source.- What is the frequency of the waves as they’re received by the
detector?
Here’s How to Crack It
- Because the detector moves toward the source, we use the +
sign in the numerator of the Doppler effect formula, and since the
source moves toward the detector, we use the − sign in the
denominator. This gives us - A train sounds its whistle as it travels at a constant speed by a
train station. Describe the pitch of the train’s whistle as heard by
someone standing on the station platform.
Here’s How to Crack It
Because the “pitch” of a sound is its frequency, we want to describe the frequency
of the train whistle as heard by someone on the platform. As the train approaches
the platform, the frequency detected will be higher than the frequency emitted by
the train; then, as the train recedes from the platform, the frequency detected will be
lower than the frequency emitted by the train. So, a person standing on the platform
hears a higher pitch as the train approaches and a lower pitch as it travels away.