Which observation best supports the idea that sound is a mechanical wave? Class 9

Which observation best supports the idea that sound is a mechanical wave? Class 9

Revise, Reflect, Refine (Page 204-206)

Question 1.

Which observation best supports the idea that sound is a mechanical wave? Class 9

(i) Sound shows reflection

(ii) Sound needs a medium to propagate

(iii) Sound has frequency

(iv) Sound carries energy

Answer:

(ii) Sound needs a medium to propagate

Explanation: Mechanical waves are defined as waves that require a material medium for propagation. The fact that sound cannot travel in vacuum (as shown by the bell jar experiment) directly supports this.

Question 2.

For a sound wave propagating in a medium, increasing its frequency will increase its: Class 9

(i) wavelength

(ii) speed

(iii) number of compressions per second

(iv) time period

Answer:

(iii) number of compressions per second

Explanation: Frequency is the number of oscillations (compressions) per second. Increasing frequency directly increases the number of compressions passing a point per second. Speed is a property of the medium (not frequency, in most media). Wavelength decreases with increasing frequency (since u = λv and v is constant). Time period decreases (T = 1/v).