A duck paddles across a still pond, bobbing as it goes and sending out ripples. The ripples ahead of it sit crowded together; the ripples behind it sit stretched apart.
The duck never changed how often it bobs. The crowding comes from where each ripple starts. The same geometry explains the train horn from last lesson.
The duck bobs once, glides forward, bobs again, glides forward again, and bobs a third time.
Each ripple spreads out as a circle centered on the spot where the duck was when it made that ripple.
The duck keeps moving, so each new circle starts from a point farther along its path.
Ahead of the duck, each new circle starts closer behind the older circles' fronts, so the wavefronts there sit squeezed together.
Behind the duck, each new circle starts farther from the older fronts, so the wavefronts there sit spread apart.
A moving train's horn does the same thing with sound wavefronts.
Ahead of the train the wavefronts arrive squeezed together, more of them each second: the received frequency is higher.
Behind the train the wavefronts arrive spread apart, fewer each second: the received frequency is lower.
The source's emission rate never changed. Only the starting points of the wavefronts moved.
A speedboat crosses a lake with its horn blaring. Explain why a kayaker directly ahead of the boat receives a higher frequency than the horn emits.
The moving boat emits each wavefront from a position farther along its path.
So ahead of the boat, the wavefronts sit squeezed together.
The wave speed is unchanged, and with shorter gaps between fronts, the fronts arrive more often each second.
The kayaker receives a higher frequency than the horn emits.
A speedboat moves away from a kayaker behind it, horn blaring. Explain why the kayaker receives a lower frequency than the horn emits.
The moving boat emits each wavefront from a position farther along its path, away from the kayaker.
So behind the boat, the wavefronts sit spread apart.
With longer gaps between fronts at the same wave speed, the fronts arrive less often each second.
The kayaker receives a lower frequency than the horn emits.
A fire engine drives toward Maren, who stands still, siren on. Why does she receive a higher frequency than the siren emits?
A jet ski speeds across a bay, sending sound wavefronts through the air. Where is the spacing between its wavefronts smallest?
Noll says: 'The ripples ahead of a swimming duck sit squeezed together because the duck makes each new ripple from a point farther along its path.' Is Noll correct?
A small plane flies straight away from the airfield fence where Pim stands, engine buzzing. Which option explains why Pim receives a lower frequency than the engine emits?
An ice-cream truck plays its jingle while driving down a street. Compared with the wavefront spacing from the same truck parked, how are the wavefronts spaced behind the moving truck?
A delivery drone hums at an emitted 300 Hz as it flies toward Ottoline. Which statement is correct?