Apparent wavelength in Doppler's effect

[Krushnaraj Pandya]

Homework Statement

a source of sound of frequency f emits sound with speed v, is at rest. If an observer is moving at towards source with speed u, then wavelength of sound waves observed by the observer will be?

Homework Equations

f' = f(u+v)/u...(i) f' is apparent frequency, f is the real frequency
fλ=v...(ii) for any wave frequency into wavelength is speed.

The Attempt at a Solution

I tried to get an answer manipulating the above two equations but the answer says its simply v/f without any change whatsoever- This doesn't seem correct though. I'd appreciate some help.

 

[gneill]

Can you show one of your attempts to manipulate the equations? Explain in advance what the variables represent.

 

[Krushnaraj Pandya]

Can you show one of your attempts to manipulate the equations? Explain in advance what the variables represent.

I edited it to show what the symbols represent. There isn't any point trying to manipulate these equations further though since the answer already says its wrong to have u in the answer

 

[ehild]

Homework Statement

a source of sound of frequency f emits sound with speed v is at rest. If an observer is moving at towards source with speed v, then wavelength of sound waves observed by the observer will be?

Homework Equations

f' = f(u+v)/u...(i)
fλ=v...(ii)

The Attempt at a Solution

I tried to get an answer manipulating the above two equations but the answer says its simply v/f without any change whatsoever- This doesn't seem correct though. I'd appreciate some help.

Imagine a train with carriers, each of length L. You stand on the platform and see the carriers passing by. You see more carriers passing in the same time if you walk in opposite direction as the train travels, but does the length of the carriers change? According to Relativity Theory, it does, but this effect is significant only at speeds comparable to the speed of light.

 

[Krushnaraj Pandya]

Imagine a train with carriers, each of length L. You stand on the platform and see the carriers passing by. You see more carriers passing in the same time if you walk in opposite direction as the train travels, but does the length of the carriers change? According to Relativity Theory, it does, but this effect is significant only at speeds comparable to the speed of light.

Wonderful example! Its crystal clear now, Thank you very much :D

 

[Krushnaraj Pandya]

Why would you have u in the answer? You need to plug in your given information. Both speeds are "v". Wait! You've edited the original problem so that now the sound is no longer propagating at the same speed as the observer. It a different problem now.

You should re-edit your post to restore the original problem statement.

Yeah, that's my mistake, Its 2 am here and I chose symbols arbitrarily while I was half asleep. Then while editing it to write the meaning to the symbols I noticed my blunder and corrected it. Really sorry about that, I really appreciate your help though :D

 

[gneill]

No problem. Get some sleep!

 

[Krushnaraj Pandya]

No problem. Get some sleep!

Can't afford any, Jee's in 20 days. Although I have the benefit of living in India, and thus knowing how to meditate deeply- so I'm getting by with 5 hours of sleep or so.