As a car drives past a person standing on a sidewalk, the driver keeps a hand on the horn. how does the pitch of the horn differ for the driver and the person standing on the sidewalk?

While the car is going towards the standing person on the sidewalk he is listening the pitch of the horn higher then it is.

When the car passes the person on the sidewalk and goes away, the horn's pitch become lower (for the person standing on the sidewalk).

For the driver, the pitch is always the same.

Explanation:

Is the doppler effect.

When the car is approaching, the sound waves must travel a smaller space than the previous one and they take less time to reach the standing person. So, sound waves reach the standing person with a higher frequency.

It happens the opposite when the car is going away: the sound waves need to travel more space and take more time to reach the person on the sidewalk. So, the frequency of the waves is lower.

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explanation:

the force on the thing and the distance the force moves the thing