# Physics: waves and sound help!

1. May 25, 2013

### pakman127

1. The problem statement, all variables and given/known data
1) A boy sitting on a dock notices that in 4.0 min, 36 wave crests pass by him. Determine the frequency and period of the water waves.

2) A string of length 2.0m is stretched between two fixed ends. When the string vibrates with a frequency of 60Hz, you notice six nodes (including the ends):

a) Which harmonic is this?

b) Determine the speed of the wave in the string

c) Determine the frequency of the fundamental node in this string

3) Two brains have identical whistles that emit a sound at 220Hz. Train A moves away from Train B at some constant speed. A woman at rest on Train B notes that while Train A is moving, the combination of the two whistles results in a beat frequency of 6.0Hz. The speed of sound in the air is 330m/s.

a) What frequency does the woman hear from the whistle of Train A?

b) What is the speed of train A?

2. Relevant equations

f= 1/T
v=f × λ
fn=nv/2L
λ-n = 2L/n
fbeat = |f1 - f2|
fo = (v/v+-vs)fs

3. The attempt at a solution

For question 6 i couldnt figure it out..kind of confusing. Question 7 i believe i have to use fn = nv/2L to find the speed...still dont know how to do it
question 8a) fbeat = f1 - f2 therefore, fw - FB = FB + 6 = 220 + 6 = 226 Hz ...is that right? :/

2. May 25, 2013

### pakman127

Please show all your work and try add all steps, so i can ask any further questions if i dont understand a step. Please and Thank you! :)

3. May 25, 2013

### barryj

No, No. This forum requires YOU to do the work,. or at least give it a good try. Then you will get help.

4. May 25, 2013

### SteamKing

Staff Emeritus
What are questions 6 and 7? You stop at question 3.