NCERT Solutions Class 9 Science Chapter 9 - Gravitation

Gravitation – Important Questions with Answers

1. What is gravitation?
Answer: Gravitation is the force of attraction between any two objects in the universe.

2. State Newton’s Universal Law of Gravitation.
Answer: Every object attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
F=Gm1m2r2F=Gr2m1​m2​​

3. What is the value of the universal gravitational constant (G)?
Answer:
G = 6.674 × 10⁻¹¹ N·m²/kg²

4. What is the SI unit of G?
Answer: Newton metre² per kilogram² (N·m²/kg²)

5. What factors affect the gravitational force between two objects?
Answer:

• Masses of the objects

• Distance between the objects

6. Why is gravitational force called a universal force?
Answer: Because it acts between all objects in the universe, irrespective of their size or distance.

7. What is free fall?
Answer: When an object falls under the influence of gravitational force alone, it is said to be in free fall.

8. What is acceleration due to gravity (g)?
Answer: It is the acceleration produced in a body due to the gravitational force of the Earth.

9. What is the value of g on Earth?
Answer: 9.8 m/s²

10. Does the value of g remain constant everywhere on Earth?
Answer: No, it varies slightly with altitude and location (e.g., higher at poles than at the equator).

11. What is the relation between g and G?
Answer:
g=GMR2g=R2GM​
Where M = mass of Earth, R = radius of Earth.

12. Define mass and weight.
Answer:

• Mass is the amount of matter in a body (constant).

• Weight is the force with which Earth attracts a body (variable).
  $W=mg$

13. What is the SI unit of weight?
Answer: Newton (N)

14. What is the SI unit of mass?
Answer: Kilogram (kg)

15. Does weight change with location?
Answer: Yes, weight changes with the value of g, which varies at different places.

16. Why do objects appear weightless in a spacecraft?
Answer: Because they are in free fall, experiencing zero apparent weight due to microgravity.

17. What is the formula for gravitational force?
Answer:
F=Gm1m2r2F=Gr2m1​m2​​

18. What is the difference between mass and weight?
Answer:

• Mass is scalar, weight is vector.

• Mass is constant, weight depends on gravity.

• Mass is measured in kg, weight in N.

19. What is the weight of a 10 kg object on Earth?
Answer:
Weight = mg = 10 × 9.8 = 98 N

20. What happens to the weight of an object on the Moon?
Answer: It becomes 1/6 of its weight on Earth due to lesser gravity on the Moon.

21. State Archimedes’ principle.
Answer: When a body is immersed in a fluid, it experiences an upward buoyant force equal to the weight of the fluid displaced by it.

22. What is buoyancy?
Answer: The upward force exerted by a fluid on a submerged object is called buoyancy.

23. What is the reason for the buoyant force?
Answer: The pressure of a fluid increases with depth, creating a net upward force on the submerged object.

24. Why do objects float or sink in water?
Answer:

• If the object’s density is less than water, it floats.

• If more, it sinks.

25. State the formula for the pressure exerted by a liquid column.
Answer:
$P=h\rho g$
Where h = height of the column, ρ = density of fluid, g = acceleration due to gravity.