Doppler Effect — Prelims Questions

An ambulance siren emits a sound of frequency $1200\,\text{Hz}$. It is moving towards a stationary observer at a speed of $30\,\text{m/s}$. The speed of sound in air is $340\,\text{m/s}$. What is the apparent frequency heard by the observer?

A train approaches a stationary observer at a speed of $20\,\text{m/s}$ and blows a whistle of frequency $400\,\text{Hz}$. What is the apparent frequency heard by the observer? (Speed of sound in air = $340\,\text{m/s}$)

A person is moving away from a stationary siren at a speed of $10\,\text{m/s}$. The siren emits sound of frequency $680\,\text{Hz}$. If the speed of sound in air is $340\,\text{m/s}$, what is the frequency heard by the person?

A source of sound is moving towards a stationary wall with a speed of $10\,\text{m/s}$. The source emits sound of frequency $500\,\text{Hz}$. The speed of sound in air is $340\,\text{m/s}$. What is the frequency of the reflected sound as heard by the source?

A car moving at $20\,\text{m/s}$ blows a horn of frequency $600\,\text{Hz}$. A cyclist is moving in the same direction at $10\,\text{m/s}$. What is the frequency heard by the cyclist when the car is behind the cyclist and approaching? (Speed of sound in air = $340\,\text{m/s}$)

A source of sound is moving away from a stationary observer at $15\,\text{m/s}$. The observer then starts moving towards the source at $5\,\text{m/s}$. If the source emits sound at $800\,\text{Hz}$ and the speed of sound is $340\,\text{m/s}$, what is the apparent frequency heard by the observer at this instant?