Gravitation – Short Answer Type Questions

CBSE Class 9 | Physics Chapter 10: Gravitation — 50 Short Answer Questions

Content Bank — Chapter 10: Gravitation

Newton’s Universal Law of Gravitation, gravitational constant G
Gravitational field and acceleration due to gravity (g)
Variation of g with altitude & depth
Mass vs Weight, apparent weight and weightlessness
Free fall, kinematics under gravity
Orbital motion of satellites, orbital speed, period, escape velocity
Kepler’s law (brief), centered force concept, solved examples

50 Short Answer Type Questions & Answers — Gravitation

Each answer is concise but explanatory (2–4 lines), aligned to NCERT level and ideal for board exam revision.

Universal Law of Gravitation

Q1. State Newton’s Universal Law of Gravitation.

Every two point masses attract each other with a force proportional to the product of their masses and inversely proportional to the square of the distance between them: F = G m₁ m₂ / r².

Q2. What is the gravitational constant G and its SI unit?

G is the universal gravitational constant that determines gravitational strength. Its value ≈ 6.67×10⁻¹¹ and unit is N·m²/kg².

Q3. Why do we use centre-to-centre distance in the formula?

The masses can be treated as concentrated at their centres (point masses). The gravitational attraction acts along the line joining their centres, so r is the centre-to-centre distance.

Q4. If the distance between two masses is halved, how does force change?

Force varies as 1/r², so if distance is halved, force becomes four times larger.

Q5. Does gravitational force depend on the medium between masses?

Gravitational force depends only on masses and separation; the medium does not affect the gravitational force in classical physics.

Gravitational Field & Acceleration due to Gravity

Q6. Define gravitational field (intensity).

Gravitational field at a point is the force experienced per unit mass placed there: g = F / m. It shows strength of gravity at that point.

Q7. Express g at distance r from Earth's centre.

g = G M / r², where M is Earth’s mass and G is the gravitational constant.

Q8. Typical value of g on Earth’s surface and why it varies slightly.

Approximately 9.8 m/s². It varies slightly with latitude, altitude and local geology because Earth's shape and mass distribution are not perfectly uniform.

Q9. Is g a vector or scalar? Give its direction near Earth.

g is a vector directed toward the centre of Earth (downward near Earth's surface).

Q10. Does value of g depend on the mass of the test object?

No. g depends on the source mass (Earth) and distance; it is independent of the test object’s mass.

Variation of g with Altitude & Depth

Q11. How does g change with altitude h above Earth's surface?

At height h: g' = g (R / (R + h))². g decreases with altitude because distance from Earth's centre increases.

Q12. For small heights, give the approximation for g.

For small h relative to R: g' ≈ g (1 − 2h/R), a linear approximation used in simple problems.

Q13. How does g vary with depth d below Earth's surface (uniform density)?

Assuming uniform density, g_depth = g (1 − d / R); g decreases linearly with depth and becomes zero at centre.

Q14. Why is g zero at the centre of Earth?

At Earth's centre, gravitational pulls from all directions cancel out symmetrically, resulting in net field zero.

Q15. If g at surface is 9.8 m/s², what is g at 2R from centre?

At r = 2R, g' = g / 4 because g ∝ 1/r²; so 9.8/4 = 2.45 m/s².

Mass, Weight & Apparent Weight

Q16. Define mass.

Mass is the amount of matter in a body, a scalar quantity measured in kilograms. It does not change with location.

Q17. Define weight and its unit.

Weight is the gravitational force on a body: W = m g. Its SI unit is newton (N).

Q18. Why is mass different from weight?

Mass is intrinsic; weight depends on local g. Mass remains constant while weight changes with location (e.g., Moon vs Earth).

Q19. What is apparent weight and how does it change in an accelerating lift?

Apparent weight is the normal reaction. In a lift accelerating upward with acceleration a, apparent weight = m(g + a); downward acceleration reduces apparent weight.

Q20. Explain weightlessness with an example.

Weightlessness occurs when there is no normal reaction (e.g., astronauts in orbit). Both the astronaut and spacecraft fall together under gravity, producing zero apparent weight.

Free Fall & Motion under Gravity

Q21. What is free fall?

Free fall is motion under gravity only (neglecting air resistance). The acceleration of a freely falling body is g downward.

Q22. Write three kinematic equations used for motion under constant g.

v = u + g t, s = u t + ½ g t², v² = u² + 2 g s, where acceleration is g downward.

Q23. If an object is thrown up with speed u, what is its acceleration during upward motion?

Acceleration is constant and equal to −g (downward), which slows the upward motion until velocity becomes zero at maximum height.

Q24. How do you find time of flight for vertical motion?

Use kinematic equations. For upward throw and return time: total time = 2u / g (ignoring air resistance).

Q25. Explain why heavier objects and lighter objects fall with same acceleration in vacuum.

