Gravitation — Class 11 Physics: 20 flashcards
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1. State Newton's universal law of gravitation.
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Every particle attracts every other particle with a force $F = G\frac{m_1 m_2}{r^2}$, directly proportional to the product of the masses and inversely proportional to the square of the distance between their centres.
2. What is the value of the universal gravitational constant $G$?
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$G = 6.67 \times 10^{-11}\ \text{N m}^2\text{kg}^{-2}$
3. Is gravitational force attractive, repulsive, or both?
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Always attractive — it never repels.
4. How does gravitational force change if the distance between two masses is doubled?
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The force becomes one-fourth of its original value, because $F \propto 1/r^2$.
5. Write the formula for acceleration due to gravity at a planet's surface.
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$g = \frac{GM}{R^2}$
6. What is the approximate value of $g$ on Earth's surface?
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$g \approx 9.8\ \text{m s}^{-2}$
7. Distinguish between mass and weight.
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Mass is the quantity of matter (kg), constant everywhere. Weight is the gravitational force $W = mg$ (newtons), which changes with $g$.
8. Why is $r$ measured centre-to-centre for spherical bodies?
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Because the entire mass of a uniform sphere behaves as if concentrated at its centre.
9. Which is the weakest of the four fundamental forces?
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Gravitational force — yet it dominates on astronomical scales because it is always attractive and has infinite range.
10. Write the expression for orbital velocity of a satellite.
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$v = \sqrt{\frac{GM}{r}}$
11. Write the expression for escape velocity from a planet.
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$v_e = \sqrt{\frac{2GM}{R}}$
12. What is Earth's escape velocity?
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About $11.2\ \text{km/s}$
13. Write the gravitational potential energy at distance $r$ from a mass $M$.
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$U = -\frac{GMm}{r}$
14. State Kepler's third law.
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$T^2 \propto a^3$ — the square of the orbital period is proportional to the cube of the semi-major axis.
15. Why does the Moon not fall into Earth?
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Its tangential velocity keeps it in orbit; gravity provides the centripetal force, so it continuously falls around Earth rather than into it.
16. What causes apparent weightlessness in an orbiting satellite?
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Free fall — the astronaut and the satellite fall together with the same acceleration, so the normal force is zero. Gravity is still present.
17. What causes tides on Earth?
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The differential gravitational pull of the Moon and the Sun on different parts of Earth's oceans.
18. Why do black holes trap light?
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Their escape velocity exceeds the speed of light, so nothing — not even light — can escape.
19. Does gravity need a medium to act?
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No — it acts through vacuum. This is why astronauts in space still feel Earth's gravity.
20. What is the relation between $G$ and $g$?
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$g = \frac{GM}{R^2}$ — $G$ is universal, $g$ is local and depends on the planet's mass and radius.
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