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Gravitation in very Easy Notes

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Gravitation Class 9 Physics – Complete Notes, Summary, MCQs & Important Questions

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Comprehensive Gravitation Class 9 notes, summary, MCQs, and important questions for exam preparation. Learn easily and score high in physics exams.

Introduction to Gravitation

Gravitation is a fundamental force of nature that governs the motion of all objects in the universe. It is the attractive force between any two masses, acting along the line joining them. The concept of gravitation is essential in understanding planetary motion, tides, satellite motion, and weight variations. This chapter explores the laws of gravitation, acceleration due to gravity, mass vs weight, free fall, and orbital motion, making it crucial for Class 9 physics exams.

Understanding gravitation not only helps in scoring well in school exams but also forms the foundation for higher studies in physics and astronomy. It is a force that keeps planets in orbit, causes objects to fall on Earth, and explains the behavior of celestial bodies.

Short Notes (Bullet Points)

  • Gravitation: Universal force of attraction between two masses.
  • Newton’s Law of Gravitation:
  • G: Universal Gravitational Constant ()
  • Acceleration due to gravity (g):
  • Weight: Force of gravity on a body;
  • Free fall: Motion under gravity only; no air resistance.
  • Mass vs Weight: Mass is constant; weight varies with location.
  • Orbit: Path of satellite/planet under gravitational pull.
  • Escape velocity: Minimum velocity to leave Earth’s gravitational field.
  • Tides: Caused by gravitational pull of Moon and Sun.

Detailed Summary of Gravitation (1000–1200 Words)

Newton’s Law of Gravitation

Newton proposed that every particle in the universe attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Formula:


F = G \frac{m_1 m_2}{r^2}
  • = Gravitational force
  • = Masses of two objects
  • = Distance between centers of masses
  • = Universal gravitational constant

Key Points:

  • Force acts along the line joining the centers of masses.
  • It is always attractive.
  • Applies universally to all matter.

Acceleration Due to Gravity

Acceleration due to gravity is the rate at which an object accelerates when falling freely under the Earth’s gravitational pull.


g = \frac{GM}{R^2}
  • = Mass of Earth
  • = Radius of Earth
  • = Universal gravitational constant

Observations:

  • Value of decreases with height and increases below Earth’s surface.
  • is approximately on Earth.

Mass and Weight

  • Mass: Quantity of matter in a body; constant everywhere; unit = kg.
  • Weight: Force exerted by gravity on a body; varies with location; unit = N.

Relation:

Free Fall

When an object falls under the influence of gravity alone (ignoring air resistance), it is in free fall.

  • Distance covered:
  • Velocity after time :

Motion of Planets and Satellites

  • Gravitational force keeps planets in orbit around the Sun and moons around planets.
  • Centripetal force required for circular motion is provided by gravity.
  • Orbital velocity:

Escape Velocity

Escape velocity is the minimum speed needed to escape Earth’s gravitational pull without further propulsion.


v_e = \sqrt{2gR} \approx 11.2 \, km/s

Tides and Gravitation

  • Tides are caused due to the differential gravitational pull of the Moon and the Sun.
  • Spring tides occur during full moon and new moon; neap tides occur during quarter moons.

Applications of Gravitation

  • Artificial satellites and GPS systems
  • Predicting planetary motion
  • Space missions and satellite launches
  • Understanding tides and ocean currents

Summary Note: Gravitation is everywhere—from falling apples to planetary motion. Understanding its laws, acceleration, mass-weight relationship, and celestial applications is vital for exams and real-world physics.

Flowchart / Mind Map (Text-based)

Gravitation
   |
   +-- Newton’s Law of Gravitation
   |      +-- Formula: F = G m1 m2 / r^2
   |      +-- Universal, Attractive, Inverse-square
   |
   +-- Acceleration Due to Gravity (g)
   |      +-- g = GM / R^2
   |      +-- Free fall equations
   |
   +-- Mass vs Weight
   |      +-- Mass constant
   |      +-- Weight varies with location (W = mg)
   |
   +-- Motion of Planets & Satellites
   |      +-- Orbital velocity v = √(GM/r)
   |      +-- Centripetal force = Gravitational force
   |
   +-- Escape Velocity
   |      +-- ve = √2gR
   |
   +-- Tides
          +-- Caused by Moon & Sun
          +-- Spring & Neap tides

Important Keywords with Meanings

KeywordMeaning
GravitationUniversal attractive force between masses
GUniversal gravitational constant
Free FallMotion under gravity alone
MassAmount of matter in a body
WeightForce of gravity on a body
OrbitPath of planet or satellite
Escape VelocityMinimum speed to escape gravitational field
TidesRise and fall of sea level due to Moon & Sun

Important Questions & Answers

Short Answer Questions (10)

  1. Define gravitation.
    Answer: Gravitation is the attractive force between any two masses in the universe.
  2. Write Newton’s law of gravitation formula.
    Answer:
  3. Difference between mass and weight.
    Answer: Mass is constant; weight varies with gravity.
  4. What is free fall?
    Answer: Motion of a body under gravity alone.
  5. Write the formula for acceleration due to gravity.
    Answer:
  6. Define escape velocity.
    Answer: Minimum speed to leave Earth’s gravity.
  7. What causes tides?
    Answer: Gravitational pull of Moon and Sun.
  8. Value of universal gravitational constant.
    Answer:
  9. Relationship between weight and mass.
    Answer:
  10. What is orbital velocity?
    Answer: Minimum velocity to keep satellite in orbit: .

Long Answer Questions (10)

  1. Explain Newton’s law of gravitation with example.
  2. Derive acceleration due to gravity formula.
  3. Discuss motion of planets using gravitation.
  4. Difference between mass and weight with examples.
  5. Explain free fall and derive s = ½ gt².
  6. Define escape velocity and derive formula.
  7. Explain tides and types of tides.
  8. Derive orbital velocity formula.
  9. Explain applications of gravitation in satellites.
  10. Discuss variation of g with height and depth.

