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MCQ practice · SSC Physics Board questions · Chapter 7: Waves and Sound

Barishal Board 2023

The unit of sound intensity is−

  1. i. W m⁻²
  2. ii. J m⁻¹
  3. iii. J s⁻¹ m⁻²

Think it through first, then pick an option.

  1. a i and ii
  2. b i and iii
  3. c ii and iii
  4. d i, ii and iii
Answer and explanation

Correct answer: b i and iii

Right. W m⁻² and J s⁻¹ m⁻² are the same unit, since 1 W = 1 J s⁻¹. J m⁻¹ has no time and no area.

Why the other options are wrong

  • a i and ii: i is right, but ii is not: J m⁻¹ is energy ÷ length, a unit of force. iii is right too, because W = J s⁻¹.
  • c ii and iii: ii is not right: J m⁻¹ is a unit of force (energy ÷ length). i and iii are right, since W = J s⁻¹ makes them the same unit.
  • d i, ii and iii: Not all three: ii is J m⁻¹, with no time and no area, so it is a unit of force.

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