What is the minimum distance required to hear a distinct echo? (Assuming speed of sound in air is 344 m/s)
A. 10 meters
B. 34.4 meters
C. 25.6 meters
D. 17.2 meters
Answer: Option D
Solution (By JKSSB Mock Tests)
The human ear retains the sensation of sound for about 0.1 seconds (persistence of hearing). To hear a distinct echo, the reflected sound must reach the ear after at least 0.1 s. Distance traveled by sound = Speed × Time = 344 m/s × 0.1 s = 34.4 m. Since this is the total distance (go and return), the minimum distance to the obstacle is 34.4 / 2 = 17.2 meters.
Explanation:
Viscosity η = Force/(Area × velocity gradient). Force [MLT⁻²], area [L²], velocity gradient [T⁻¹]. So η = [MLT⁻²]/([L²][T⁻¹]) = [ML⁻¹T⁻¹]. Often asked.
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