Transverse wave particle motion in water waves

Steps:

• Recognize water surface waves as transverse, with particles oscillating vertically perpendicular to horizontal propagation.
• Note wave travels left to right, so analyze snapshot positions relative to wave crests and troughs.
• Identify particles at or just past troughs (lowest points) as those starting upward motion.
• Compare positions: Q and S are at troughs, while P and R are at crests or descending slopes.

Why D is correct:

• In transverse waves, particles at troughs reverse from downward to upward velocity, per simple harmonic motion phase.

Why the others are wrong:

• A: P and R at crests start downward motion.
• B: P at crest moves down; S at trough moves up, but pair mismatched.
• C: R at crest moves down; S at trough moves up, but pair mismatched.

Final answer: D