Gravitation – Very Short Answer Type Questions

50 Very Short Answer Type Questions & Answers — Gravitation

Instructions: Each answer is concise and aligned to NCERT-level expectations — ideal for quick revision and board exam practice.

1. Q1: State Newton’s Universal Law of Gravitation.
   Every point mass attracts every other point mass 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.
2. Q2: What does the constant G represent?
   G is the universal gravitational constant, value ≈ 6.67×10⁻¹¹ N·m²/kg², determining the strength of gravitational interaction.
3. Q3: Is gravitational force attractive or repulsive?
   Gravitational force is always attractive between masses.
4. Q4: How does gravitational force change if distance is doubled?
   If distance r is doubled, force becomes one-fourth (since F ∝ 1/r²).
5. Q5: How does gravitational force change if one mass is tripled?
   If one mass is tripled, gravitational force triples (force ∝ product of masses).

6. Q6: Define gravitational field (intensity).
   Gravitational field at a point is gravitational force experienced per unit mass placed there: g = F/m.
7. Q7: Give the formula for acceleration due to gravity at distance r from Earth's centre.
   g = GM / r², where M is Earth's mass and G is the gravitational constant.
8. Q8: Typical value of g at Earth’s surface?
   Approximately 9.8 m/s² (often approximated as 9.8 or 10 m/s² depending on the problem).
9. Q9: What is the direction of g near Earth's surface?
   g is directed towards the centre of Earth (downward).
10. Q10: Does g depend on mass of the object?
    No. g depends on Earth’s mass and radius, not on the test object's mass.

11. Q11: How does g change with altitude?
    g decreases with altitude: g' = g (R/(R+h))², where h is height above surface.
12. Q12: How does g change with small altitude (approx)?
    For small h, g' ≈ g (1 − 2h/R) (approximate linear decrease).
13. Q13: How does g vary with depth inside Earth (uniform density)?
    For depth d, gdepth = g (1 − d/R), decreasing linearly with depth.
14. Q14: Why does g decrease inside Earth?
    Because only the mass enclosed within radius (R−d) contributes; effective attracting mass decreases as you go deeper.
15. Q15: At Earth's centre, what is g?
    At centre g = 0 (net gravitational field cancels in all directions).

16. Q16: Define mass.
    Mass is the amount of matter in a body, a scalar quantity measured in kg; it is invariant.
17. Q17: Define weight.
    Weight is the gravitational force on a body: W = m g, measured in newtons (N).
18. Q18: How do mass and weight differ?
    Mass is intrinsic; weight depends on local g and changes with location (e.g., Moon vs Earth).
19. Q19: What is apparent weight?
    Apparent weight is the normal reaction on a body in a non-inertial frame; equals m(g ± a) depending on acceleration.
20. Q20: What causes weightlessness?
    Weightlessness occurs when a body and its support accelerate equally under gravity (e.g., free-falling elevator or orbiting spacecraft), giving zero normal reaction.

21. Q21: What is free fall?
    Free fall is motion under gravity only, with negligible air resistance and acceleration equal to g downward.
22. Q22: Which kinematic equation is used for distance under constant g?
    s = ut + ½ g t² (with acceleration g directed downward).
23. Q23: If an object is dropped from rest, what is its initial velocity?
    Initial velocity u = 0.
24. Q24: What is the velocity after time t in free fall from rest?
    v = g t.
25. Q25: Give the relation between v² and displacement s under constant g.
    v² = u² + 2 g s.

26. Q26: What provides centripetal force for a satellite in circular orbit?
    Gravitational force between Earth and satellite provides the required centripetal force.
27. Q27: Formula for orbital speed in a circular orbit of radius r?
    v = √(GM / r), where M is Earth's mass.
28. Q28: Formula for orbital period T of a circular orbit?
    T = 2π √(r³ / GM).
29. Q29: Define escape velocity.
    Escape velocity is the minimum speed required to escape Earth’s gravitational pull without further propulsion: vₑ = √(2GM / R).
30. Q30: Numerical value of Earth’s escape velocity (approx)?
    Approximately 11.2 km/s at Earth's surface (neglecting atmosphere).

31. Q31: State Kepler’s third law in simple form.
    Square of orbital period is proportional to cube of semi-major axis: T² ∝ r³ (for circular orbits use radius r).
32. Q32: Is Newton’s law of gravitation applicable only to planets?
    No — it applies universally to all masses, from particles to galaxies.
33. Q33: What happens to orbital speed if orbit radius increases?
    Orbital speed decreases with increasing radius: v ∝ 1/√r.
34. Q34: What is a geostationary satellite (brief)?
    A satellite that orbits Earth with period 24 hours over the equator, appearing stationary relative to Earth’s surface.
35. Q35: Is weightlessness the same as zero gravity?
    Not exactly — weightlessness occurs because no normal force; gravity still acts but occupants accelerate with the spacecraft.

36. Q36: Two masses attract each other with force F. If both masses are doubled, what is new force?
    Force becomes four times (since product of masses increases by factor 4): 4F.
37. Q37: If gravitational constant G were larger, what happens to g on Earth?
    g would increase proportionally, making gravity stronger at all distances.
38. Q38: Does gravitational force obey superposition?
    Yes — net gravitational force is vector sum of forces due to individual masses.
39. Q39: Why do we use centre-to-centre distance in gravitational formula?
    Because gravitational attraction acts between centres of mass; r is distance between centres for point-mass approximation.
40. Q40: For two identical spheres touching each other, what r do we use?
    Use distance between centres equal to sum of their radii (r = r₁ + r₂).

41. Q41: If g = 9.8 m/s², what is weight of 2 kg mass on Earth?
    W = m g = 2 × 9.8 = 19.6 N.
42. Q42: If g on Moon ≈ 1/6 Earth, weight of 12 kg object on Moon?
    Weight on Moon = 12 × (9.8/6) ≈ 12 × 1.63 ≈ 19.6 N (approx).
43. Q43: If g at surface is 9.8 m/s², what is g at altitude equal to Earth's radius?
    At r = 2R, g' = g/4 ≈ 9.8/4 = 2.45 m/s².
44. Q44: An object in orbit moves in a circle; what force keeps it in orbit?
    Gravitational force acts as centripetal force keeping it in circular orbit.
45. Q45: Why is atmosphere ignored in escape velocity formula?
    Escape velocity formula assumes no air resistance and no further propulsion; atmosphere adds drag making required speed larger in practice.

46. Q46: What is the relation between gravitational potential energy and distance (qualitative)?
    Gravitational potential energy decreases (becomes more negative) as two masses come closer; it varies inversely with distance.
47. Q47: If a body is thrown vertically upward, what is acceleration during its motion (neglect air resistance)?
    Acceleration is constant and downward equal to −g throughout the motion.
48. Q48: How does weight change in a uniformly accelerating elevator moving upward with acceleration a?
    Apparent weight increases to m(g + a).
49. Q49: How does weight change in an elevator accelerating downwards with acceleration a?
    Apparent weight decreases to m(g − a); becomes zero if a = g (free fall).
50. Q50: Give one exam tip for this chapter.
    Memorise key formulas, practice variation-of-g and orbital problems, and draw neat free-body diagrams for conceptual questions.

Note: These 50 very short questions and answers are crafted strictly as per NCERT-level topics for Class 9 Physics — Chapter 10: Gravitation. Use them for quick revision, flashcards, and last-minute board exam preparation.