CDS Physics · Free notes

Gravitation: CDS Physics

Gravitation has 12 past-year questions in CDS General Knowledge, from 2019 (I) to 2025 (I): 8 EASY, 3 MODERATE and 1 HARD. Eight of them are about Newton's law and weight: four are a single scaling step (double a mass, halve a distance, change g), and the rest ask what weight and the law really say. The other four are recall about the sky, from the nearest star to the Lagrange points. So square the distance factor every time, keep mass and weight apart, and learn the handful of space facts as a short table.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

CDS home All formulas on one page

Subtopic notes

Formula & revision sheet

2 formulas · 2 reference tables · 11 gotchas across all subtopics — the exam-eve cheat-sheet

Gravitation and Weight

Formulas (2)

Newton's law of gravitation · Universal law of gravitation $F = \dfrac{Gm_1m_2}{r^{2}} \qquad F' = F \times \dfrac{(\text{mass factors})}{(\text{distance factor})^{2}}$
Weight, mass and g · Weight and surface gravity $W = mg \qquad g = \dfrac{GM}{R^{2}}$

Watch out for (6)

The distance is squared
→ Newton's law of gravitation
Gravitation is its own law
→ Newton's law of gravitation
G is not g
→ Newton's law of gravitation
Weight is a force, mass is not
→ Weight, mass and g
g needs both mass and radius
→ Weight, mass and g
Weight scales with g, not with its square
→ Weight, mass and g

Stars and Space

Reference tables (2)

Nearby stars and constellations5 rows

Star or term	Fact
Sun	The nearest star; light takes about 8 min 20 s to reach us
Proxima Centauri	Nearest star after the Sun, about 4.24 light yearsQ CDS 2019 (I): of Sirius, Arcturus, Spica and Proxima Centauri, the nearest is Proxima Centauri.
Sirius	Brightest star of the night sky, about 8.6 light years
Arcturus and Spica	Bright stars much farther away (tens to hundreds of light years)
Constellation	A pattern of stars on the sky; the stars may be at very different distancesQ CDS 2019 (I): a constellation is a pattern of stars that may not be equidistant from the earth.

Lagrange points and the GMRT4 rows

Place or instrument	Fact
Sun-earth L1	On the Sun-earth line, between them; home of Aditya-L1
Sun-earth L2	On the line, beyond the earth; home of the James Webb Space Telescope
Sun-earth L4 (and L5)	Sun, earth and the point form an equilateral triangleQ CDS 2024 (II): the Sun, the earth and L4 form an equilateral triangle.
GMRT	30 parabolic dishes, each 45 m across, near PuneQ CDS 2024 (I): the GMRT consists of multiple parabolic antennas.

Watch out for (5)

Brightest is not nearest
→ Nearby stars and constellations
A constellation is not a group in space
→ Nearby stars and constellations
The Sun is a star too
→ Nearby stars and constellations
L4 and L5 are off the line
→ Lagrange points and the GMRT
The GMRT is many dishes, not one
→ Lagrange points and the GMRT

PYQ weightage by concept

4 concepts · 12 PYQs — where the marks actually sit, so you know what to drill first

Gravitation and Weight8 PYQs · 67%

Concept	PYQs	Share
Newton's law of gravitation	4	33%
Weight, mass and g	4	33%

Stars and Space4 PYQs · 33%

Concept	PYQs	Share
Nearby stars and constellations	2	17%
Lagrange points and the GMRT	2	17%

Test yourself on a real paper

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