Question

A replica of longitudinal waves can be best observed
A) By swinging a pendulum
B) By pressing and releasing a horizontal spring
C) By striking a guitar string
D) By rotating a top

Answer 1

In longitudinal waves, the direction of displacement of each individual particle or point in the wave is in a direction parallel to or anti parallel to the direction of propagation of the wave but for a transverse wave, the displacement of a point on the wave is perpendicular to the direction of propagation of the wave.

Complete answer:
The defining characteristic of a longitudinal wave is that the direction of displacement of each point or particle in the wave is in the parallel or antiparallel direction of propagation of the wave.
Now, keeping this characteristic property in mind, let us analyze the options one by one.
For a swinging pendulum, the motion exhibited is simple harmonic motion (SHM).
For pressing and releasing a horizontal spring, there are alternate areas of compression and rarefaction in the spring, that is, individual points of the spring get displaced in the same direction as that of the wave. Hence, by pressing and releasing a horizontal spring, we can observe a replica of longitudinal waves.
When we strike a guitar string, the points on the guitar string get displaced in a perpendicular direction as to that of the direction of the length of the string. Hence, this is a replica of transverse waves and not longitudinal waves.
The rotation of a top is the representation of simple rotational motion and is not the replica of longitudinal waves.

Therefore, the correct option is B) By pressing and releasing a horizontal spring.

Note:
Students might think that since sound is a longitudinal wave and plucking a guitar string produces sound, so plucking a guitar string must be the representation of the replica of longitudinal waves. However, this is not so as explained in the answer that the points on the guitar string move perpendicular to the length of the spring. So, even though the sound produced is a longitudinal wave, the actual movement and motion of the guitar string is a representation of the replica of transverse waves and not longitudinal waves.