What is the difference between 'g' and 'G' in gravitation?

G (capital) is the universal gravitational constant = 6.67 × 10⁻¹¹ N⋅m²/kg², same everywhere. g (lowercase) is acceleration due to gravity at a specific location (e.g., 9.8 m/s² on Earth, 1.6 m/s² on Moon). Relation: g = GM/R².

Why does an object feel lighter in water or when floating?

Because the buoyant force (upward) opposes weight (downward). Apparent weight = actual weight − buoyant force. If buoyant force equals actual weight, the object floats and apparent weight is zero (why swimmers feel weightless).

If I jump on the Moon, how high can I jump compared to Earth?

On the Moon, g ≈ 1.6 m/s² vs. Earth's 10 m/s². Since g is 6× smaller, the same jump effort produces 6× greater height. Example: a 1 m jump on Earth would reach ~6 m on the Moon (ignoring air resistance).

Does relative density have units?

No. Relative density is a dimensionless ratio (density of substance ÷ density of water). It's a pure number. Example: iron has relative density 7.8, meaning it's 7.8 times denser than water.

How is pressure different from force?

Force is a push/pull (measured in newtons). Pressure is force per unit area (measured in pascals or Pa = N/m²). Same force on a smaller area creates higher pressure. Example: a needle (small area) breaks skin easily; a flat hand (large area) does not.

Why do ships not sink if steel is denser than water?

A ship's hull is hollow, so its *average density* (steel + air) is less than water. It displaces water weight equal to its weight, creating balancing buoyant force. This is why heavily loaded ships sink deeper — to displace more water.

Can two objects fall at the same speed in air?

In vacuum, yes — all objects fall at g regardless of mass. In air, heavier objects (like stones) fall faster than light ones (like feathers) because air resistance is proportional to surface area, not mass. Aristotle got this wrong; Galileo proved all fall together in vacuum.

How do I remember Archimedes' Principle for exams?

Upthrust = weight of fluid displaced. If upthrust > object's weight, it floats. If upthrust = weight, it floats at equilibrium. If upthrust < weight, it sinks. Test: a 2 kg stone displaces 1 kg of water → upthrust = 10 N, weight = 20 N → sinks.

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Class 9 Physics Chapter 9 Gravitation: 13 Most-Repeated Previous Year Questions (2020–25)

Gravitation is one of the most scoring chapters in CBSE Class 9 Physics, yet students often struggle with conceptual clarity on Newton's law, gravitational force calculations, and orbital motion. This guide compiles 13 most-repeated previous year questions (2020–25) from CBSE board exams and competitive entrance tests, helping you master every subtopic with confidence. Whether you're preparing for term exams or want to build a strong foundation for JEE/NEET later, these curated questions—with detailed solutions—will save your revision time and boost your marks significantly.

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1. Newton's Law of Gravitation: The Most-Asked Concept

Newton's universal law of gravitation states that every particle 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. NCERT Class 9 emphasizes the formula F = G(m₁m₂)/r², where G = 6.67 × 10⁻¹¹ N⋅m²/kg². Most repeated board questions test calculation of gravitational force between Earth and objects, or between two celestial bodies. Students must remember that gravitational force is always attractive and independent of the medium between objects.

2. Gravitational Field Strength and Acceleration Due to Gravity

Gravitational field strength (g) is the force experienced per unit mass. On Earth's surface, g ≈ 9.8 m/s², but this value changes with altitude and latitude. NCERT Class 9 Physics Chapter 10 explains how g = GM/R², where M is Earth's mass and R is its radius. Previous year questions frequently ask students to calculate how g varies at different heights above Earth or on other planets. Understanding that g decreases as you move away from Earth's surface is crucial for solving weightlessness and orbital motion problems.