Multiple Choice Questions

  1. Gravitation is
    a) Repulsive
    b) Attractive ✅
    c) Both
    d) None
  2. Newton’s law of gravitation formula is
    a) F = m1 + m2
    b) F = G m1 m2 / r² ✅
    c) F = ma
    d) F = mg
  3. Value of g on Earth
    a) 10 m/s²
    b) 9.8 m/s² ✅
    c) 8.9 m/s²
    d) 9.0 m/s²

Exam Tips / Value-Based Questions

  1. Always use SI units in calculations.
  2. Draw diagrams for free fall, orbit, and tides.
  3. Practice numerical problems on g, weight, orbital & escape velocity.
  4. Remember differences: mass vs weight, spring vs neap tides.
  5. Revise formulas regularly before exams.

Value-Based Questions Example:

  1. Why is understanding gravitation important for satellite launches?
    Answer: To calculate orbital and escape velocity accurately for successful satellite placement.
  2. How does gravitation affect daily life?
    Answer: Keeps us grounded, causes tides, and ensures stable planetary motion.
  3. Why weight varies but mass doesn’t?
    Answer: Weight depends on gravitational pull, which changes with location.
  4. Why free fall is studied ignoring air resistance?
    Answer: To simplify calculations and understand pure gravitational acceleration.
  5. How can tides help in renewable energy?
    Answer: By generating tidal energy for electricity.

Conclusion (SEO Friendly, 1500 Words)

Gravitation is one of the most important chapters in Class 9 physics. It is not only fundamental to understanding the motion of objects on Earth but also crucial for astronomical phenomena. This chapter teaches the laws of universal gravitation, the difference between mass and weight, the concept of free fall, and the principles behind satellite motion and tides.

By mastering gravitation, students can solve numerical problems related to weight, free fall, orbital velocity, and escape velocity with confidence. The concepts of gravitation are universally applicable—from an apple falling from a tree to the motion of planets around the Sun.

This chapter also develops analytical skills by connecting real-world observations with scientific laws, such as understanding why Moon causes tides or why satellites orbit Earth instead of falling down. Practicing MCQs, short answers, long answers, and numerical problems ensures a strong grasp of the concepts.

Students are advised to use the flowchart/mind map for revision and memorize important formulas. Understanding gravitation also prepares students for higher studies in physics, competitive exams, and practical applications like GPS, satellite communication, and space missions.

In conclusion, Gravitation Class 9 is a chapter that combines theory, calculations, and applications in a student-friendly and exam-oriented way. A strong foundation in this chapter helps not only in board exams but also in Olympiads, competitive exams, and practical physics understanding. Regular practice of MCQs, numerical problems, and value-based questions will ensure conceptual clarity and confidence in exams. Gravitation is everywhere, from falling objects on Earth to celestial motions, making it an indispensable topic for Class 9 students.

Long Answer Questions & Answers – Gravitation Class 9

1. Explain Newton’s law of gravitation with example.

Answer:
Newton’s law of gravitation states that every particle in the universe attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

Formula:


F = G \frac{m_1 m_2}{r^2}

Where:

  • = Gravitational force
  • = Masses of the objects
  • = Distance between centers
  • = Universal gravitational constant

Example:
If an apple of mass 0.2 kg is near the Earth (mass kg, radius m), the gravitational force on the apple is:


F = G \frac{m_1 m_2}{r^2} = 6.67 \times 10^{-11} \frac{(0.2 \times 6 \times 10^{24})}{(6.37 \times 10^6)^2} \approx 1.96 \, N

This is the weight of the apple.

2. Derive the formula for acceleration due to gravity.

Answer:
The gravitational force on a body of mass due to Earth of mass is:


F = G \frac{Mm}{R^2}

By Newton’s second law, , here (acceleration due to gravity).


mg = G \frac{Mm}{R^2} \implies g = \frac{GM}{R^2}

Where:

  • = Acceleration due to gravity
  • = Radius of Earth
  • = Universal gravitational constant

3. Discuss motion of planets using gravitation.

Answer:
Planets move around the Sun due to the gravitational attraction of the Sun. This force acts as a centripetal force, keeping the planet in its orbit.

Orbital velocity of a planet:


v = \sqrt{\frac{GM}{r}}

Where = mass of Sun, = orbital radius.

The balance between gravitational pull and centrifugal effect of motion keeps planets in nearly circular or elliptical orbits.

Example: Earth remains in orbit because the gravitational force of the Sun pulls it inward while its motion tends to move it outward.

4. Difference between mass and weight with examples.

Answer:

PropertyMassWeight
DefinitionAmount of matter in a bodyForce of gravity on a body
UnitkgN (Newton)
SymbolmW
VariationConstant everywhereChanges with location
ExampleMass of a person = 50 kgWeight on Earth = 50 × 9.8 = 490 N

Conclusion: Mass is intrinsic, weight depends on gravity.

5. Explain free fall and derive .

Answer:
Free fall is the motion of a body under gravity alone, ignoring air resistance.

  • Initial velocity
  • Acceleration

Equation of motion:


s = ut + \frac{1}{2}at^2


s = 0 \cdot t + \frac{1}{2} g t^2 = \frac{1}{2} g t^2

Velocity after time :

6. Define escape velocity and derive formula.

Answer:
Escape velocity is the minimum velocity required by a body to escape Earth’s gravity without further propulsion.

  • Total energy = 0 at infinity:

\frac{1}{2}mv^2 = \frac{GMm}{R} \implies v = \sqrt{\frac{2GM}{R}} = \sqrt{2gR}

Where:

  • = radius of Earth
  • = acceleration due to gravity

Escape velocity from Earth ≈ 11.2 km/s.

7. Explain tides and types of tides.

Answer:
Tides are the periodic rise and fall of sea levels due to gravitational pull of Moon and Sun.

  • Spring tide: Occurs during new moon and full moon; maximum tidal range.
  • Neap tide: Occurs during quarter moons; minimum tidal range.

Mechanism:

  • Gravitational pull causes bulges in oceans.
  • Earth’s rotation moves locations under bulges → high tide and low tide.

8. Derive orbital velocity formula.

Answer:
For a satellite of mass in circular orbit of radius around Earth of mass :

Gravitational force = Centripetal force:


\frac{GMm}{r^2} = \frac{mv^2}{r} \implies v^2 = \frac{GM}{r} \implies v = \sqrt{\frac{GM}{r}}

This is the orbital velocity.

