Introduction to Gravitation

Gravitation is one of the fundamental forces of nature and plays a key role in the motion of celestial bodies and objects on Earth. This chapter is crucial for JEE Main as it explains the universal law of gravitation, gravitational field, acceleration due to gravity, and orbital motion.

Understanding gravitation allows students to solve problems related to falling bodies, satellites, planets, and energy in gravitational fields, which frequently appear in JEE Main physics exams.

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Importance of Gravitation in JEE Main

Studying gravitation helps students:

• Analyze motion under gravity

• Apply Newton’s law of gravitation to planetary motion

• Solve satellite and orbital motion problems

• Understand energy and force concepts in gravitational fields

Questions from this chapter can be numerical or conceptual and often test understanding of multiple interconnected concepts, making it a scoring topic.

Newton’s Law of Universal Gravitation

Newton’s law states:

• Every particle in the universe attracts every other particle with a force F = G m₁ m₂ / r²

  • G = universal gravitational constant

  • m₁, m₂ = masses of particles

  • r = distance between them

Key points for JEE Main:

• Force is mutual and attractive

• Acts along the line joining the centers of two bodies

• Obeys inverse-square law

Applications:

• Determining gravitational forces between Earth and objects

• Calculating forces in multi-body systems

• Problems involving planets and satellites

Acceleration Due to Gravity (g)

• Near Earth’s surface: g = G M / R²

  • M = mass of Earth, R = radius of Earth

• Variation of g with altitude: g’ = g (1 − h/R)²

• Variation of g with depth: g’ = g (1 − d/R)

JEE Main numericals often include:

• Gravitational acceleration at different altitudes or depths

• Comparative problems between planets

Gravitational Field and Potential

• Gravitational field: g = F/m

• Gravitational potential: V = −G M / r

• Work done by gravitational force: W = m g h

• Potential energy: U = −G m₁ m₂ / r

Applications:

• Energy of satellites and planets

• Escape velocity calculations

• Problems with multiple gravitating bodies

Escape Velocity

• Minimum velocity to escape Earth’s gravitational field:

  • v_e = √(2 G M / R) = √(2 g R)

• Independent of mass of the object

• Key JEE Main application: rocket and satellite launch problems

Orbital Motion of Satellites

• Centripetal force provided by gravity: G M m / r² = m v² / r

• Orbital velocity: v = √(G M / r)

• Time period of satellite: T = 2 π r / v = 2 π √(r³ / G M)

• Geostationary satellites: T = 24 hours, orbit above equator

JEE Main numericals often require calculations of:

• Satellite velocity

• Orbital radius

• Time period of revolution

Kepler’s Laws and Gravitational Applications

1. First law: Orbits of planets are ellipses

2. Second law: Line joining planet and sun sweeps equal areas in equal time

3. Third law: T² ∝ r³, relates orbital period and radius

Applications:

• Planetary motion

• Satellite calculations

• Relative motion of celestial bodies

Gravitational Potential Energy of a System

• For two masses m₁ and m₂ separated by distance r:

  • U = −G m₁ m₂ / r

• Work done against gravity = change in potential energy

• Applications in calculating energy required for moving satellites

Combined Motion under Gravitation

• Two-body problems: consider reduced mass

• Motion of planets and satellites: central force problem

• JEE Main numericals involve:

  • Motion of Earth-Moon system

  • Orbits of satellites

  • Gravitational energy and forces

Common Mistakes Students Make

• Confusing g and G

• Ignoring radius variation in g-related problems

• Forgetting negative sign in potential energy

• Using incorrect formulas for orbital velocity or period

• Applying gravitational formulas to non-point masses without simplification

Avoiding these mistakes ensures high accuracy in JEE Main numericals.

Weightage of Gravitation in JEE Main

• 1–3 questions per exam

• Often integrated with circular motion, Newton’s Laws, and satellite motion

• Conceptually moderate but numerically high scoring

Gravitation is a strategically important chapter for securing marks in mechanics and astronomy-related problems.

Preparation Tips for JEE Main Students

• Memorize formulas for g, potential energy, orbital velocity, and escape velocity

• Solve numericals using step-by-step method and diagrams

• Practice problems on satellites and planetary motion

• Understand application of Kepler’s laws

• Use dimensional analysis to verify results

Consistency and careful practice improve accuracy and speed in exams.

Why Study Gravitation from Studentbro.in

Studentbro.in provides:

• Step-by-step explanations for gravitation problems

• Solved examples for satellites, planetary motion, and escape velocity

• Conceptual clarity and problem-solving strategies

• Chapter-wise structure tailored for JEE Main

This ensures students can solve gravitation numericals confidently and efficiently.

Conclusion

Gravitation is a fundamental Class 11 Physics chapter that explains forces acting at a distance, motion of celestial bodies, and energy in gravitational fields. Mastery of this chapter allows students to solve both conceptual and numerical problems efficiently. With structured guidance and practice from Studentbro.in, students can excel in JEE Main Physics and tackle gravitation-related questions with confidence.