An ambulance moves away from an accident scene at a constant speed with its siren producing sound waves at a frequency of 890 Hz - NSC Physical Sciences - Question 6 - 2017 - Paper 1

As the ambulance moves away from the scene/detector, the wavelengths become longer, resulting in fewer waves reaching the detector per unit time. Therefore, the frequency decreases, causing the detector to measure a frequency that is 90% of the original emitted frequency.

To find the speed of the ambulance, we use the formula for the Doppler effect:
$f' = f_s \frac{v + v_o}{v + v_s}$
Given that the detector measures 90% of the frequency:
$f' = 0.90 \times 890 ext{ Hz} = 801 ext{ Hz}$
Here, $v = 340 ext{ m/s}$ (speed of sound), and $f_s = 890 ext{ Hz}$.
Substituting values, we rearrange the formula to find $v_s$:
$801 = 890 \frac{340}{340 + v_s}$
Cross-multiplying gives us:
$801(340 + v_s) = 890(340)$
$801 \cdot v_s = 890 imes 340 - 801 imes 340$
$v_s = \frac{890 \cdot 340 - 801 \cdot 340}{801}$
Calculating the above:
$v_s = \frac{299800 - 272340}{801} = \frac{27460}{801} \approx 34.3 ext{ m/s}$
Thus, the speed of the ambulance is approximately 34.3 m/s.

The Doppler flow meter is used to determine whether arteries are clogged or narrowed, or to measure the rate of blood flow in patients.

The nearby star is moving towards Earth.

The nearby star is moving faster than the distant star as observed from Earth.