# Nextstep diagnostic #13 C/P

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#### akimhaneul

##### Full Member
2+ Year Member
I understand why the correct answer is the correct one. But I just have this random question...Why is it that increasing speed of the sound always changes frequency, not wavelength?

v=wavelength times frequency so why always only frequency?

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#### laczlacylaci

##### Full Member
2+ Year Member
Bump. Since the equation lambda=C/V...

#### theonlytycrane

##### Full Member
7+ Year Member
The two big rules for waves are: 1. same medium ~ same velocity and 2. different medium ~ same frequency.

So if the medium is constant, velocity will stay constant. This means a change to wavelength will change frequency to keep v the same.

If we change mediums, frequency stays the same. So changes to velocity changes wavelength.

Bump. Since the equation lambda=C/V...

The c / v relationship is for index of refraction. I think you might mean n = c / v.

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#### akimhaneul

##### Full Member
2+ Year Member
The two big rules for waves are: 1. same medium ~ same velocity and 2. different medium ~ same frequency.

So if the medium is constant, velocity will stay constant. This means a change to wavelength will change frequency to keep v the same.

If we change mediums, frequency stays the same. So changes to velocity changes wavelength.

The c / v relationship is for index of refraction. I think you might mean n = c / v.

But isn't medium changing from air to helium in the problem? Why does the pitch change?

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#### laczlacylaci

##### Full Member
2+ Year Member
But isn't medium changing from air to helium in the problem? Why does the pitch change?

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No, because as the person is talking, the medium is always air. You can think of the medium as the solvent, and He as the solute.

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#### theonlytycrane

##### Full Member
7+ Year Member
TBH, immediately after reading the q-stem I identify this as a POE type question. What I mean is that the q-stem is vague as to what's actually going on, and it's a scenario that we can't study for. The only important thing to take from the question stem is that we pick an answer where frequency increases since higher pitch ~ higher frequency.

For what it's worth, the medium might be changing slightly to helium, but the person who hears the sound won't be in a helium filled room so the medium will still be normal air in general.

(a). Assuming the medium is the same, increased wavelength would mean decreased frequency. This would be wrong. Even if we assumed the medium changed, meaning frequency is the same, this would still be wrong.

(c). This says frequency decreases which would be wrong.

(d). This statement is incorrect and speed does not have to do with pitch.

Pick (b) because it's "least wrong". Unsatisfying, I know. But that's basically the MCAT in a nutshell, especially CARS.