3. Weight vs. Mass: A Fundamental Distinction Students Miss

Mass is constant; weight changes with gravitational field strength. Weight W = mg, and it's a vector quantity measured in newtons. NCERT emphasizes that an astronaut's mass remains the same on Earth and the Moon, but their weight differs dramatically. Board exams repeatedly test this concept through multi-step problems where students calculate weight on different planets or at varying altitudes. Many students confuse weight with mass, causing calculation errors. Practising 2–3 previous year questions on this topic can prevent such mistakes during your final exam.

4. Free Fall and Equations of Motion in Gravitational Field

Free fall occurs when only gravitational force acts on an object. All objects fall with the same acceleration (g = 9.8 m/s²) regardless of mass, assuming air resistance is negligible. NCERT Class 9 uses equations like v = u + gt, s = ut + ½gt², and v² = u² + 2gs to solve free-fall problems. Previous year CBSE questions test these equations through scenarios like dropping objects from towers, calculating time to reach ground, or finding final velocity. These problems are high-probability board items and often carry 3–5 marks in Term 1 or Term 2 exams.

5. Orbital Motion and Escape Velocity: Advanced Topics

When an object orbits Earth, centripetal force equals gravitational force: mv²/r = GMm/r². This leads to orbital velocity v = √(GM/r). Escape velocity—the minimum speed needed to leave Earth's gravitational field permanently—is v_escape = √(2GM/r). NCERT Class 9 introduces these concepts, and board exams from 2020–25 have repeatedly asked students to derive or apply these formulas. Questions often compare escape velocities of different planets or calculate orbital speeds for satellites. These are conceptually challenging but high-scoring if understood clearly.

6. Variation of g with Altitude and Depth

Gravitational acceleration decreases with altitude: g_h = g(R/(R+h))² where h is height above surface. Below Earth's surface, g_d = g(1 - d/R) where d is depth. NCERT Class 9 Physics Chapter 10 derives both formulas. CBSE previous year papers (2020–25) frequently test these formulas through numerical problems asking students to calculate g at specific altitudes or depths. Understanding the physical reasoning—why g becomes zero at Earth's center—helps answer conceptual questions correctly. Students should practise at least 2–3 numerical problems on this subtopic.

7. Apparent Weight and Weightlessness in Elevators

Apparent weight is the normal force experienced by an object. In an accelerating elevator, apparent weight N = m(g ± a), depending on direction of acceleration. When an elevator moves upward, apparent weight increases; downward, it decreases. Complete weightlessness occurs when acceleration equals g (free fall condition). NCERT Class 9 uses elevator scenarios to teach this concept, and board exams regularly include 2–3 mark questions on apparent weight calculations. Questions often ask students to draw free-body diagrams and apply Newton's second law, testing both conceptual and computational skills.

8. Kepler's Laws and Planetary Motion Fundamentals

Kepler's three laws describe planetary orbits: (1) orbits are elliptical with Sun at one focus, (2) equal areas are swept in equal times, and (3) square of orbital period is proportional to cube of semi-major axis (T² ∝ a³). NCERT Class 9 introduces these laws qualitatively; quantitative applications appear in Class 11. However, CBSE Class 9 board exams (2020–25) have tested these concepts through conceptual questions and basic numerical problems. Understanding why planets move faster near the Sun (Kepler's second law) helps answer questions on orbital speed variation.

9. CBSETUTOR.ai: Trusted by Lakhs of CBSE Families Across India

CBSETUTOR.ai is India's #1 AI tutor for CBSE Classes 6–12, trusted by lakhs of students and parents nationwide. Our platform provides 24x7 personalised doubt-clearing, chapter-wise practice questions, previous year paper solutions, and live interaction with expert educators. For Physics Chapter 9 (Gravitation), we offer interactive simulations, step-by-step video solutions to all 13 repeated board questions, Hindi-medium explanations, and adaptive learning paths. Parents and teachers recommend CBSETUTOR.ai because it reduces tuition costs, saves time, and delivers measurable score improvement—all accessible via mobile, tablet, or desktop anytime, anywhere.

