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IB · C · multiple choice

Wave behaviour · Question 4

Wave behaviour · Original GioPhysics question with a detailed, mark-by-mark answer guide.

Demand
routine
Marks
1
Topics
1
Answer
Complete
An ambulance sounding its siren travels towards a stationary observerambulance, siren of frequency 512 Hz25 m s⁻¹stationary observerspeed of sound in air = 340 m s⁻¹
Figure 2Side view along a straight road, drawn as a hatched ground line. On the left an ambulance stands on the road, labelled 'ambulance, siren of frequency 512 Hz'. A horizontal arrow leaves the front of the ambulance and points to the right, labelled 25 m s⁻¹. Well ahead of it, on the same road, a person stands still, labelled 'stationary observer'. A note below the road reads 'speed of sound in air = 340 m s⁻¹'.
multiple choice1 mark

An ambulance siren emits a note of frequency 512 Hz. The ambulance travels at 25 m s⁻¹ towards a stationary observer. The speed of sound in air is 340 m s⁻¹. What frequency does the observer hear?

  1. A

    477 Hz

  2. B

    512 Hz

  3. C

    553 Hz

  4. D

    590 Hz

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Answer overviewCorrect answer: C — 553 Hz
01

Answer and reasoning

1 mark
How to approach it

Predict from the governing principle before looking for a matching option. Then reject alternatives by checking direction, units, limiting behaviour or the physical meaning of each statement.

  1. 1

    The correct choice is C: 553 Hz

  2. 2

    A is the frequency heard after the ambulance has passed, obtained by using the wrong sign. Any answer below 512 Hz can be discarded for an approaching source before the arithmetic begins.

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