Grade 9th : Gravitation

1. Law of Universal Gravitation

• Define Newton’s Law of Universal Gravitation.
• State the formula: F = G·M·m / r²
• Solve numerical problems using the universal law.
• Compare gravitational forces between different pairs of objects.
• Justify how Newton’s law helped understand celestial motion.
2. Newton’s Third Law and Gravitation
• Explain that gravitational forces work in action-reaction pairs (Newton’s Third Law).
• Analyze how gravitational interaction between two bodies is mutual.
• Evaluate a situation where mass difference leads to different observable effects.
3. Mass of Earth
• Describe how Earth’s mass can be calculated from gravitational equations.
• Calculate the Earth’s mass using gravitational force formula.
• Propose a simple model or setup to estimate mass of Earth.
4. Gravitational Field
• Define gravitational field and field strength (g).
• Explain how gravitational field is a force field (non-contact force).
• Calculate field strength using g = F / m and g = GM / r²
• Illustrate field lines around a massive body (e.g., Earth).
5. Gravitational Field Strength (Free-fall Acceleration)
• State the value of g at Earth’s surface.
• Investigate the effect of altitude and mass on weight.
• Predict the behavior of an object’s motion on another planet with different g.
6. Variation of ‘g’ with Altitude
• Explain why and how g decreases with altitude.
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• Calculate g at different heights using g = G
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• Interpret how this affects satellite orbits and projectile motion.
7. Motion of Artificial Satellites
• Describe the motion of artificial satellites in circular orbits.
• Calculate satellite speed using v = √ GM /r
• Solve problems involving satellite altitude and orbital speed.
• Assess the importance of satellites in communication and space science.
• Design a conceptual model for launching and maintaining a satellite in orbit.
Multiple Choice Questions (MCQs)
(Options are given in one line: A, B, C, D)
1. Newton’s law of gravitation is applicable to:
A) Only Earth B) Only Moon C) All objects with mass D) Only satellites
2. The force of gravity is:
A) Repulsive B) Attractive C) Electrostatic D) Magnetic
3. The value of G is:
A) 6.67×10⁻⁸ Nm²/kg² B) 9.8 m/s² C) 6.67×10⁻¹¹ Nm²/kg² D) 3×10⁸ m/s
4. Gravitational force between two objects increases when:
A) Their distance increases B) Their masses decrease C) Their masses increase D) None of these
5. What is the SI unit of gravitational constant G?
A) Nm/kg B) Nm²/kg² C) m/s² D) kg.m/s²
6. Weight of an object is:
A) Scalar quantity B) Vector quantity C) Constant D) Same as mass
7. The value of acceleration due to gravity (g) on Earth is:
A) 10 m/s² B) 9.8 m/s² C) 9.8 km/s² D) 6.67 m/s²
8. Value of g changes with:
A) Mass B) Volume C) Altitude D) Shape
9. What happens to the value of g as we go higher above Earth’s surface?
A) Increases B) Remains constant C) Decreases D) Becomes zero
10. Weightlessness is experienced when:
A) No gravity acts B) In vacuum C) In free fall D) In water
11. Gravitational force is weakest:
A) On mountains B) On Earth’s surface C) Between galaxies D) At poles
12. The mass of an object:
A) Changes with altitude B) Increases with speed C) Remains constant D) Depends on temperature
13. Gravitational force between two masses is directly proportional to:
A) Their speed B) Their product of masses C) Their distance D) Their volume
14. Newton’s third law is satisfied in gravity because:
A) Objects don’t react B) Both masses attract each other equallyC) Only Earth pulls B) Gravity is one-sided
15. Gravitational field strength (g) is:
A) Force per unit area B) Force per unit mass C) Mass per unit force D) None of these
16. Weight of an object on moon is:
A) Zero B) Same as Earth C) More than Earth D) Less than Earth
17. If mass of Earth becomes half, the gravitational force will:
A) Double B) Become half C) Remain same D) Be zero
18. A satellite moves in circular orbit due to:
A) Centrifugal force B) Inertia C) Gravitational pull D) Friction
19. Mass of Earth can be calculated using:
A) Newton’s second law B) Newton’s first law C) Law of gravitation D) Archimedes’ principle