9. Explain applications of gravitation in satellites.

Answer:

  • Artificial satellites: Earth observation, weather forecasting, communication.
  • GPS satellites: Accurate navigation using orbital motion.
  • Space exploration: Calculation of escape velocity for spacecraft.
  • Predicting tides: Based on Moon and Sun’s gravitational influence.

Gravitation helps plan satellite orbits and ensures stable motion in space.

10. Discuss variation of g with height and depth.

Answer:

  1. Variation with height :

g_h = g \left(1 - \frac{2h}{R}\right) \approx \frac{GM}{(R+h)^2}
  1. Variation with depth :

g_d = g \left(1 - \frac{d}{R}\right)

Conclusion: Acceleration due to gravity is maximum at the surface and decreases both with height above and depth below Earth.

30 Very Short Questions & Answers – Gravitation Class 9

  1. What is gravitation?
    A: Universal attractive force between two masses.
  2. Who formulated the law of gravitation?
    A: Sir Isaac Newton.
  3. State Newton’s law of gravitation.
    A: Force ∝ product of masses, ∝ 1/(distance)².
  4. Write the formula for gravitational force.
    A:
  5. What is G?
    A: Universal gravitational constant, .
  6. What is acceleration due to gravity?
    A: Rate at which a body accelerates under Earth’s gravity.
  7. Symbol for acceleration due to gravity.
    A: g
  8. Value of g on Earth.
    A:
  9. Difference between mass and weight?
    A: Mass constant, weight varies with location.
  10. Formula for weight.
    A:
  11. What is free fall?
    A: Motion of a body under gravity alone.
  12. Initial velocity in free fall?
    A: Usually zero.
  13. Distance formula in free fall.
    A:
  14. Velocity after time t in free fall?
    A:
  15. What keeps planets in orbit?
    A: Gravitational force of the Sun.
  16. Formula for orbital velocity.
    A:
  17. What is escape velocity?
    A: Minimum speed to leave Earth’s gravity.
  18. Escape velocity from Earth.
    A: 11.2 km/s
  19. Cause of tides?
    A: Gravitational pull of Moon and Sun.
  20. Types of tides?
    A: Spring and neap tides.
  21. Which tide occurs during full moon?
    A: Spring tide.
  22. Which tide occurs during quarter moon?
    A: Neap tide.
  23. Does weight vary with altitude?
    A: Yes, decreases with height.
  24. Does mass vary with location?
    A: No, mass is constant.
  25. Centripetal force for orbit comes from?
    A: Gravitational force.
  26. Unit of gravitational force.
    A: Newton (N)
  27. Why do objects fall on Earth?
    A: Due to Earth’s gravitational pull.
  28. What is a satellite?
    A: Body moving around a planet due to gravity.
  29. What is a geostationary satellite?
    A: Satellite that remains fixed relative to Earth’s surface.
  30. Relation between g and G?
    A:

Absolutely! Here’s a comprehensive set of 100 MCQs with answers for Gravitation – Class 9 Physics. I’ve ensured they are exam-oriented, diverse, and cover theory, formulas, and numericals.

100 MCQs with Answers – Gravitation Class 9

1–20: Basic Concepts

  1. Gravitation is a:
    a) Repulsive force
    b) Attractive force ✅
    c) Neutral force
    d) None
  2. Newton’s law of gravitation is:
    a) F ∝ m₁ + m₂
    b) F ∝ m₁ × m₂ / r² ✅
    c) F ∝ m₁ / m₂
    d) F ∝ r²
  3. Unit of gravitational force:
    a) kg
    b) N ✅
    c) m/s²
    d) J
  4. G stands for:
    a) Gravity
    b) Gravitational constant ✅
    c) Mass
    d) Distance
  5. Value of G:
    a) ✅
    b)
    c) 10 m/s²
    d) 1 N
  6. Acceleration due to gravity symbol:
    a) G
    b) g ✅
    c) a
    d) w
  7. Acceleration due to gravity on Earth:
    a) 8 m/s²
    b) 9.8 m/s² ✅
    c) 10 m/s²
    d) 11 m/s²
  8. Weight formula:
    a) W = m + g
    b) W = mg ✅
    c) W = m/g
    d) W = g/m
  9. Mass is:
    a) Constant everywhere ✅
    b) Varies with height
    c) Varies with shape
    d) None
  10. Weight is:
    a) Constant
    b) Depends on gravity ✅
    c) Always zero
    d) None
  11. Free fall occurs when:
    a) Air resistance acts
    b) Only gravity acts ✅
    c) Force applied
    d) Friction present
  12. In free fall, initial velocity u =
    a) 0 m/s ✅
    b) 5 m/s
    c) g m/s²
    d) 10 m/s
  13. Distance in free fall:
    a) s = ut + ½gt² ✅
    b) s = gt²
    c) s = u/g
    d) s = 2gt²
  14. Velocity after t seconds:
    a) v = u + g ✅
    b) v = gt ✅
    c) v = u + gt ✅
    d) v = g/t
  15. Formula for g in terms of G:
    a) g = GM²/R²
    b) g = GM/R² ✅
    c) g = G/MR²
    d) g = MR²/G
  16. Gravitational force acts:
    a) Perpendicular to distance
    b) Along line joining centers ✅
    c) Opposite to motion
    d) Random
  17. Law of gravitation applies to:
    a) Only Earth
    b) Only planets
    c) All matter ✅
    d) Only satellites
  18. A body weighs less on Moon because:
    a) Mass decreases
    b) Gravity is less ✅
    c) Air resistance
    d) Shape changes
  19. Moon’s gravity ≈
    a) 9.8 m/s²
    b) 1.6 m/s² ✅
    c) 0.98 m/s²
    d) 2.0 m/s²
  20. The force between two objects depends on:
    a) Mass only
    b) Distance only
    c) Mass & distance ✅
    d) Shape & density

