The figure represents the instantaneous displacement-position graph of a longitudinal harmonic wave travelling along the negative $x$-axis. Identify the points of maximum compression.

An engine approaches a hill with a constant speed. When it is at a distance of $0.9 \, km$,it blows a whistle whose echo is heard by the driver after $5 \, s$. If the speed of sound in air is $330 \, m/s$,then the speed of the engine is .... $m/s$.

What is the distance between consecutive condensation and rarefaction in longitudinal waves?

$A$ man sets his watch by a whistle that is $2\, km$ away. How much will his watch be in error? (Speed of sound in air = $330\, m/s$)

$A$ man standing on a cliff claps his hands and hears the echo after $1 \, s$. If the sound is reflected from another mountain and the velocity of sound in air is $340 \, m/s$,then the distance between the man and the reflection point is ..... $m$.

$A$ firecracker exploding on the surface of a lake is heard as two sounds a time interval $t$ apart by a man on a boat close to the water surface. Sound travels with a speed $u$ in water and a speed $v$ in air. The distance from the exploding firecracker to the boat is