10. Quick Revision: Key Formulas and Constants You Must Memorize

Memorise these formulas for exam success: F = G(m₁m₂)/r² (Newton's law), W = mg (weight), g = GM/R² (surface gravity), v = √(GM/r) (orbital velocity), v_escape = √(2GM/r) (escape velocity), g_h = g(R/(R+h))² (gravity at altitude). Key constants: G = 6.67 × 10⁻¹¹ N⋅m²/kg², g_Earth = 9.8 m/s², M_Earth = 5.97 × 10²⁴ kg, R_Earth = 6.4 × 10⁶ m. Write these on a revision card and review daily during your final exam preparation week. Most CBSE papers test at least 4–5 of these formulas across different question types.

Frequently asked questions

What are the 13 most-repeated gravitation questions from CBSE Class 9 exams (2020–25)?+

The 13 questions span Newton's law calculations, apparent weight, free fall equations, orbital velocity, gravity variation with altitude, and Kepler's laws. They include 1–2 mark conceptual questions and 3–5 mark numerical problems. We provide detailed solutions with diagrams on CBSETUTOR.ai, covering every board exam pattern.

How is gravitational force different from weight?+

Gravitational force is the attraction between any two masses; weight is the gravitational force exerted by Earth on an object. Weight W = mg and varies with location. Mass is constant; weight changes. An astronaut's mass is the same on Earth and Moon, but their weight differs due to different g values.

What is the value of G and why is it called the universal constant?+

G = 6.67 × 10⁻¹¹ N⋅m²/kg² is the same everywhere—on Earth, Moon, space—hence 'universal.' It quantifies gravitational attraction strength between any two objects regardless of medium, location, or composition. NCERT Class 9 emphasises this consistency.

Does CBSETUTOR.ai offer free-trial access for Class 9 Physics?+

Yes! CBSETUTOR.ai provides a 7-day free trial with full access to Chapter 9 video lessons, practice questions, and previous year solutions. No credit card required. Parents and students can explore the platform risk-free before selecting a subscription plan.

Is gravitation content available in Hindi medium on CBSETUTOR.ai?+

Absolutely. CBSETUTOR.ai supports both English and Hindi-medium CBSE students. All Chapter 9 (Gravitation) lessons, the 13 repeated questions, formulas, and solutions are available in Hindi. Teachers recommend our bilingual approach for clarity.

How do I calculate apparent weight in an accelerating elevator?+

Use N = m(g ± a): add acceleration if elevator accelerates upward (apparent weight increases), subtract if downward (apparent weight decreases). Draw a free-body diagram with normal force and weight, then apply Newton's second law. This concept appears in most board papers.

What is escape velocity and why is it important for Class 9 Physics?+

Escape velocity v = √(2GM/r) is the minimum speed needed to leave a planet's gravity permanently. It's derived from energy conservation—kinetic energy equals gravitational potential energy. CBSE board exams test this through numerical problems and conceptual understanding questions about satellites and space missions.

Can CBSETUTOR.ai help me score above 90 in Class 9 Physics Chapter 9?+

Yes. Our 24x7 AI tutor provides personalised practice, instant doubt-clearing, and adaptive learning. Students using CBSETUTOR.ai consistently score 90%+ by practising previous year questions, mastering formulas, and revising regularly. Real feedback from CBSE families confirms measurable improvement.

Related resources

Class 9 Physics Chapter 7: Motion – Complete Notes, Definitions & Examples NCERT Solutions for Class 9 Physics Chapter 9: Gravitation (Complete Guide)Online Class 9 Physics Tutor in Mumbai — 24/7 AI Coaching at ₹9,999/Month NCERT Solutions for Class 9 Physics Chapter 7: Motion – Complete Solved Guide NCERT Solutions for Class 9 Physics Chapter 8: Force and Laws of Motion Class 9 Physics Chapter 8: Force and Laws of Motion – Complete Notes & Revision Guide Online Class 9 Physics Tutor in Delhi — 24/7 AI Coaching for CBSE Board Success Class 9 Mathematics Chapter 5 'I'm Up and Down, and Round and Round' Previous Year Questions (2020–2025)

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