21–40: Numerical / Formula Based

  1. Gravitational force formula:
    a) F = Gm/r²
    b) F = G m₁ m₂ / r² ✅
    c) F = mg
    d) F = m₁ m₂ / r
  2. Universal constant G: 6.67 × 10⁻¹¹ N·m²/kg². T/F?
    A: True ✅
  3. Weight on Moon W_m = ?
    A: W_m = m × g_m ✅
  4. Escape velocity formula:
    a) v = √gR
    b) v = √2gR ✅
    c) v = gR
    d) v = g/2
  5. Orbital velocity:
    a) v = √(GM/r) ✅
    b) v = √2GM/r
    c) v = GM/r²
    d) v = G/r
  6. g decreases with:
    a) Height ✅
    b) Depth
    c) Mass
    d) Radius
  7. g decreases below Earth?
    a) Linearly ✅
    b) Quadratically
    c) Constant
    d) Random
  8. Spring tide occurs:
    a) Full moon ✅
    b) New moon ✅
    c) Quarter moon
    d) None
  9. Neap tide occurs:
    a) Quarter moon ✅
    b) Full moon
    c) New moon
    d) None
  10. Formula for weight:
    a) W = mg ✅
    b) W = g/m
    c) W = m/g
    d) W = mg²
  11. A 10 kg body weight on Earth:
    A: W = 10 × 9.8 = 98 N ✅
  12. Gravitational force between two 1 kg bodies at 1 m:
    A: F = G ×1×1 /1² = 6.67 × 10⁻¹¹ N ✅
  13. Satellite’s orbit depends on:
    A: Mass of planet & radius ✅
  14. g at height h:
    g_h = g(1 – 2h/R) ✅
  15. g at depth d:
    g_d = g(1 – d/R) ✅
  16. Acceleration in free fall after 5s: v = gt = 9.8×5 = 49 m/s ✅
  17. Distance covered in 3s: s = ½gt² = 0.5×9.8×9 = 44.1 m ✅
  18. Weight on Jupiter = 3 × Earth’s gravity? True/False?
    A: True ✅
  19. A body weighs 100 N on Earth, mass?
    m = W/g = 100/9.8 ≈ 10.2 kg ✅
  20. Satellite remains in orbit because:
    Gravitational force = Centripetal force ✅

41–60: Conceptual

  1. Gravitation is:
    a) Contact force
    b) Non-contact force ✅
  2. If distance doubles, force?
    a) Same
    b) Halved
    c) Quarter ✅
    d) Doubled
  3. Mass of body changes with location?
    A: No ✅
  4. Weight of body on Moon:
    Weight ↓ because g ↓ ✅
  5. Force is always:
    a) Attractive ✅
    b) Repulsive
    c) Neutral
    d) None
  6. Moon causes:
    a) Gravity
    b) Tides ✅
    c) Earthquakes
    d) Volcano
  7. Gravitational force formula is inverse-square law?
    A: True ✅
  8. Objects fall due to:
    A: Gravity ✅
  9. Acceleration due to gravity is vector?
    A: True ✅
  10. Mass in kg, weight in?
    A: Newton ✅
  11. Escape velocity independent of?
    A: Mass of satellite ✅
  12. Weightlessness occurs in?
    A: Free fall ✅
  13. Gravitational pull acts:
    A: At a distance ✅
  14. Artificial satellite:
    A: Moves due to gravity ✅
  15. Mass = 50 kg, weight on Moon =?
    g_m = 1.6 → W = 50×1.6 = 80 N ✅
  16. Free fall formula s = ½gt² derived from?
    A: Equations of motion ✅
  17. v = gt in free fall if u = 0 ✅
  18. Tides caused by?
    A: Moon & Sun ✅
  19. Orbital motion requires?
    A: Centripetal force = gravity ✅
  20. Satellites in circular orbit?
    A: v = √(GM/r) ✅

61–80: Advanced / Numerical

  1. Body mass 20 kg, weight on Earth?
    W = 20×9.8 = 196 N ✅
  2. Satellite orbit radius = 7000 km, v = ?
    v = √(GM/r) ✅
  3. Escape velocity from Earth ≈ 11.2 km/s ✅
  4. Gravitational constant G = 6.67 ×10⁻¹¹ N·m²/kg² ✅
  5. Distance doubles → F becomes?
    F/4 ✅
  6. Mass of planet doubles, radius same → g doubles ✅
  7. Depth = 500 m, g? g_d = g(1 – d/R) ✅
  8. Height = 1000 m, g? g_h = g(1 – 2h/R) ✅
  9. Weight on Mars (g = 3.7 m/s²) mass = 60 kg → W = 222 N ✅
  10. Satellite remains in orbit due to?
    Centripetal force by gravity ✅
  11. Moon orbit period = 27 days, velocity? v = √(GM/r) ✅
  12. Satellite in geostationary orbit → period = 24 hrs ✅
  13. Satellite in orbit → velocity depends on radius ✅
  14. Tidal energy → renewable energy source ✅
  15. Free fall acceleration independent of mass ✅
  16. Newton proved law using?
    Planetary motion and apple observation ✅
  17. Escape velocity formula ve = √2gR ✅
  18. Weightless condition → free fall ✅
  19. Weight = mass × local gravity ✅
  20. Mass = 0.5 kg, height = 10 m, g = 9.8 → F = mg = 4.9 N ✅

81–100: Mix / Higher Order

  1. g at poles higher than equator? ✅
  2. Moon has no atmosphere → objects fall freely ✅
  3. Gravitational force formula applicable in space ✅
  4. Satellite energy = K.E. + P.E ✅
  5. Geostationary satellite → fixed above equator ✅
  6. Gravity acts on all matter ✅
  7. Artificial satellites → communication, GPS ✅
  8. Mass & radius affect g ✅
  9. Gravity decreases with height above Earth ✅
  10. Weight = 0 → free fall or outer space ✅
  11. Neap tide → small tidal range ✅
  12. Spring tide → large tidal range ✅
  13. g = GM/R² → constant for Earth surface ✅
  14. Objects fall due to Earth’s gravity ✅
  15. Moon’s gravity → 1/6 of Earth ✅
  16. Free fall → neglect air resistance ✅
  17. Gravitational force is always attractive ✅
  18. v = √(GM/r) for circular orbit ✅
  19. Escape velocity independent of mass ✅
  20. Tides → useful for tidal energy ✅

50 Assertion-Reason Questions – Gravitation Class 9

Instructions:

  • A: Both Assertion (A) and Reason (R) are correct, and R explains A.
  • B: Both are correct but R does not explain A.
  • C: Assertion is wrong, Reason is correct.
  • D: Both are wrong.

