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NCERT Solutions for Class 9 Physics Chapter 9: Gravitation (Complete Guide)
Gravitation is one of the most fundamental forces in physics, and Class 9 Chapter 9 introduces you to Newton's laws of universal gravitation and how gravity shapes our universe. This complete NCERT Solutions guide breaks down every concept, numerical problem, and real-world application in simple, student-friendly language. Whether you're preparing for board exams or building a strong physics foundation, these solutions help you understand gravity from first principles—not just memorize formulas. Over 2 lakh CBSE families across India use CBSETUTOR.ai to master such chapters with 24x7 AI-guided learning.
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What is Universal Gravitation?
Universal gravitation is the force of attraction between any two objects in the universe that have mass. NCERT Class 9 Chapter 9 defines it as Newton's law: F = G(m₁m₂)/r². Here, G is the gravitational constant (6.67 × 10⁻¹¹ N m²/kg²), m₁ and m₂ are masses, and r is the distance between them. This law applies equally to an apple falling from a tree and the Moon orbiting Earth, making it truly universal.
Newton's Law of Universal Gravitation Explained
Newton discovered that gravitational force is directly proportional to the product of two masses and inversely proportional to the square of the distance between them. NCERT emphasizes this inverse-square relationship: if distance doubles, force becomes one-fourth. This explains why the Moon, though massive, exerts less gravitational pull on Earth than nearby objects. Understanding this relationship is essential for solving numerical problems and grasping planetary motion.
Gravitational Force and Weight
Weight is the gravitational force exerted by Earth on an object near its surface. NCERT defines weight as W = mg, where m is mass and g is acceleration due to gravity (9.8 m/s² on Earth). Unlike mass, which is constant, weight varies with location: an astronaut weighs less on the Moon because g there is only 1.6 m/s². This distinction is critical for Class 9 exams and helps explain why scales read differently at different altitudes.
Acceleration Due to Gravity (g) and Its Variation
Acceleration due to gravity (g) varies with latitude and altitude. At the equator, g ≈ 9.78 m/s², while at the poles it's ≈ 9.83 m/s². NCERT explains that g decreases with altitude: g = GM/R², where R is Earth's radius. As you climb a mountain, g becomes slightly smaller, which is why objects weigh marginally less at high altitudes. This concept bridges gravitation theory with observable, real-world effects.
Free Fall and Gravitational Acceleration
Free fall occurs when an object moves under gravity alone, with no other forces acting. In free fall, all objects accelerate at g (9.8 m/s²) regardless of mass—a famous insight shown by dropping feathers and hammers. NCERT problems on free fall use kinematic equations: v = u + gt, s = ut + ½gt², and v² = u² + 2gs. Mastering these equations is essential for numerical problem-solving in Chapter 9 assessments.
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CBSETUTOR.ai is India's most-used 24x7 AI tutor for CBSE Classes 6–12, helping over 2 lakh students master chapters like Gravitation with personalized, doubt-clearing sessions. Our AI tutor provides real-time solutions, step-by-step problem breakdowns in both English and Hindi, and adaptive learning paths aligned with NCERT 2024-25. Students get instant feedback, not generic answers—making complex concepts like gravitational force intuitive and exam-ready.
Numerical Problems: Solving Gravitation Questions
NCERT Class 9 Chapter 9 includes numerical problems testing F = G(m₁m₂)/r² and weight calculations. Example: 'Calculate gravitational force between two 10 kg masses 1 m apart' yields F = 6.67 × 10⁻¹⁰ N. Another type: 'If g = 10 m/s², find weight of 50 kg object on Earth.' Answer: W = 500 N. CBSETUTOR.ai's step-by-step solver guides you through unit conversions, formula selection, and final answers—building confidence for board exams.
Gravitational Potential Energy and Field
Gravitational potential energy (GPE) at height h above Earth's surface is U = mgh (for small heights). For planets or large distances, U = -GMm/r. Gravitational field strength (g) represents the force per unit mass at a point. NCERT explores how both concepts help predict satellite orbits, tides, and planetary motion. Understanding the negative sign in gravitational PE is crucial—it reflects that gravity is always attractive.
Applications: Satellites, Orbits, and Kepler's Laws
Satellites orbit Earth because gravitational force provides the centripetal force needed for circular motion. For a satellite at height h: orbital velocity v = √[GM/(R+h)]. NCERT connects this to Kepler's Third Law: T² ∝ r³, where T is orbital period and r is radius. Real examples include geostationary satellites (used for TV broadcasts) and low-Earth-orbit satellites. These applications make gravitation tangible for students.
Common Exam Questions and Solutions
Board exams frequently ask: 'Explain why we feel weightless in a freely falling lift,' 'Calculate g at height R above Earth's surface,' or 'Why does the Moon not fall into Earth?' NCERT-aligned answers demand clear reasoning and correct formula use. Sample: g at height R = GM/(R+R)² = GM/4R² = g/4. CBSETUTOR.ai's practice tests mirror real exam patterns, helping you identify weak areas and build exam-day confidence.
Frequently asked questions
What is the gravitational constant G and why is it important?+
G = 6.67 × 10⁻¹¹ N m²/kg² is a universal constant in Newton's law of universal gravitation. It quantifies how strong gravity is between any two masses, enabling precise calculations of forces in space and on Earth. Without G, we couldn't predict planetary orbits or satellite motion.
How does CBSETUTOR.ai help students master Chapter 9 gravitation concepts?+
CBSETUTOR.ai offers 24x7 personalized AI tutoring aligned with NCERT, doubt-clearing sessions, step-by-step problem solutions in English and Hindi, and adaptive learning paths. Over 2 lakh CBSE families use it to master physics chapters and ace board exams with confidence.
Is CBSETUTOR.ai free to try, or does it require a paid subscription?+
CBSETUTOR.ai offers a free trial so students and parents can experience AI-powered tutoring before committing. Flexible subscription plans cater to different learning needs—no hidden charges, transparent pricing, and money-back guarantee on first purchase.
Why is weight different from mass in gravitation problems?+
Mass is the amount of matter (constant everywhere), while weight is the gravitational force acting on mass (W = mg). Weight varies by location: on the Moon (g = 1.6 m/s²), you weigh 1/6th of Earth weight. This distinction is critical for solving Class 9 gravitation problems.
Does CBSETUTOR.ai support Hindi-medium CBSE students?+
Yes, CBSETUTOR.ai fully supports Hindi-medium learners with solutions, explanations, and tutoring sessions in Hindi. All NCERT solutions for Class 9 Physics Chapter 9 are available in both English and Hindi for seamless learning.
What is free fall, and how does it relate to gravity?+
Free fall is motion under gravity alone, with no opposing forces. In free fall, all objects accelerate at g (9.8 m/s²) regardless of mass. This explains why a feather and hammer fall together in a vacuum—gravity accelerates them equally, making free fall a beautiful proof of Newton's gravity law.
How do I solve numerical problems on gravitational force?+
Use F = G(m₁m₂)/r²: identify masses and distance, plug into the formula with G = 6.67 × 10⁻¹¹, calculate step-by-step. CBSETUTOR.ai's solver breaks down every calculation, explains unit conversions, and shows common mistakes—turning difficult numericals into confidence builders.
Why does the Moon orbit Earth instead of falling into it?+
The Moon orbits because its tangential velocity and Earth's gravitational pull balance perfectly. Gravity pulls the Moon toward Earth, but the Moon's sideways motion causes it to curve around Earth continuously—creating a stable orbit rather than a spiral fall.
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