Authentic Classroom Explanation by IAS Monk

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📍 The Question

Consider the following statements:

1. In a seismograph, P waves are recorded earlier than S waves.
2. 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?

(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2

Correct Answer: (c) Both 1 and 2

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🎯 Theme of the Question

Physical Geography | Earth Interior | Seismic Waves | NCERT Concepts

This is a direct NCERT-based question testing the fundamental differences between P-waves and S-waves.

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🧠 Classroom Explanation

Earthquake waves are broadly of two types:

• Body waves (travel through Earth’s interior)
  • P waves
  • S waves
• Surface waves (travel along Earth’s surface, most destructive)

Now, examine each statement.

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🔹 Statement 1: P waves are recorded earlier than S waves. ✅

• P-waves (Primary waves) are the fastest seismic waves
• They reach recording stations first
• S-waves (Secondary waves) arrive later due to lower speed

✔️ Statement 1 is correct

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🔹 Statement 2: Particle motion in P and S waves is as described. ✅

• P-waves (compressional / longitudinal):
  • Particle vibration is parallel to wave direction
  • Motion is to and fro (push–pull)
  • Creates compression and rarefaction
  • Travels through solids, liquids, and gases
• S-waves (shear / transverse):
  • Particle vibration is perpendicular to wave direction
  • Motion is up and down / side to side
  • Travels through solids only (cannot pass through liquid outer core)

✔️ Statement 2 is correct

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📊 Final Assessment

Statement	Status
P waves recorded before S waves	✅ Correct
P: parallel motion; S: perpendicular motion	✅ Correct

👉 Correct answer: (c)

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❌ UPSC Elimination Logic

UPSC tests:

• Speed difference (P faster than S)
• Type of vibration
• Often linked with:
  • Earth’s core composition (S-waves don’t pass through liquids)

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🧩 Memory Hook

“P pushes forward; S shakes sideways.”

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🧠 Prelims Strategy Insight

Always remember:

• P = Primary = fastest = through solids + liquids
• S = Secondary = slower = only through solids
• Surface waves = maximum damage

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🧭 IAS Monk Whisper

The Earth speaks in waves, and P always speaks first.