Question 61: Waves

Question

When a car is approaching the observer, the frequency of horn is $$100 \mathrm{~Hz}$$. After passing the observer, it is $$50 \mathrm{~Hz}$$. If the observer moves with the car, the frequency will be $$\frac{x}{3} \mathrm{~Hz}$$ where $$x=$$ ________________.

Accepted Answer

$$100 = {v_0}{v \over {v - {v_c}}}$$
$$50 = {v_0}{v \over {v + {v_c}}}$$
$$2 = {{v + {v_c}} \over {v - {v_c}}}$$
$$2v - 2{v_c} = v + {v_c}$$
$${v_c} = {v \over 3}$$
$$100 = {v_0}{{v 	imes 3} \over {2v}} \Rightarrow {v_0} = {{200} \over 3} = {x \over 3}$$
$$ \Rightarrow x = 200$$

Waves

When a car is approaching the observer, the frequency of horn is $100 \mathrm{~Hz}. After passing the observer, it is 50 \mathrm{~Hz}. If the observer moves with the car, the frequency will be \frac{x}{3} \mathrm{~Hz} where x=$ ________________.

Integer / Numeric Answer Type