Question

You are uptown one Saturday night and blow air across the top of your half-full water bottle and hear a noticeable hum. Then you drink the rest of your water and again blow across the top and again hear a noticeable hum. The pitch of this second hum will be ___ than/as the first.

A) Lower
B) Higher
C) Same
D) Unpredictable

Answer 1

The pitch of the second hum will be lower than the first hum because a larger air column in the bottle leads to a lower resonant frequency and thus a lower pitch.

Step-by-step explanation:

When a student blows across the top of a half-full water bottle, they create a resonant frequency within the air column in the bottle. The pitch of the hum you hear corresponds to this resonant frequency. After drinking the rest of the water and blowing across the bottle again, the resonant frequency changes because there is now more air in the bottle which can vibrate.

As the length of the air column increases (which happens when you drink more of the water), the resonant frequency decreases. This means that the pitch of the second hum will be lower than that of the first hum. This is analogous to having a longer string on a musical instrument, which also produces a lower pitch when compared to a shorter string.

Therefore, the correct answer is A) Lower.