Summary
Chapter 7 of the Class 11 Physics NCERT textbook, "Gravitation", explains the force of attraction between all material objects, inversely proportional to the square of the distance between them. Newton's law of universal gravitation governs both terrestrial and celestial phenomena, with gravitational constant G = 6.67×10⁻¹¹ N m²/kg².
- One law for heaven and Earth — The chapter traces the historical path from Ptolemy through Copernicus to Newton, whose universal law of gravitation unified falling apples and orbiting planets. Kepler's laws of planetary motion emerge naturally within this single framework.
- Gravity near and around Earth — Acceleration due to gravity is not fixed—it changes with altitude and depth. The chapter examines how g varies, alongside gravitational potential energy, providing the basis for describing motion under Earth's pull.
- Escape speed and orbital energy — Escape speed sets the threshold to leave Earth's gravity, and orbiting satellites carry negative total mechanical energy. These ideas turn gravitation into a working model for launches and satellite orbits.
Key points & formulas
- 01All planets move in elliptical orbits with the Sun at one focus; the line joining a planet to the Sun sweeps equal areas in equal times; and the square of orbital period is proportional to the cube of semi-major axis (Kepler's laws)
- 02Gravitational force: F = Gm₁m₂/r² where G = 6.67×10⁻¹¹ N m²/kg²; independent of intervening matter and the nature of objects
- 03Acceleration due to gravity decreases with altitude: g(h) ≈ g(1 − 2h/Rₑ) for h << Rₑ and decreases with depth: g(d) = g(1 − d/Rₑ)
- 04Gravitational potential energy at distance r from Earth's center: W(r) = −GMₑm/r; total mechanical energy of orbiting satellites is negative
- 05Escape speed from Earth's surface: vₑ = √(2GMₑ/Rₑ) = √(2gRₑ) ≈ 11.2 km/s; independent of projectile mass or direction
- 06A uniform spherical shell exerts gravitational force on external objects as if all mass is concentrated at the centre; exerts zero force on internal objects
Frequently asked questions
01What is Newton's law of universal gravitation?
Every body in the universe attracts every other body with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between them. Mathematically: F = Gm₁m₂/r², where G = 6.67×10⁻¹¹ N m²/kg².
02How does gravitational acceleration change with altitude and depth?
At altitude h above Earth's surface, g(h) = GMₑ/(Rₑ+h)² ≈ g(1 − 2h/Rₑ) for small heights. At depth d below the surface, g(d) = g(1 − d/Rₑ). Thus, gravity is maximum on Earth's surface and decreases both upward and downward.
03What is the escape speed and how is it derived?
Escape speed is the minimum speed required for an object to escape Earth's gravitational influence. Using energy conservation, when kinetic energy equals the magnitude of gravitational potential energy: vₑ = √(2GMₑ/Rₑ) = √(2gRₑ) ≈ 11.2 km/s from Earth's surface. It is independent of the projectile's mass, direction, or where it is launched from.
04Is the NCERT Class 11 Physics Chapter 7 PDF free to download?
Yes, the NCERT Class 11 Physics Chapter 7 (Gravitation) PDF is free to download. NCERT textbooks are published by India's National Council of Educational Research and Training and are freely available to all students.
More chapters in Physics Part I
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