# 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

Ambitwistor
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.

timtng

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.