A wire of length 400cm is stretched between two fixed points. When plucked, its fundamental frequency is 150Hz. Calculate the speed of the wave produced.

Analyze the question: A stretched wire fixed at both ends vibrates like a string. In its fundamental mode, it forms one loop (one antinode between two nodes).

Concept: For the fundamental mode, the length of the wire equals half a wavelength:

L = λ/2, so λ = 2L

The wave speed is v = fλ.

Solution:

• L = 400 cm = 4 m
• λ = 2 × 4 = 8 m
• f = 150 Hz

v = fλ = 150 × 8 = 1200 m/s

Answer: C. 1200 m/s

Tip: Always convert cm to m before calculating. For a string fixed at both ends, the fundamental has λ = 2L, so you can use the shortcut v = 2Lf. If you forgot to convert units, you would get 120,000, which is not among the options, so that tells you something went wrong.