Looking for reliable SEBA Class 9 Science Chapter 10 (Gravitation) extra questions and answers for the 2026–27 academic year? Access important short questions, conceptual explanations on Universal Law of Gravitation, mass vs. weight, free fall, buoyancy, Archimedes’ Principle, and worked numericals to score top marks.
Extra question and answer
- What is gravitation?
Answer: Gravitation is the force with which every object in the universe attracts every other object.
2. State the Universal Law of Gravitation.
Answer:It states that every object attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
3. What is centripetal force?
Answer:It is the force acting towards the center of a circular path that keeps an object moving in a circle.
4. Why does an apple fall to the ground?
Answer:Because the Earth attracts the apple with gravitational force.
5. Does the apple attract the Earth?
Answer:Yes, but we cannot notice Earth’s movement because Earth’s mass is extremely large compared to the apple.
6. Define free fall.
Answer:Free fall is the motion of an object when only gravity acts on it.
7. What is the value of acceleration due to gravity (g) on Earth?
Answer:Approximately g = 9.8 m/s^2.
8. Write the formula for gravitational force.
Answer:F = G * (M * m / d^2)
9. What is weight?
Answer:Weight is the force with which Earth attracts an object towards its center. W = m * g.
10. Does mass change from place to place?
Answer:No, mass remains constant everywhere in the universe.
11. Why does weight change from place to place?
Answer:Because weight depends on acceleration due to gravity (g), and g varies depending on location.
12. What is thrust?
Answer:Thrust is the force acting perpendicular to a surface.
13. Write the formula for pressure.
Answer:Pressure = Thrust / Area
14. Why does a person sink more while standing on sand?
Answer:Because the entire weight acts on a smaller surface area (feet), resulting in higher pressure.
15. What is buoyancy?
Answer:Buoyancy is the net upward force exerted by a fluid on an object immersed in it.
16. Why do objects feel lighter in water?
Answer: Objects feel lighter in water because the water exerts an upward buoyant force (upthrust) on the object, which opposes gravity and reduces its apparent weight.
17. When does an object float?
Answer: An object floats when the upward buoyant force exerted by the liquid is greater than or equal to the total weight of the object (or when its density is less than or equal to the fluid’s density).
18. State Archimedes’ Principle.
Answer: Archimedes’ Principle states that when a body is immersed fully or partially in a fluid, it experiences an upward buoyant force that is equal to the weight of the fluid displaced by it.
19. Name one device based on Archimedes’ principle.
Answer: Lactometer (used to test the purity of milk) or Hydrometer (used to measure the density of liquids).
20. What happens to pressure with depth in a fluid?
Answer: Pressure increases as depth increases because the weight of the fluid column above increases with depth.
Class 9 Science Chapter 10 Important Short Questions
21. Why is gravitational force called a universal force?
Answer: Gravitational force is called a universal force because it exists between every pair of objects in the universe, regardless of their size, mass, or location.
22. Explain why Earth does not move toward an apple when it falls.
Answer: According to Newton’s second law, acceleration is inversely proportional to mass (a = F / m). Since Earth has an enormously large mass compared to the apple, the acceleration produced in the Earth by the gravitational force is extremely small and unnoticeable.
23. Write the formula for value of g near Earth.
Answer: g = (G * M) / R^2, where G is the universal gravitational constant, M is the mass of the Earth, and R is the radius of the Earth.
24. Why is g maximum at poles and minimum at equator?
Answer: Earth is not a perfect sphere and is flattened at the poles. Since the radius at the poles (R_p) is smaller than the radius at the equator (R_e), and g is inversely proportional to R^2, the value of g is maximum at the poles and minimum at the equator.
25. Why do all bodies fall with the same acceleration in free fall?
Answer: All bodies fall with the same acceleration because acceleration due to gravity (g = GM / R^2) depends only on the mass and radius of the Earth, not on the mass of the falling object.
26. Differentiate between mass and weight.
Answer: Mass is the amount of matter in an object, measured in kg, and remains constant everywhere.
Weight is the gravitational force acting on the object (W = m * g), measured in Newtons (N), and changes depending on the value of g.
27. What happens to weight on the Moon? Explain.
Answer: An object’s weight on the Moon becomes 1/6th of its weight on Earth because the Moon’s mass and radius produce a gravitational acceleration that is only 1/6th of Earth’s gravity. However, the object’s mass remains unchanged.
28. Explain why lying down reduces pressure on sand.
Answer: Pressure is inversely proportional to area (Pressure = Force / Area). When lying down, the total weight of the body is distributed over a much larger surface area, significantly reducing the pressure exerted on the sand and preventing sinking.
29. What is pressure in fluids?
Answer: Pressure in fluids is the force exerted per unit area by the fluid particles as they collide with the surface of an object or the walls of a container. Unlike solids, fluids transmit this pressure equally in all directions.
30. What is buoyant force? What factors affect it?
Answer: Buoyant force (upthrust) is the net upward force exerted by a fluid on an object immersed in it. It depends on:
- The density of the fluid.
- The volume of the fluid displaced (volume of the submerged portion of the object).
- Acceleration due to gravity (g).
SEBA Class 9 Science Chapter 10 Practice Numericals
31. When does an object sink in a fluid?
Answer: If its density is greater than the density of the fluid.\
32. State two applications of Archimedes’ principle.
Answer: (1) Hydrometers (liquid density)
(2) Ships and submarines float using displacement of water.