1–10: Basic Concepts

  1. A: Gravitation is a force that attracts all objects towards each other.
    R: Gravitation is a contact force.
    Answer: C
  2. A: Weight of a body changes with location.
    R: Mass of a body changes with location.
    Answer: D
  3. A: Free fall occurs due to gravity alone.
    R: Air resistance is neglected in free fall.
    Answer: A
  4. A: Newton formulated the law of gravitation.
    R: Law of gravitation states force ∝ product of masses / square of distance.
    Answer: A
  5. A: g on Earth is approximately 9.8 m/s².
    R: g = GM/R², where M is Earth’s mass, R is radius.
    Answer: A
  6. A: Mass of an object decreases on Moon.
    R: Moon’s gravity is less than Earth.
    Answer: C
  7. A: Weightless condition occurs in free fall.
    R: Weight is force due to gravity on a body.
    Answer: A
  8. A: Tides occur due to gravitational pull of Moon and Sun.
    R: Earth’s rotation has no effect on tides.
    Answer: B
  9. A: Gravitational force is always attractive.
    R: Gravitational force acts along the line joining centers.
    Answer: A
  10. A: Escape velocity from Earth is 11.2 km/s.
    R: Escape velocity is minimum velocity to leave Earth’s gravitational field.
    Answer: A

11–20: Numerical / Formula Based

  1. A: Weight = mg.
    R: Mass is measured in Newtons.
    Answer: C
  2. A: Orbital velocity formula: v = √(GM/r).
    R: Gravitational force provides centripetal force for circular motion.
    Answer: A
  3. A: g decreases with height.
    R: g = GM/(R+h)².
    Answer: A
  4. A: g decreases with depth below surface.
    R: g = g(1 – d/R).
    Answer: A
  5. A: Free fall distance s = ½gt².
    R: Free fall assumes u = 0 and a = g.
    Answer: A
  6. A: Weight of body depends on local gravity.
    R: Mass varies with height.
    Answer: C
  7. A: Escape velocity formula: ve = √2gR.
    R: Ve is independent of mass of object.
    Answer: A
  8. A: Centripetal force is required for circular orbit.
    R: Orbital velocity formula: v = √(GM/r).
    Answer: A
  9. A: Moon causes spring and neap tides.
    R: Spring tide occurs at quarter moon.
    Answer: C
  10. A: Gravitational force decreases with square of distance.
    R: Doubling distance reduces force to one-fourth.
    Answer: A

21–30: Conceptual

  1. A: Satellite remains in orbit due to gravity.
    R: Centripetal force equals gravitational force.
    Answer: A
  2. A: Weight of object on Moon is less than Earth.
    R: Gravity on Moon ≈ 1.6 m/s².
    Answer: A
  3. A: Mass of body is constant everywhere.
    R: Weight depends on g at location.
    Answer: A
  4. A: Free fall is accelerated motion.
    R: Acceleration = g.
    Answer: A
  5. A: Objects fall at same rate in vacuum.
    R: Mass affects free fall acceleration.
    Answer: C
  6. A: Geostationary satellites remain fixed over equator.
    R: Orbital period = Earth rotation period.
    Answer: A
  7. A: Gravity acts at a distance.
    R: It requires contact.
    Answer: C
  8. A: Tidal energy is renewable.
    R: Tides occur due to Moon and Sun.
    Answer: A
  9. A: Escape velocity depends on radius of planet.
    R: Ve depends on mass of satellite.
    Answer: C
  10. A: Weightlessness occurs in orbit.
    R: Satellite experiences free fall.
    Answer: A

31–40: Advanced

  1. A: g on poles > g at equator.
    R: Earth is oblate spheroid; centrifugal effect.
    Answer: A
  2. A: Gravitational force acts along line joining centers.
    R: Force is attractive.
    Answer: B
  3. A: Moon has weaker gravity → smaller weight.
    R: Moon has smaller mass than Earth.
    Answer: A
  4. A: Free fall acceleration independent of mass.
    R: Gravity acts equally on all masses.
    Answer: A
  5. A: Tides vary daily.
    R: Earth rotates under tidal bulges.
    Answer: A
  6. A: Weight formula: W = mg.
    R: Mass measured in kg, weight in N.
    Answer: A
  7. A: Gravitational potential energy decreases as object falls.
    R: Work done by gravity = ΔPE.
    Answer: A
  8. A: v = √(GM/r) formula valid for circular orbit.
    R: Centripetal force = gravitational force.
    Answer: A
  9. A: Satellite in orbit experiences weight.
    R: Satellite in free fall.
    Answer: C
  10. A: g decreases with height from surface.
    R: g ∝ 1/(R+h)².
    Answer: A

41–50: Miscellaneous

  1. A: Gravity exists between all matter.
    R: Massless particles are not affected.
    Answer: C
  2. A: Weight = 0 → free fall.
    R: Acceleration = g.
    Answer: A
  3. A: Mass of body = 50 kg → weight = 490 N.
    R: g = 9.8 m/s².
    Answer: A
  4. A: Satellite orbit depends on planet mass.
    R: Satellite mass affects orbit.
    Answer: C
  5. A: Artificial satellites used for GPS.
    R: Orbital velocity keeps satellite in orbit.
    Answer: A
  6. A: Tides useful for tidal energy.
    R: Moon’s gravity causes tides.
    Answer: A
  7. A: g at center of Earth = 0.
    R: Inside Earth, g decreases linearly with depth.
    Answer: A
  8. A: Moon orbit period = 27 days.
    R: Orbital velocity = √(GM/r).
    Answer: B
  9. A: Spring tide → large tidal range.
    R: Occurs at full & new moon.
    Answer: A
  10. A: Gravitational force decreases with distance.
    R: Force ∝ 1/r².
    Answer: A

✅ These 50 Assertion-Reason questions cover:

  • Basic concepts
  • Numerical/formula understanding
  • Motion of satellites
  • Free fall and tides
  • Mass, weight, and gravity variations

Absolutely! Here’s a set of 30 Case-Based Questions (CBQs) with answers for Gravitation – Class 9 Physics. These are exam-oriented, scenario-based, and perfect for board and competitive exams.

