A tornado siren sits on a pole, wailing at its one fixed frequency. Maren rides her bike straight toward it, and the wail sounds higher to her than it does to Noll, standing still beside the road.
This time the source never moves. The observer does. The shift follows the observer's motion, with its own rule.
The siren on the pole is the source, and it is stationary. Maren, moving, is the observer.
While Maren moves toward the siren, the frequency she receives is higher than the frequency the siren emits: a higher pitch.
When she turns and rides away from the siren, the received frequency is lower than the emitted one: a lower pitch.
Standing still, like Noll, gives no shift: he receives exactly the emitted frequency.
Keep track of which thing moved. Last lesson the source moved; here the observer moves. Either motion shifts the received pitch, and each case has its own mechanism.
Pim jogs toward a parked car whose alarm is blaring. Compare the pitch Pim receives with the pitch the alarm emits.
The source is stationary, and the observer moves toward it.
Observer moving toward the source means a higher received pitch.
Pim receives a higher pitch than the alarm emits.
On a moving walkway, Ottoline glides away from a wall speaker playing a steady tone. What happens to the pitch she receives?
The source is stationary, and the observer moves away from it.
Observer moving away from the source means a lower received pitch.
She receives a lower pitch than the speaker emits.
A school bell rings on a wall at its fixed pitch. Noll runs toward it. Is the pitch he receives higher or lower than the bell emits?
A foghorn sounds from a stationary lighthouse. A sailboat glides straight away from it. Is the pitch received on the boat higher or lower than the foghorn emits?
Ottoline says: 'Riding toward a stationary siren raises the pitch you receive, and riding away from it lowers that pitch.' Is Ottoline correct?
A speaker on a pole plays one steady tone. In which case does the listener receive a higher frequency than the speaker emits?
A buzzer on a fence post buzzes steadily while Maren stands still, listening. Which statement is correct?
A parked truck's horn beeps steadily. In which case does the skater receive a lower frequency than the horn emits?