NEET Physics — Gravitation Complete Chapter Guide

Chapter Overview & Weightage

Gravitation brings together Newton’s universal law with orbital mechanics and the variation of $g$ . NEET tests this chapter with 1-2 questions, usually from escape velocity, orbital velocity, or variation of $g$ with height/depth.

Gravitation carries 2-3% weightage in NEET. While the question count is lower (1-2), the formulas are direct and the questions are typically easy-to-medium. This is a high-return-on-investment chapter.

Year	NEET Q Count	Key Topics Tested
2025	1	Escape velocity
2024	2	Orbital velocity, variation of g
2023	1	Kepler’s third law
2022	2	Gravitational PE, escape velocity
2021	1	g at height and depth

graph TD
    A[Gravitation] --> B[Newton's Law of Gravitation]
    A --> C[Variation of g]
    A --> D[Orbital Mechanics]
    A --> E[Energy in Orbits]
    C --> F[g at Height]
    C --> G[g at Depth]
    D --> H[Orbital Velocity]
    D --> I[Time Period]
    D --> J[Kepler's Laws]
    E --> K[Escape Velocity]
    E --> L[Binding Energy]

Key Concepts You Must Know

Tier 1 (Always asked)

Escape velocity: $v_e = \sqrt{2gR}$
Orbital velocity: $v_o = \sqrt{gR}$ (for orbit close to surface)
Variation of $g$ with height: $g' = g\left(1 - \frac{2h}{R}\right)$ for $h \ll R$
Variation of $g$ with depth: $g' = g\left(1 - \frac{d}{R}\right)$

Tier 2 (Frequently asked)

Kepler’s third law: $T^2 \propto r^3$
Gravitational potential energy: $U = -\frac{GMm}{r}$
Relation between $v_e$ and $v_o$ : $v_e = \sqrt{2} \cdot v_o$

Tier 3 (Occasional)

Geostationary satellite conditions
Gravitational field due to a uniform sphere (shell theorem)
Energy of a satellite in orbit

Important Formulas

F = \frac{GMm}{r^2}

On the surface of Earth: $g = \dfrac{GM}{R^2}$ , so $GM = gR^2$ . This substitution eliminates $G$ and $M$ from almost every problem.

At height $h$ above surface (exact): $g_h = \dfrac{g}{\left(1 + \frac{h}{R}\right)^2}$

At height $h$ (approximate, $h \ll R$ ): $g_h \approx g\left(1 - \dfrac{2h}{R}\right)$

At depth $d$ below surface: $g_d = g\left(1 - \dfrac{d}{R}\right)$

At the centre of Earth: $g = 0$

Orbital velocity (radius $r$ ): $v_o = \sqrt{\dfrac{GM}{r}} = \sqrt{\dfrac{gR^2}{r}}$

Close to surface: $v_o = \sqrt{gR} \approx 7.9$ km/s

Escape velocity: $v_e = \sqrt{\dfrac{2GM}{R}} = \sqrt{2gR} \approx 11.2$ km/s

Key relation: $v_e = \sqrt{2} \cdot v_o$

Third law: $T^2 = \dfrac{4\pi^2}{GM}r^3$

Total energy of satellite: $E = -\dfrac{GMm}{2r}$

KE $= \dfrac{GMm}{2r}$ , PE $= -\dfrac{GMm}{r}$

Note: Total energy = $-$ KE = $\dfrac{\text{PE}}{2}$

The substitution $GM = gR^2$ is a universal shortcut. It converts every formula from “gravitational constant and mass of Earth” form into “surface gravity and radius” form — which is what NEET problems typically give you.

Solved Previous Year Questions

PYQ 1 — NEET 2024

Problem: If the radius of Earth were doubled keeping mass the same, by what factor would the escape velocity change?

Solution:

v_e = \sqrt{\frac{2GM}{R}}

If $R \to 2R$ :

v_e' = \sqrt{\frac{2GM}{2R}} = \frac{1}{\sqrt{2}} \cdot v_e

Escape velocity becomes $\mathbf{\dfrac{1}{\sqrt{2}}}$ times the original.

PYQ 2 — NEET 2023

Problem: A satellite orbits Earth at a height equal to the radius of Earth. Find the orbital velocity in terms of escape velocity at the surface.

Solution:

At height $h = R$ , the orbital radius $r = R + R = 2R$ .

v_o = \sqrt{\frac{GM}{2R}}

Escape velocity at surface: $v_e = \sqrt{\frac{2GM}{R}}$

Ratio:

\frac{v_o}{v_e} = \sqrt{\frac{GM/2R}{2GM/R}} = \sqrt{\frac{1}{4}} = \frac{1}{2}

So $v_o = \mathbf{\dfrac{v_e}{2}}$ .

PYQ 3 — NEET 2022

Problem: At what depth below the Earth’s surface is the value of $g$ equal to that at a height of 10 km? (Radius of Earth = 6400 km)

Solution:

At height $h$ : $g_h = g\left(1 - \dfrac{2h}{R}\right) = g\left(1 - \dfrac{20}{6400}\right)$

At depth $d$ : $g_d = g\left(1 - \dfrac{d}{R}\right)$

Setting $g_h = g_d$ :

1 - \frac{2h}{R} = 1 - \frac{d}{R}

d = 2h = 2 \times 10 = \mathbf{20 \text{ km}}

This is a classic result: the depth required equals twice the height for equal values of $g$ (when $h \ll R$ ). Memorise $d = 2h$ — it saves calculation time in NEET.

Difficulty Distribution

Difficulty	% of Questions	What to Expect
Easy	45%	Direct formula — escape velocity, orbital velocity
Medium	40%	Variation of g, Kepler’s law applications
Hard	15%	Energy in orbit, satellite transfer problems

Expert Strategy

Week 1: Memorise the five key formulas: $F = GMm/r^2$ , $v_o$ , $v_e$ , $g_h$ , $g_d$ . Always practise with the $GM = gR^2$ substitution. Solve 15 problems on variation of $g$ .

Week 2: Kepler’s laws and satellite problems. The time period formula and energy relations are formulaic — once you memorise them, these questions become direct substitution.

Week 3: Practise PYQs. Gravitation has limited variety in NEET — the same 4-5 question types repeat. After solving 20 PYQs, you will have seen every pattern.

Common Traps

Trap 1 — Using the approximate formula when $h$ is not small. The formula $g' = g(1 - 2h/R)$ works only when $h \ll R$ . For $h$ comparable to $R$ , use the exact formula $g' = g/(1 + h/R)^2$ . NEET sometimes gives $h = R$ or $h = R/2$ to test this.

Trap 2 — Escape velocity depends on mass of the planet, not the object. $v_e = \sqrt{2GM/R}$ has no term for the mass of the escaping object. A 1 kg ball and a 1000 kg satellite have the same escape velocity from Earth.

Trap 3 — Sign of gravitational PE. $U = -GMm/r$ . It is always negative (for bound systems). As $r$ increases, $U$ increases (becomes less negative). Students often confuse “increases” with “becomes more negative.”

Trap 4 — Total energy of a satellite is negative. $E = -GMm/(2r)$ . If a question asks “what happens to total energy when orbit radius increases” — the answer is it increases (becomes less negative, i.e., closer to zero). This is counterintuitive but crucial.