20. Which of the following is a field force?
A) Friction B) Gravity C) Tension D) Contact
21. SI unit of weight is:
A) Newton B) Joule C) kg D) Pascal
22. Weight of a body is maximum at:
A) Poles B) Equator C) Space D) Inside Earth
23. Which of the following is not a unit of force?
A) N B) dyne C) kg D) lb
24. The cause of centripetal force in satellites is:
A) Gravity B) Magnetic force C) Friction D) Nuclear force
25. A geostationary satellite completes one revolution in:
A) 12 hours B) 48 hours C) 24 hours D) 1 hour
26. The formula for gravitational force is:
A) F = Gm/r² B) F = Gm₁m₂/r C) F = Gm₁m₂/r² D) F = ma
27. The value of G is:
A) Same everywhere B) Varies on Earth C) Depends on mass D) Zero in vacuum
28. If the radius of Earth becomes double, g will:
A) Increase B) Become half C) Become one-fourth D) Double
29. The free-fall acceleration is:
A) Constant B) Depends on mass C) Directionless D) Always zero
30. Satellites move in orbit due to:
A) Balanced forces B) Constant thrust C) Unbalanced gravity D) Zero mass
31. The object in orbit is:
A) In vacuum B) Not accelerating C) Constantly falling toward Earth D) Not under gravity
32. Weight is:
A) Independent of gravity B) Equal to mass C) Due to gravity D) Always zero
33. The force that keeps Moon in orbit is:
A) Magnetic B) Electric C) Nuclear D) Gravity
34. G is called:
 A) Local gravity B) Universal constant C) Specific weight D) Space constant
35. A falling object has:
A) Increasing mass B) Increasing weight C) Constant g D) Decreasing g
36. A 5 kg object weighs:
A) 49 N B) 50 N C) 5 N D) 25 N
37. Gravitational field is:
A) Uniform always B) Same everywhere C) Strong near massive objects D) Weak on surface
38. The direction of weight is:
A) Toward Moon B) Upward C) Toward center of Earth D) Horizontal
39. Orbital speed depends on:
A) Object mass B) Radius and Earth’s mass C) Shape of object D) Temperature
40. As we go above Earth’s surface, gravity:
A) Increases B) Decreases C) Remains constant D) First increases then decreases
Conceptual Questions
1. Why does a heavier and lighter object fall at the same rate in vacuum?
2. How does Newton’s third law apply to gravitational force between Earth and a falling apple?
3. Why does your weight decrease as you go higher above Earth?
4. If Earth’s mass doubled, what would happen to the weight of objects on its surface?
5. Why do astronauts feel weightless in orbit despite Earth’s gravity acting on them?
6. If Moon had no mass, would it still orbit Earth? Why or why not?
7. What causes the gravitational force to act over long distances?
8. Why does the Moon not fall into the Earth despite Earth’s gravity pulling it?
9. If gravitational field strength is zero at the center of Earth, what force acts there?
10. How would your weight change if you moved to a planet with higher g?
11. Why does g vary with location on Earth (e.g., poles vs. equator)?
12. If gravitational force is always attractive, how are artificial satellites kept from crashing?
13. Can a gravitational field be created in a laboratory? Why or why not?
14. What would happen to Earth’s orbit if the Sun suddenly lost half its mass?
15. Why does gravitational potential energy become more negative as you move closer to Earth?
16. How does Earth’s rotation affect the shape of the gravitational field?
17. Why does g not depend on the falling object’s mass?
18. Can two objects repel each other gravitationally? Why or why not?
19. How does a geostationary satellite stay above the same point on Earth?
20. Why is G a universal constant, but g is not?
LONG QUESTIONS INCLUDING NUMERICAL PROBLEMS
Theory-Based Long Questions
1. State and explain Newton’s Law of Universal Gravitation.
Derive the mathematical formula and explain each term with units and significance.
2. Differentiate between mass and weight.
Also explain why weight changes from place to place but mass does not.
3. What is a gravitational field?
Describe its characteristics and give examples of how it acts as a field of force.
4. What is the significance of Newton’s Law of Gravitation in satellite motion?
Explain the role of gravitational force in keeping satellites in orbit.
5. Explain how gravitational forces are consistent with Newton’s Third Law.
Give real-life examples to support your explanation.
6. Describe the variation of acceleration due to gravity (g) with altitude.
Derive the formula and explain how g decreases with height.
7. What is weightlessness?
Explain the concept with reference to free-falling objects and orbiting satellites.
8. Discuss the importance of artificial satellites.
Explain how gravitational force controls their motion.
9. Why do astronauts feel weightless in space even though gravity is still acting on them?
Explain using the concept of free fall and orbital motion.
10. Explain how to determine the mass of Earth using Newton’s Law of Gravitation.
Derive the expression and discuss the assumptions made.
12. Find the weight of a 50 kg object on the Moon, where g = 1.6 m/s².
Also compare it to its weight on Earth.
13. A body weighs 500 N on the surface of Earth.
Find its mass and weight on a planet where g = 3.5 m/s².
14. Calculate the gravitational field strength (g) at a point 640 km above Earth’s surface.
(Radius of Earth = 6400 km, G = 6.67 ×10-11M, M = 6.0 ×1024kg
15. A satellite revolves around Earth in a circular orbit of radius 7000 km.
Find the orbital speed of the satellite.
(G = 6.67 ×10-11M = 6.0 ×1024kg
16. Calculate the value of g at the surface of the Moon.
(Mass of Moon = 7.4 ×1022kg, radius = 1.74 ×106 m
17. The gravitational force between two objects is 400 N.
If the distance between them is doubled, what will be the new force?
18. A 1000 kg satellite is moving in a circular orbit 500 km above the Earth’s surface.
Find:
a) Gravitational force acting on it
b) Orbital speed
19. Derive the formula to calculate g using Newton’s Law of Gravitation.
Use values for G, M, and R to compute g.
20. A 70 kg astronaut is orbiting Earth. What is his weight:
a) On Earth’s surface?
b) In a satellite orbiting 400 km above Earth?
Compare and explain the difference.