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Concepts Of Physics MCQ Edition [Volume 1]
Exam
Concepts Of Physics MCQ Edition [Volume 1]
Subject
Physics
Chapter
Wave Motion and Waves on a String

Concepts Of Physics MCQ Edition [Volume 1] · Physics · Wave Motion and Waves on a String

A string fixed at both ends is vibrating in its third harmonic. The...

A string fixed at both ends is vibrating in its third harmonic. The equation for its vibration is given by y=(0.4 cm)sin⁡[(0.314 cm−1)x]cos⁡[(600π s−1)t]y = (0.4 \text{ cm}) \sin[(0.314 \text{ cm}^{-1}) x] \cos[(600\pi \text{ s}^{-1})t]. Determine the wavelength and the speed of the two travelling waves that can interfere to produce this vibration.

Options

  1. A20 cm20 \text{ cm} and 120 m/s120 \text{ m/s}
  2. B10 cm10 \text{ cm} and 30 m/s30 \text{ m/s}
  3. C20 cm20 \text{ cm} and 60 m/s60 \text{ m/s}
  4. D30 cm30 \text{ cm} and 60 m/s60 \text{ m/s}

Correct answer

C. 20 cm20 \text{ cm} and 60 m/s60 \text{ m/s}

Step-by-step solution

The given equation of the standing wave is y=0.4sin⁡(0.314x)cos⁡(600πt)y = 0.4 \sin(0.314 x) \cos(600\pi t).

Comparing this with the standard equation of a standing wave y=2Asin⁡(kx)cos⁡(ωt)y = 2A \sin(kx) \cos(\omega t), we get:

k=0.314 cm−1k = 0.314 \text{ cm}^{-1}

ω=600π s−1\omega = 600\pi \text{ s}^{-1}

The wavelength λ\lambda is given by:

λ=2πk=2×3.140.314=20 cm\lambda = \dfrac{2\pi}{k} = \dfrac{2 \times 3.14}{0.314} = 20 \text{ cm}

The speed vv of the travelling waves is given by:

v=ωk=600π0.314=600×3.140.314=6000 cm/s=60 m/sv = \dfrac{\omega}{k} = \dfrac{600\pi}{0.314} = \dfrac{600 \times 3.14}{0.314} = 6000 \text{ cm/s} = 60 \text{ m/s}
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