Previous Year Question
Consider the following statements:
In a seismograph, P waves are recorded earlier than S waves.
In P waves, the individual particles vibrate to and fro in the direction of wave propagation whereas in S waves, the particles vibrate up and down at right angles to the direction of wave propagation.
Which of the statements given above is/are correct?
Correct Answer: C
Detailed Solution
Statement 1 is correct:
P waves are called primary waves because they travel faster than all other seismic waves.
Therefore, P waves are recorded first on a seismograph, followed by the comparatively slower S waves.
Statement 2 is correct:
P waves are compressional or longitudinal waves. Particles of the medium vibrate back and forth parallel to the direction of wave propagation, producing successive compression and expansion of the material.
S waves are shear or transverse waves. Particle vibration occurs perpendicular to the direction of wave propagation.
The statement describes S-wave vibration as “up and down.” This is a simplified representation of transverse motion; more precisely, particles may vibrate in any transverse direction at right angles to the direction of wave propagation.
P waves can travel through solids, liquids and gases. S waves can pass only through solids because liquids and gases lack shear rigidity.
Thus, both statements are correct.