30 Case-Based Questions – Gravitation Class 9

Case 1 – Free Fall & Weightlessness

Case:
A 5 kg object is dropped from the top of a tower of height 45 m. Ignore air resistance.

Questions:

  1. What is the time taken to reach the ground?
  2. What is the velocity just before hitting the ground?
  3. Calculate the weight of the object during free fall.

Answers:

  1. → →
  2. In free fall, apparent weight = 0 N

Case 2 – Mass vs Weight

Case:
A person has mass 60 kg.

Questions:

  1. Find weight on Earth.
  2. Find weight on Moon (g = 1.6 m/s²).
  3. Does mass change on Moon?

Answers:

  1. W = mg = 60 × 9.8 = 588 N
  2. W_moon = 60 × 1.6 = 96 N
  3. Mass remains 60 kg

Case 3 – Variation of g with Height

Case:
A satellite is orbiting at height 400 km above Earth. R_earth = 6370 km, g = 9.8 m/s².

Questions:

  1. Find acceleration due to gravity at satellite.
  2. Compare with g on surface.

Answers:

  1. Slightly less than 9.8 m/s²

Case 4 – Gravitational Force

Case:
Two 2 kg masses are 1 m apart.

Question:
Calculate gravitational force between them.

Answer:

Case 5 – Orbital Motion

Case:
A satellite orbits Earth at radius 7000 km. M_earth = 6 × 10²⁴ kg.

Question:
Find orbital velocity.

Answer:

Case 6 – Escape Velocity

Case:
Find escape velocity from Earth. g = 9.8 m/s², R = 6370 km

Answer:

Case 7 – Tides

Case:
High tides occur at full moon and new moon.

Questions:

  1. Name these tides.
  2. Why do they occur?

Answers:

  1. Spring tides
  2. Gravitational pull of Moon and Sun in same line

Case 8 – Weight on Another Planet

Case:
g on Jupiter = 24.8 m/s². Mass = 70 kg

Question:
Find weight on Jupiter.

Answer:
W = mg = 70 × 24.8 = 1736 N

Case 9 – Free Fall Time

Case:
Drop object from 19.6 m.

Question:
Time to reach ground.

Answer:
s = ½gt² → t² = 2s/g = 39.2/9.8 = 4 → t = 2 s

Case 10 – Mass vs Weight Confusion

Case:
Weight of astronaut in space?

Answer:
Apparent weight = 0, mass unchanged

Cases 11–30 (Summary Table)

CaseScenarioQuestionsAnswers
11Object dropped from tower, height = 80 mTime to fall, velocityt = 4.04 s, v = 39.6 m/s
12Moon gravity, mass = 50 kgWeight on MoonW = 50 × 1.6 = 80 N
13Satellite radius = 36000 kmOrbital velocityv = √(GM/r) ≈ 3.07 km/s
14Two masses 5 kg, r = 2 mForce between themF = 6.67×10⁻¹¹ × 25 /4 ≈ 4.17×10⁻¹¹ N
15Height = 500 mg at heightg_h ≈ 9.69 m/s²
16Depth = 2 kmg at depthg_d ≈ 9.797 m/s²
17Escape velocity, Moon R=1.74×10^6, g=1.62ve?v_e = √2gR ≈ 2.1 km/s
18Tides questionType of tideNeap or spring
19Geostationary satellitePeriod?24 hrs
20Weightless astronautForce?0 N
21Planet with g = 15 m/s²Weight 60 kg?900 N
22Free fall from 45 mVelocity?v ≈ 29.7 m/s
23Satellite at 500 kmg?≈ 8.87 m/s²
24Two planets 1×10^6 m apart, masses = 10^24 kgF?F ≈ 6.67×10¹⁰ N
25Object on equator vs polesCompare weightWeight at poles > equator
26Height 1000 mg?g_h ≈ 9.76 m/s²
27Satellite circular orbitVelocity formula?v = √(GM/r)
28Weight of 80 kg on Mars g=3.7W?296 N
29Spring tide scenarioCause?Moon-Sun alignment
30Moon orbit around EarthPeriod?27.3 days

✅ These 30 case-based questions cover:

  • Free fall and weightlessness
  • Mass vs weight on different planets
  • Satellites and orbital motion
  • Escape velocity
  • Tides and Earth-Moon-Sun system
  • g variation with height/depth

Class 9 Physics – Gravitation Sample Paper (80 Marks)

Time: 3 Hours
Maximum Marks: 80

General Instructions:

  1. All questions are compulsory.
  2. Use g = 9.8 m/s² wherever needed.
  3. Show all steps in calculations.

Section A – Very Short Answer Questions (1 mark each, 10 × 1 = 10)

  1. Define gravitation.
  2. Unit of gravitational force.
  3. Symbol for acceleration due to gravity.
  4. Mass vs weight: which is constant everywhere?
  5. Value of g on Earth.
  6. Write the formula for gravitational force.
  7. Free fall occurs due to which force?
  8. Formula for weight.
  9. What is escape velocity?
  10. Name the two types of tides.

Section B – Short Answer Questions (2 marks each, 10 × 2 = 20)

  1. Mass of a body = 50 kg. Find weight on Earth.
  2. What is the apparent weight of a body in free fall?
  3. Find the acceleration due to gravity at a height of 400 km above Earth.
  4. What is the difference between mass and weight?
  5. Write the formula for orbital velocity of a satellite.
  6. Two 2 kg masses are placed 1 m apart. Calculate gravitational force between them.
  7. Why is weight on Moon less than on Earth?
  8. Why do all objects fall with the same acceleration in vacuum?
  9. What is a geostationary satellite?
  10. Name any two applications of gravitation in daily life.

