Schooling · Class 11 · Physics · Gravitation · Chapter on a page
CBSE Class 11 · Physics · NCERT Physics Part-I
Gravitation: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
Fact-checked on 21 Oct 2026: an AI-assisted check of this note against cited sources; the errors it found were corrected that day. Some errors may remain.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- Universal gravitation
- Acceleration due to gravity (g)
- Gravitational potential energy
- Kepler's laws
- Orbital velocity
- Escape velocity
- Geostationary satellites
- Formulas and facts
- Newton's law of universal gravitation
- Acceleration due to gravity at…
- Variation of g with altitude h (when…
- Variation of g with depth d
- Gravitational potential energy
- Orbital velocity for circular orbit
- Escape velocity from Earth's surface
- Watch out for
- Confusing mass and weight
- Using g = 9.8 m/s² for all situations
- Forgetting the negative sign in…
- Thinking satellites stay up because…
- Applying Kepler's third law with…
What this chapter is about
Gravitation is the fundamental force of attraction that exists between any two masses in the universe. This chapter builds on Newton's laws of motion to explain how the same force that makes an apple fall to the ground also keeps the Moon in orbit around the Earth and the planets in orbit around the Sun. You will learn Newton's law of universal gravitation, which provides a single mathematical description for all gravitational phenomena.
Key ideas
- Universal gravitation: Every mass attracts every other mass with a force proportional to the product of their masses and inversely proportional to the square of the distance between their centres. This force acts along the line joining them.
- Acceleration due to gravity (g): Near Earth's surface, all objects experience the same gravitational acceleration of approximately 9.8 m/s², regardless of their mass. This value varies slightly with altitude, depth, and latitude.
- Gravitational potential energy: The work done against gravity to bring a mass from infinity to a point in a gravitational field. It is always negative, with zero reference at infinity.
- Kepler's laws: 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; the square of the orbital period is proportional to the cube of the semi-major axis.
- Orbital velocity: The horizontal speed needed for an object to remain in a stable circular orbit around a planet without falling or escaping.
- Escape velocity: The minimum speed an object must have at Earth's surface to escape Earth's gravitational pull completely, without any further propulsion.
- Geostationary satellites: Satellites that orbit Earth with a period of 24 hours, appearing stationary relative to a point on Earth's surface, used for communication purposes.
Formulas and facts to remember
- Newton's law of universal gravitation:F = G × m₁ × m₂ / r²G = 6.67 × 10⁻¹¹ N m² kg⁻² (universal gravitational constant)
- Acceleration due to gravity at Earth's surface:g = G × M / R²where M is Earth's mass and R is Earth's radius; g ≈ 9.8 m/s²
- Variation of g with altitude h (when h << R):g' = g × (1 − 2h/R)
- Variation of g with depth d:g' = g × (1 − d/R)
- Gravitational potential energy:U = −G × M × m / rNegative sign indicates bound system; zero at infinity.
- Orbital velocity for circular orbit:v₀ = √(G × M / r) = √(g × R) for orbit near surface
- Escape velocity from Earth's surface:vₑ = √(2 × G × M / R) = √(2 × g × R) ≈ 11.2 km/s
- Kepler's third law:T² ∝ a³, or T² = (4π² / G × M) × a³where T is orbital period and a is semi-major axis.
- Time period of satellite in circular orbit:T = 2π × √(r³ / G × M)
Common mistakes
- Confusing mass and weight → Mass is the amount of matter (constant everywhere), while weight is the gravitational force on that mass (varies with location and g).
- Using g = 9.8 m/s² for all situations → Remember that g varies with altitude, depth, and latitude; use the appropriate formula when the situation demands.
- Forgetting the negative sign in gravitational potential energy → The negative sign is essential; it shows that energy must be supplied to separate the masses to infinity.
- Thinking satellites stay up because gravity is zero in orbit → Gravity acts on satellites; they are in continuous free fall but their horizontal velocity keeps them in orbit.
- Applying Kepler's third law with wrong units → When using T² ∝ a³, ensure consistent units; mixing years with metres or days with kilometres gives incorrect results.
Quick revision
- Newton's law: F = Gm₁m₂/r², where G = 6.67 × 10⁻¹¹ N m² kg⁻².
- Acceleration due to gravity: g = GM/R² ≈ 9.8 m/s² at Earth's surface.
- Escape velocity from Earth: vₑ = √(2gR) ≈ 11.2 km/s.
- Orbital velocity for satellite near surface: v₀ = √(gR) ≈ 7.9 km/s.
- Gravitational potential energy is negative and increases (becomes less negative) as distance increases.
- Kepler's third law: T² ∝ a³ applies to all bodies orbiting the same central mass.