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Q7.1: GRAVITATION Solutions — NCERT Physics - Part 1
⚠️ Common Exam Traps & Conceptual Pitfalls
Students often incorrectly assume that because the Sun exerts a stronger total gravitational force than the Moon, it must also cause stronger tides. You must distinguish that tidal effects depend on the gradient of the field, which scales with $1/r^3$, rather than the force itself, which scales with $1/r^2$.
Answer the following: (a) You can shield a charge from electrical forces by putting it inside a hollow conductor. Can you shield a body from the gravitational influence of nearby matter by putting it inside a hollow sphere or by some other means? (b) An astronaut inside a small space ship orbiting around the earth cannot detect gravity. If the space station orbiting around the earth has a large size, can he hope to detect gravity? (c) If you compare the gravitational force on the earth due to the sun to that due to the moon, you would find that the Sun's pull is greater than the moon's pull. However, the tidal effect of the moon's pull is greater than the tidal effect of sun. Why?
Step-by-Step Verified Solution
**Answer:** (a) No. (b) Yes, if the size is large enough. (c) Tidal effect depends on inverse of distance's cube.
**Step-by-Step Explanation:**
(a) Gravitational shielding is not possible because there is no known material that can block gravitational force. Unlike electrical forces, gravitational force cannot be shielded by any hollow sphere or other means.
(b) If the space station is large enough, the astronaut will be able to detect the change in Earth's gravity. In a small spaceship, the astronaut is in a state of free fall and feels weightlessness. But in a large space station, the gravitational force at different points may differ enough to be detected [citation:1].
(c) The tidal effect depends on the inverse of the cube of the distance, while the gravitational force depends on the inverse of the square of the distance. Since the distance between the Moon and the Earth is much shorter than the distance between the Sun and the Earth, the tidal influence of the Moon's pull is higher than the tidal effect of the Sun's influence [citation:1].
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