An echo returned in 0.5 s. If speed of sound is 340 m/s, the distance of reflecting surface is MCQ with Answer and Explanation

An echo returned in 0.5 s. If speed of sound is 340 m/s, the distance of reflecting surface is
A. 340 m
B. 170 m
C. 680 m
D. 85 m
Answer: Option D
Solution (By JKSSB Mock Tests)
d = (v×t)/2 = (340×0.5)/2 = 170/2 = 85 m. Minimum distance for echo perception is about 17 m. Remember divide by 2 for one-way distance.

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Question #1
The kinetic energy of a body becomes four times its initial value. The new momentum will be:
A. Four times the initial
B. Half the initial
C. Same as initial
D. Twice the initial

Correct Answer: Option D


Explanation:
Kinetic energy KE = p²/(2m), where p is momentum. Thus p = √(2m·KE). If KE becomes 4 times, p_new = √(2m·4KE) = 2√(2m·KE) = 2p_initial. Hence momentum doubles. This relationship shows momentum scales with square root of kinetic energy for constant mass. Memory tip: p ∝ √KE when mass constant. This conceptual link between momentum and kinetic energy is frequently tested in competitive exams to assess depth of understanding beyond rote formula memorization. Always derive relationships when direct formulas aren't recalled.

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Question #2
A transformer has 100 turns in primary and 200 turns in secondary. If primary voltage is 120 V, the secondary voltage is:
A. 120 V
B. 480 V
C. 240 V
D. 60 V

Correct Answer: Option C


Explanation:
Transformer voltage ratio: V_s/V_p = N_s/N_p, where N is number of turns. Thus V_s = V_p × (N_s/N_p) = 120 V × (200/100) = 240 V. This assumes ideal transformer (no losses). Step-up transformer (N_s > N_p) increases voltage. Memory aid: 'Turns ratio = voltage ratio'. This direct formula application is frequent in competitive exams. Always verify if transformer is step-up or step-down from turn counts. Note: Current ratio is inverse: I_s/I_p = N_p/N_s for power conservation (V_pI_p = V_sI_s).

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Question #3
At the magnetic poles of the Earth, the angle of dip (inclination) is exactly:
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B. 90 degrees
C. 0 degrees
D. 45 degrees

Correct Answer: Option B


Explanation:
The angle of dip is the angle the Earth's total magnetic field vector makes with the horizontal surface. At the magnetic equator, the magnetic field lines are perfectly parallel to the ground (0 degrees). At the magnetic poles, the field lines plunge strictly vertically downward (or upward) into the ground, making a 90-degree angle with the horizontal.

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