Doppler's Effect PS Section on GS 6

[Sailor Senshi Dermystify]

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A sound wave emanating from a stationary observer bounces off an object approaching with velocity v. The original wave, relative to the reflected wave, has:

a higher wavelength but a lower velocity.
a lower wavelength and the same velocity.
a higher wavelength and the same velocity.
a lower wavelength but a higher velocity.

I kind of understand the concept of Doppler Effect and I use kind of very lightly. In the TBR Physics book they explained it but it wasn't in depth. I also watched Chad's videos on this topic but idk.

This is how I looked at this problem but I don't know if my reasoning is right, so this is why I posted this question.

The observer is stationary, so it doesn't have any velocity. The source is approaching the observer with some type of velocity, so we can also think of the equation v=wavelength x frequency. Because the frequency will be higher for the source it's wavelength will be shorter. For the observer, it's wavelength will be longer with a lower frequency. This is my reasoning but as far as using the equation is where I get lost.

Thanks again!

 

[BB8730]

Your logic looks good except for the fact that the source and the observer are the same thing. Here's how I would approach the problem.

First of all, sound waves in still air always have the same speed, so the first and last answers are eliminated. Next, when sound waves bounce off an object that is moving towards the observer, the frequency increases. It's the same rationale as if it were the sound source moving towards the observer. Increased frequency = decreased wavelength. Since the reflected wave has a shorter wavelength, the original wave will have a longer wavelength. That makes the 3rd answer choice the best.

You could use an equation, but I would stress the importance of being able to think a problem like this through without having to.