Paper II · Part B — Physics

Gravitation

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Concept

Every mass attracts every other mass with a force described by Newton's law of gravitation. Near the Earth this gives a body its weight and a free-fall acceleration g.

Mass is constant; weight depends on g and so varies with location.

Key Points & Formulas

Gravitational force F = G m₁m₂ / r².
Weight W = mg; g ≈ 9.8 m/s² near the Earth's surface.
g is greatest at the poles, least at the equator; decreases with altitude.
Weight on the Moon ≈ 1/6 of that on the Earth.

Worked Example

Q. A body has mass 10 kg. Find its weight on Earth (g = 9.8 m/s²).

W = mg.

= 10 × 9.8 = 98 N.

Common traps

Reading the prompt too quickly and missing a qualifier.
Memorising the result without checking when it applies.
Skipping units, signs, chronology or the final reasonableness check.

NDA shortcuts

State the governing rule before substituting values.
Eliminate impossible options before doing full calculation.

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5 NDA questions

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Question 1

NDA Practice 2026

The acceleration of a freely falling body near the Earth's surface is about

Question 2

NDA I 2025

Two bodies of unequal masses are dropped from a tower. At any instant, they have equal

Question 3

NDA II 2024

An astronaut whose weight on the Earth is 600 N experiences weightlessness on International Space Station orbiting around the Earth. It means that

Question 4

NDA I 2024

Escape speed from the Earth is close to 11.2 km s^-1. On another planet whose radius is half of the Earth’s radius and whose mass density is four times that of the Earth, the escape speed in km s^-1 will be close to:

Question 5

NDA II 2024

The masses of two planets are in the ratio of 1 : 7. The ratio between their diameters is 2 : 1. The ratio of forces which they exert on each other is

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