Gravitational acceleration is independent of object mass because both gravitational force and inertia scale with mass, canceling out: a = F/m = g.

Satellites, Orbital Motion & Escape Velocity

Q26. What provides centripetal force for a satellite in circular orbit?

Gravitational attraction between Earth and satellite provides the centripetal force needed for circular motion.

Q27. Derive orbital speed for a circular orbit (brief).

Equate centripetal force m v² / r to gravitational force G m M / r². Cancel m and solve: v = √(GM / r).

Q28. Write the formula for orbital period T.

For circular orbit: T = 2π √(r³ / GM), where r is orbit radius and M is Earth’s mass.

Q29. Define escape velocity.

Escape velocity is the minimum speed needed to move far away from Earth’s gravity without further propulsion: vₑ = √(2GM / R) at surface.

Q30. Why is atmosphere ignored in calculating escape velocity?

Escape velocity formula assumes no air resistance and no energy loss. In reality, atmosphere causes drag, so practical requirements differ.

Kepler’s Laws & Related Ideas

Q31. State Kepler’s third law in simple form.

For planets orbiting the Sun, the square of the orbital period is proportional to the cube of the semi-major axis: T² ∝ r³.

Q32. Is Newton’s law of gravitation universal? Explain.

Yes. It applies to all masses everywhere — from everyday objects to planets and stars — hence 'universal'.

Q33. How does orbital speed vary with orbital radius?

Orbital speed decreases as radius increases: v ∝ 1 / √r, from v = √(GM / r).

Q34. What is a geostationary satellite?

A satellite that orbits above the equator with a 24-hour period, appearing fixed relative to a point on Earth’s surface.

Q35. Can two satellites at different radii have the same period?

No for circular orbits around the same central body: from T² ∝ r³, period changes with radius.

Short Numericals & Calculation Concepts

Q36. Calculate weight of 5 kg mass on Earth (use g = 9.8 m/s²) — method only.

Use W = m g. Substitute m = 5 kg and g = 9.8 to get weight in newtons (W = 49 N approx.).

Q37. How to find time taken for a body dropped from height h to reach ground?

Use s = ½ g t² with s = h, solve for t: t = √(2h / g).

Q38. If g on Moon ≈ 1/6 g on Earth, how does weight change for same mass?

Weight on Moon is ≈ one-sixth of weight on Earth because W = m g and g is smaller on Moon.

Q39. A satellite orbits at radius r. How would you show that its centripetal acceleration equals g at that r?

Centripetal acceleration a = v² / r. With v = √(GM / r), substitute to get a = GM / r² which equals g at that radius.

Q40. What assumption is commonly made when using g = GM/R² for Earth?

Assume Earth is a perfect sphere with mass concentrated at its centre (point-mass approximation) and neglect rotation and atmosphere for basic calculations.

Conceptual Questions & Applications

Q41. Explain the superposition principle for gravitational forces.

Net gravitational force on a mass due to multiple bodies is the vector sum of individual gravitational forces from each body (superposition principle).

Q42. Why do tides occur (brief link to gravitation)?

Tides arise due to differential gravitational pull of Moon and Sun on different parts of Earth; variation in gravitational force across Earth leads to tidal bulges.

Q43. How does rotation of Earth affect measured weight?

Centrifugal effect due to Earth's rotation slightly reduces apparent weight, especially at the equator compared to poles.

Q44. Why do astronauts feel weightless in an orbiting spacecraft even though gravity acts on them?

They are in continuous free fall around Earth along with the spacecraft, so there is no normal reaction; hence they feel weightless though gravity still acts.

Q45. A stone dropped into a well speeds up. Which physical law explains this?

Newton’s laws and constant acceleration under gravity explain the stone’s increasing speed; specifically kinematic equations with acceleration g describe motion.

Final Revision Points & Exam Tips

Q46. List three formulae every student must memorise from this chapter.

F = G m₁ m₂ / r², g = GM / r², and v = √(GM / r) (orbital speed).

Q47. How should diagrams be used in answers for this chapter?

Draw neat labelled diagrams: free-body diagrams for weights, orbit sketches for satellites, and centre-to-centre distance illustrations for gravitational forces.

Q48. What is a good strategy to solve numericals under exam conditions?

Write known formulas, substitute values with units, simplify stepwise, and check units at the end. Show intermediate steps for clarity.

Q49. Give one common trick question students face in this chapter.

Comparing mass and weight in different locations (Earth vs Moon) — students often forget to multiply by correct g values; use W = mg each time.

Q50. One final exam tip for quick revision.

Make flashcards of formulas and key concept definitions, practice 10–12 numericals (free fall, variation of g, orbital speed) and revise diagrams.

Note: These 50 Short Answer Questions are prepared strictly as per the NCERT syllabus for Class 9 Physics — Chapter 10: Gravitation. Use them for chapter tests, revision notes and quick practice before exams.