Section C – Long Answer Questions (4 marks each, 5 × 4 = 20)

  1. Derive the formula for acceleration due to gravity (g) using Newton’s law of gravitation.
  2. A 5 kg object falls from a height of 45 m. Find:
    a) Time of fall
    b) Velocity just before hitting the ground
    c) Distance covered in first 2 s
  3. Derive the formula for escape velocity from Earth.
  4. Explain motion of planets using gravitation.
  5. Explain the causes and types of tides with examples.

Section D – Multiple Choice Questions (1 mark each, 10 × 1 = 10)

  1. Gravitation is:
    a) Repulsive force
    b) Attractive force ✅
    c) Neutral force
    d) None
  2. Value of universal gravitational constant G is:
    a) N·m²/kg² ✅
    b) 9.8 N/kg
    c) 10 m/s²
    d) 1 N
  3. Weight formula is:
    a) W = mg ✅
    b) W = m + g
    c) W = mg²
    d) W = m/g
  4. Escape velocity from Earth:
    a) 5 km/s
    b) 11.2 km/s ✅
    c) 9.8 km/s
    d) 7 km/s
  5. Free fall distance:
    a) s = gt
    b) s = ut + ½gt² ✅
    c) s = gt²
    d) s = ½g
  6. Orbital velocity:
    a) v = √(GM/r) ✅
    b) v = √2GM/r
    c) v = GM/r²
    d) v = G/r
  7. Neap tides occur during:
    a) Full moon
    b) Quarter moon ✅
    c) New moon
    d) None
  8. Mass of object changes with:
    a) Height
    b) Depth
    c) Location
    d) None ✅
  9. Satellite in orbit experiences:
    a) Weight
    b) Apparent weight = 0 ✅
    c) Mass change
    d) None
  10. Spring tides occur when:
    a) Sun-Moon aligned ✅
    b) Sun-Moon perpendicular
    c) Moon alone
    d) Sun alone

Section E – Assertion-Reason Questions (2 marks each, 5 × 2 = 10)

Choose the correct option (A/B/C/D) for each:

  1. A: Weight of a body changes with location.
    R: Mass of a body changes with location.
  2. A: Gravitational force is always attractive.
    R: Gravitational force acts along line joining centers.
  3. A: Free fall acceleration is independent of mass.
    R: Gravity acts equally on all masses.
  4. A: Tidal energy can be used as renewable energy.
    R: Tides are caused by gravitational pull of Moon and Sun.
  5. A: Escape velocity from a planet is independent of mass of object.
    R: Escape velocity formula ve = √2gR.

Section F – Case-Based Questions (5 × 2 = 10)

Case 1:
A 60 kg person is standing on Moon (g = 1.6 m/s²).

  1. Find the weight of the person.
  2. Does the mass of the person change on Moon?

Case 2:
A satellite orbits Earth at radius 7000 km. M_earth = 6 × 10²⁴ kg.

  1. Find the orbital velocity.
  2. Explain why the satellite does not fall on Earth.

Case 3:
A ball is dropped from a height of 19.6 m.

  1. Find the time taken to reach the ground.
  2. Find the velocity just before hitting the ground.

Section G – Numerical / Formula Based (5 × 1 = 5)

  1. Two masses 3 kg and 6 kg are 2 m apart. Find the gravitational force.
  2. A body of mass 10 kg is dropped from height 20 m. Find distance covered in first 1 s.
  3. Find escape velocity from Moon (R = 1.74×10⁶ m, g = 1.62 m/s²).
  4. Satellite orbit radius = 36000 km. Find period of geostationary satellite.

Marking Scheme / Distribution

SectionNo. of QuestionsMarks
Very Short Answer10 × 110
Short Answer10 × 220
Long Answer5 × 420
MCQs10 × 110
Assertion-Reason5 × 210
Case-Based5 × 210
Numerical5 × 15
Total5080

Perfect! Here’s the complete solution for the 80-mark Gravitation Sample Paper I just created. All calculations, formulas, and stepwise explanations are included for clarity.

Class 9 Physics – Gravitation Sample Paper Solutions (80 Marks)

Section A – Very Short Answer Solutions (1 mark each)

  1. Gravitation: Universal attractive force between any two masses.
  2. Unit: Newton (N)
  3. Symbol for acceleration due to gravity: g
  4. Mass vs weight: Mass is constant everywhere; weight depends on gravity.
  5. Value of g on Earth: 9.8 m/s²
  6. Gravitational force formula:
  7. Free fall force: Gravity
  8. Weight formula:
  9. Escape velocity: Minimum velocity to leave gravitational field of a planet without further propulsion
  10. Two types of tides: Spring tide, Neap tide

Section B – Short Answer Solutions (2 marks each)

  1. Weight on Earth:
  2. Apparent weight in free fall: 0 N
  3. g at height 400 km:

g_h = g \left(\frac{R}{R+h}\right)^2 = 9.8 \left(\frac{6370}{6370+400}\right)^2 \approx 8.64 \, m/s^2
  1. Mass vs Weight difference: Mass = matter content, constant; Weight = force due to gravity, changes with location.
  2. Orbital velocity formula:

v = \sqrt{\frac{GM}{r}}
  1. Gravitational force between two 2 kg masses, 1 m apart:

F = G \frac{m_1 m_2}{r^2} = 6.67×10^{-11} × 2 × 2 / 1^2 ≈ 2.668 × 10^{-10} N
  1. Weight on Moon less: g_moon ≈ 1/6 of Earth’s g
  2. Objects fall at same rate in vacuum: No air resistance; gravity accelerates all objects equally.
  3. Geostationary satellite: Orbits Earth in 24 hrs, appears stationary above equator.
  4. Applications of gravitation: Tides, satellite orbits, planetary motion, GPS, space exploration.

Section C – Long Answer Solutions (4 marks each)

  1. Derive g formula:

Gravitational force on mass m by Earth mass M:


F = G \frac{Mm}{R^2}


\boxed{g = \frac{GM}{R^2}}
  1. Object falling from 45 m, m = 5 kg

a) Time: → 45 = 0.5 × 9.8 × t² → t ≈ 3.03 s

b) Velocity before impact:

c) Distance in first 2 s:

  1. Escape velocity:

\frac{1}{2} m v^2 = \frac{GMm}{R} \implies v = \sqrt{\frac{2GM}{R}} = \sqrt{2 g R}

Substitute for Earth: →

  1. Motion of planets:

Gravitational attraction of Sun → centripetal force:


\frac{GMm}{r^2} = \frac{mv^2}{r} \implies v = \sqrt{\frac{GM}{r}}

Planets move in elliptical/circular orbits; gravity balances centrifugal effect.

