = (343)/(343+vs) * frequency emitted
and then solve for vs but that doesn't seem to be right
Maybe we can all combine our methods on how we thought to do it to figure it out?

Okay so for #2, I'm not really sure why but you do

(frequency registered by you - frequency emitted) / (frequency registered by you + frequency emitted) = x

And then do X * frequency of sounds which is 343 m/s

You approach a wall while emitting sound at 344.3 Hz. The sound is reflected from the wall back at you and you register a reflected frequency of 541.1 Hz. What is your speed (use 343 m/s for speed of sound)?


So for example, it would be 541.1 - 344.2 / 541.1 + 344.2 = .2222 * 343 = 76.2394 m/s

longest wavelength * 2 =L

F= 4*(velocity/(2*L))

Answer is negative

that doesnt work for me for ques 4

yeah i just realized it doesn't work for the maple ta but it gave me the right answer 2 times on the practice quiz

actually it does work i just typed in the number wrong the first time i did it on maple ta so that should work

here is the problem given worked out maybe this will help

1.603*2 = 3.206 =L

v/(2L)

124.48/ (2*3.206)= 19.41

19.41*4=77.65

yes, thank you. see i multiplied the length by 2 for L, but then i didn't multiply it by two again under the division sign. Quick question though, why is it multipled by four though? Like, how did you know where to get that from?

I have no idea I played around with it and I got the right answer when i multiplied it by four. the sheet says something about n being the number of wavelengths so i just started multiplying by each number until i got the answer because i wasn't sure how to find out how many wavelengths.

how about 3
any help?

#3 is simple,
just take the meters which is .5817 times it with 2 and that would be your answer (nagative answer)
.5817*2= (-)1.1634

hope this helps. so sleep tight now:)