UPSC NDA 2 2016 General Ability Solved Paper

Concept:A spring balance measures weight (force), not mass directly.Its reading depends on the local acceleration due to gravity $g$.The extension of the spring is proportional to $mg$ (Hooke's law).Thus the reading is accurate only when $g$ is the same as at the calibration location.Explanation:The spring balance works by Hooke's law: $F = -kx$, where $F = mg$.So the extension $x \propto mg$. The scale is originally marked assuming a specific $g$ value.At a different location with a different $g$, the same mass gives a different extension.Hence the reading is correct only where $g$ matches the calibration value.Option A is incorrect because it implies the reading is correct only at the exact calibration place, but it would be correct at any place with the same $g$.Option B is false because $g$ varies with altitude and latitude.Option D is false because a spring balance can measure mass if the local $g$ is known and accounted for.Only option C correctly states the condition for accurate mass measurement.Answer:Option C