Parallel-faced slab: refraction at entry and exit cancel angular deviation, but ray shifts laterally. No net direction change. Memory tip: 'Slab: parallel emergent ray; Prism: deviated emergent ray'. Ray optics concept frequently tested in competitive exams.
Explanation:
The gravitational potential V inside a uniform solid sphere at a distance r from the center is V = -GM * (3R^2 - r^2) / (2R^3). At the exact center of the Earth, r = 0. Substituting r = 0 gives V = -GM(3R^2) / 2R^3 = -3GM / 2R. The potential is strictly non-zero and highly negative.
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