33. Why does an iron ship float but a small iron nail sinks?
Answer: The ship’s shape increases volume and displaces more water, creating enough upthrust to float.
34. Write the equations of motion under gravity.
Answer: v = u + gt
s = ut + ½gt²
v² = u² + 2gs
35. Why does a freely falling body gain speed?
Answer: Because gravity continuously accelerates it downward.
36. What is inertia? On what does it depend?
Answer: Inertia is resistance to change in motion. It depends on mass.
37. Why does a stone feel lighter in water?
Answer: Because water exerts upward buoyant force, reducing its apparent weight.
38. Define density in relation to floating and sinking.
Answer: Objects with density less than fluid float; greater density sinks.
39. What happens to buoyant force if the density of liquid increases?
Answer: Buoyant force increases because more weight of fluid is displaced.
40. Why does pressure increase with depth in liquids?
Answer: Because the weight of the liquid above increases as depth increases.
Gravitation Class 9 SEBA Extra Question Answer
41. Explain the Universal Law of Gravitation with an example.
Answer: The law states F = G(Mm/d²).
Example: Earth attracts the Moon. Due to this force, the Moon stays in orbit. Similarly, Earth attracts falling objects like apples.
42. Derive the expression for weight of an object on Earth.
Answer: Gravitational force F = GMm/R².
Weight W = mg.
Comparing, g = GM/R²,
So W = m × g = m(GM/R²).
43. Calculate g using G, M, and R. Show steps.
Answer: Using formula g = GM/R².
Substituting values:
G = 6.7 ×10⁻¹¹, M = 6×10²⁴, R = 6.4×10⁶
g ≈ 9.8 m/s².
44. Discuss motion during free fall using equations of motion.
Answer: During free fall, u may be zero or non-zero.
Acceleration = g.
Equations used:
v = u + gt (velocity increases)
s = ut + ½gt² (distance increases)
v² = u² + 2gs.
45. Explain with reason:
Answer: Mass is constant but weight changes.
Mass is amount of matter → does not depend on gravity.
Weight = mg → depends on acceleration due to gravity.
Since g varies on Earth, weight varies.
46. How does Archimedes’ principle help ships float?
Answer: A ship is shaped to displace a large volume of water.
Upthrust = weight of water displaced.
When upthrust equals ship’s weight, ship floats.
47. Explain buoyancy in detail with an example.
Answer: When an object is immersed in water, water pushes it upward.
Example: A stone in water appears lighter because buoyant force reduces its apparent weight.
48. Explain why objects float or sink using density.
Answer: If ρobject < ρfluid → upthrust ≥ weight → floats If ρobject > ρfluid → weight > upthrust → sinks.
49. Describe an experiment showing upthrust using a spring balance.
Answer: Weigh object in air using spring balance.
Then immerse object in water.
Reading decreases → shows upward buoyant force.
Once fully immersed → reading becomes constant.
50. What is pressure? Explain factors affecting pressure with examples.
Answer: Pressure = Force/Area.
More area → less pressure (lying on sand).
Less area → more pressure (standing on sand).
In fluids, depth increases → pressure increases.
Class 9 Science Chapter 10 Important Questions SEBA 2026-27
51. Compare centripetal force and gravitational force.
Answer: Centripetal: Acts towards centre of circular path.
Gravitational: Acts between masses.
Moon stays in orbit due to gravitational force providing centripetal force.
52. Explain why astronauts feel weightless in orbit.
Answer: Because they are in continuous free fall with the spacecraft.
Their weight becomes effectively zero.
53. Describe the discovery of Archimedes’ principle.
Answer: Archimedes noticed water overflowing when he entered a bathtub.
He understood that weight of displaced water equals upward force.
He then tested the purity of gold using this principle.
54. Why does pressure increase in deeper parts of the sea?
Answer: More water is above deeper layers.
So weight of water increases → pressure increases continuously with depth.
55. Explain the relation between upthrust and volume displaced.
Answer: Upthrust = weight of displaced fluid = ρ × g × Vsubmerged.
More volume submerged → more upthrust.
56. How do submarines rise and sink?
Answer: They use ballast tanks.
Fill tanks with water → density increases → sinks.
Fill tanks with air → density decreases → rises.
57. Explain with formula how weight changes on Moon.
Answer: g_moon = GM_moon/R_moon² ≈ 1.62 m/s²
W_m = m × g_moon
Since g_moon is 1/6 of Earth’s, weight becomes 1/6.
58. Explain fluid pressure in all directions with an example.
Answer: In a water-filled container, holes on all sides let water escape equally.
This shows pressure acts in all directions.
59. What happens to an object of equal density when placed in water?
Answer: If ρobject = ρwater, object remains suspended in water.
Neither sinks nor floats, because weight = upthrust.
60. Explain why hollow objects can float even if made of heavy materials.
Answer: Hollow objects like ships enclose air, increasing overall volume and reducing density.
Thus average density becomes less than water, so they float.
SEBA Class 9 Science Chapter 10 Extra Questions
🔬 Updated Extra Questions Guide Notice: This page features an extensive collection of SEBA Class 9 Science Chapter 10 (Gravitation) Extra Questions and Detailed Answers updated for the 2026–27 academic session. All conceptual notes, acceleration due to gravity ($g$) numericals, and buoyancy explanations strictly follow the latest revised Assam Board (SEBA) curriculum.
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