  1. Tides:
  • Caused by gravitational pull of Moon & Sun
  • Spring tide: New & full moon, large tidal range
  • Neap tide: Quarter moon, small tidal range

Section D – MCQs Solutions (1 mark each)

  1. b
  2. a
  3. a
  4. b
  5. b
  6. a
  7. b
  8. d
  9. b
  10. a

Section E – Assertion-Reason Solutions (2 marks each)

  1. D – Weight changes, mass constant
  2. A – Both correct, R explains A
  3. A – Both correct, R explains A
  4. A – Both correct, R explains A
  5. A – Both correct, R explains A

Section F – Case-Based Solutions (2 marks each)

Case 1: Mass = 60 kg on Moon (g = 1.6)

  1. Weight = mg = 60 × 1.6 = 96 N
  2. Mass remains = 60 kg

Case 2: Satellite orbit radius = 7000 km, M_earth = 6×10²⁴ kg

  1. Orbital velocity:
  2. Satellite does not fall due to balance: Gravitational force = Centripetal force

Case 3: Ball dropped from 19.6 m

  1. Time:
  2. Velocity:

Section G – Numerical / Formula Based Solutions (1 mark each)

  1. F = G m1 m2 / r² = 6.67×10^-11 × 3×6 / 4 ≈ 3×10^-10 N
  2. s = ½ g t², t = 1 s → s = ½ × 9.8 ×1² ≈ 4.9 m
  3. Escape velocity from Moon: ve = √2gR = √(2 ×1.62 × 1.74×10^6) ≈ 2.1 km/s
  4. Geostationary satellite, r = 36000 km → T = 24 hrs

Gravitation – Quick Revision (Class 9 Physics)

1. Introduction

Gravitation is the universal attractive force acting between any two masses. It explains why objects fall on Earth, why planets orbit the Sun, and why tides occur. Sir Isaac Newton formulated the Universal Law of Gravitation in 1687, which states:

“Every particle of matter in the universe attracts every other particle with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.”

Mathematically,


F = G \frac{m_1 m_2}{r^2}
  • F = Gravitational force (N)
  • G = Universal gravitational constant
  • m₁, m₂ = Masses of the objects (kg)
  • r = Distance between their centers (m)

2. Mass and Weight

  • Mass (m): Amount of matter in an object. Constant everywhere, measured in kg.
  • Weight (W): Force of gravity on a body. Varies with location, measured in Newtons.

W = mg
  • Weight decreases with height above Earth and increases with depth below surface:

3. Free Fall

  • Motion of a body under gravity alone, ignoring air resistance.
  • Formulas:

v = u + gt, \quad s = ut + \frac{1}{2}gt^2
  • In free fall, apparent weight = 0 (weightlessness).
  • All objects fall at the same rate in vacuum.

4. Acceleration due to Gravity (g)

  • Acceleration experienced by a body under gravity on Earth’s surface.
  • Derived from Newton’s law:

g = \frac{GM}{R^2}
  • Latitude (higher at poles, lower at equator)
  • Altitude (decreases with height)
  • Depth (decreases linearly inside Earth)

5. Universal Law of Gravitation

  • Force is always attractive.
  • Acts along the line joining centers of mass.
  • Inverse square law: Doubling distance → Force reduces to 1/4.

6. Motion of Satellites

  • Satellite remains in orbit due to centripetal force = gravitational force.
  • Orbital velocity:

v = \sqrt{\frac{GM}{r}}


v_e = \sqrt{2gR}

7. Tides

  • Caused by gravitational pull of Moon and Sun on Earth’s oceans.
  • Types:
    • Spring tides: Full moon & new moon → large tidal range
    • Neap tides: Quarter moon → small tidal range
  • Useful for tidal energy, navigation, and fishing.

8. Important Formulas (Quick Reference)

QuantityFormula
Gravitational force
Weight
g at height h
g at depth d
Orbital velocity
Escape velocity
Free fall distance
Free fall velocity

9. Quick Numerical Tricks

  1. Free fall: s = ½ g t², v = g t
  2. Weight: W = mg, decreases with altitude, increases with depth
  3. Satellite: Use v = √(GM/r), ve = √(2gR)
  4. Tides: Spring → large, Neap → small

10. Important Keywords

  • Gravitation: Universal attractive force
  • Mass: Quantity of matter (kg)
  • Weight: Force due to gravity (N)
  • Free fall: Motion under gravity only
  • g: Acceleration due to gravity
  • Orbital velocity: Speed to remain in orbit
  • Escape velocity: Minimum speed to leave gravitational field
  • Tides: Rise and fall of ocean water due to Moon/Sun

11. Key Points for Exams

  • g = GM/R² → derive questions from this formula
  • Weight = mg → solve all mass-weight problems
  • Free fall formulas s = ½ gt², v = gt → memorize
  • Satellite questions → use v = √(GM/r)
  • Escape velocity → ve = √(2gR)
  • Tides → memorize types, causes, spring & neap timings
  • Always consider distance in meters, mass in kg, force in Newtons

12. Common Exam Tips

  • Use stepwise method for numerical problems.
  • Remember units: Mass (kg), Force (N), Distance (m)
  • In multiple-choice questions, check dimensions to avoid mistakes.
  • For Assertion-Reason, read carefully: R may be correct but not the explanation.
  • For case-based questions, write given, formula, solution format.

Conclusion (Revision Summary)

  • Gravitation governs falling objects, planetary motion, satellite orbits, tides.
  • Weight changes with location; mass is constant.
  • Free fall is uniform acceleration due to gravity.
  • g decreases with height, depth, or distance from Earth.
  • Satellite motion balances gravitational and centripetal forces.
  • Tides result from Moon-Sun gravitational pull; spring → large, neap → small.
  • Key formulas: , , , .

This quick revision ensures you can recall all major formulas, concepts, and applications of Gravitation in less than 30 minutes before exams.

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