Gravitational force: choose your answer

Answer: B — 32 N. Use the usual spherical-Earth model, treating the stated surface weight as gravitational force and neglecting rotation. At height h = R/2, the distance from Earth’s centre is R + h = 3R/2.

F / 72 = [R / (R + h)]² = (2/3)² = 4/9.

Therefore F = 72 × 4/9 = 32 N, directed towards Earth’s centre.

A gives 48 N, which can be produced by using the distance ratio without squaring: 72 × 2/3. This is a possible incorrect calculation, not evidence of any particular learner’s reasoning. C (30 N) and D (24 N) do not follow from the stated inverse-square calculation.

Correction to the stored explanation: substituting R/2 into the uniform-Earth depth expression would give 72 × (1 − 1/2) = 36 N, not 24 N. That below-surface expression is inapplicable here. The recorded choice alone does not identify the cause of an error. The small-height approximation also should not replace the full formula at h = R/2.

Observed September 21 set, frozen September 22 10:48 IST: 18 first answers in this exact content/form group: 6 correct, 8 wrong and 4 skips. Of 14 non-skipped answers, 8 were wrong (57.1%). A:4, B:6, C:3, D:1; skips:4. These are account responses, not a representative estimate of all NEET students. Data is subject to late arrivals; answer language was not recorded.

Physics source: OpenStax, Gravitation Near Earth’s Surface

The original question and answer key are retained; the explanation is corrected here. This does not claim that the app explanation has been updated.

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