# Wave Question

#### timtng

A sound wave of velocity 600 m/s has a wavelength of 3 m. At a certain instant one of the molecules of the medium is at its normal position. How long will it be before this same molecule is again at its normal position?

v=f&lambda;, f=v/&lambda; =(600m/s)/3m = 200Hz
T=1/f=1/200=.005s

t=T/2 = .0025s Is this answer correct?

Thanks

Related Introductory Physics Homework Help News on Phys.org

Yes.

#### timtng

Another problem of the same sort.

Standing waves are produced on a string for which the velocity of transverse waves is 200 m/s. The frequency of vibration is 500 Hz. How far apart are the nodes?

I can only find &lambda;, &lambda;=v/f=200/500=.4m

I don't know how to finish the rest of the problem

#### Ambitwistor

That's like the previous problem. The nodes are the places where the transverse displacement of the string is zero, so they occur every half-wavelength.

#### timtng

So the answer should be &lambda;/2?

#### Ambitwistor

Originally posted by timtng
So the answer should be &lambda;/2?
Yes, that's what "half-wavelength" means.

#### Ambitwistor

Originally posted by timtng
No, why?

#### timtng

I figured since they ask how far apart are the nodes, L has to be a contributing factor.

#### Ambitwistor

Originally posted by timtng
I figured since they ask how far apart are the nodes, L has to be a contributing factor.
L will be some integer multiple of &lambda;/2, but if you already know &lambda;, it doesn't matter what L is. L just dictates how many nodes there are.

#### HallsofIvy

Homework Helper
I figured since they ask how far apart are the nodes, L has to be a contributing factor.
Did you notice that there was no "L" given in the problem?

We were given, instead, the frequency.

If you were given the wavespeed (determined by the mass and tension of the string) and length, then you could find the frequency. Here, that's not necessary.

### Physics Forums Values

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving