UPSEE 2019 Solved Paper

Key-Idea If source and observer are moving towards a wall, then apparent frequency after the reflection from wall is same as the apparent frequency by the source and observer moving towards each other.
Here, speed of sound, vs=320m∕s, actual frequency, v=8000Hz and speed of source vs=20m∕s
Since, both the observer and source are moving with same speed.
⇒vs=20m∕s
Apparent frequency,
v′=(

v+v0

v−vs

)v
⇒v′=(

320+20

320−20

)×8000
=9066.66≈9.1kHz
Hence, the frequency of siren heard by car driver is 9.1kHz