Gravitation
About 7 min · Science · English
Time worked out from the lesson's 513 words at 150 words a minute, plus a minute for each question.
Gravitation is a fundamental force that governs the motion of celestial bodies and falling objects on Earth. For SOF NSO Class 9-10, this topic bridges everyday observations (objects falling, weight on a scale) with universal physical laws. Students must master Newton's universal law of gravitation, understand the distinction between mass and weight, explain free fall motion, and apply thrust-pressure concepts to solve numerical problems.
Key ideas
- Universal Law of Gravitation: Every object in the universe attracts every other object with a force directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. This explains planetary orbits, tides, and why objects fall to Earth.
- Gravitational Constant (G): The value 6.67 × 10⁻¹¹ N m²/kg² is a universal constant. It remains the same everywhere in the universe, unlike acceleration due to gravity (g) which varies with location.
- Free Fall: Motion of an object under the sole influence of gravity (ignoring air resistance). All objects, regardless of mass, fall with the same acceleration g = 9.8 m/s² near Earth's surface.
- Mass vs Weight: Mass is the amount of matter in an object (scalar, measured in kg, constant everywhere). Weight is the gravitational force on an object (vector, measured in newtons, varies with location).
- Acceleration due to Gravity (g): On Earth's surface g ≈ 9.8 m/s² or 10 m/s² (for quick calculations). It decreases with altitude and differs on other planets/moons based on their mass and radius.
Quick reference
- F = G(m₁m₂)/r² with G = 6.67 × 10⁻¹¹ N m²/kg² — universal gravitation
- W = mg where g = 9.8 m/s² on Earth — weight formula
- g on Moon = g on Earth ÷ 6 — remember the 1/6 ratio
- Free fall: all objects fall at same rate (9.8 m/s²), independent of mass
- Pressure = Force/Area — smaller area means higher pressure
Check yourself: 3 questions
1. The universal law of gravitation states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centers. If the distance between two objects is doubled while keeping their masses constant, what happens to the gravitational force between them?
2. A person weighing 600 N on Earth travels to the Moon where the acceleration due to gravity is 1.6 m/s². What will be the mass and weight of the person on the Moon? (Take g on Earth = 10 m/s²)
3. A stone is dropped from the top of a building and falls freely under gravity. Ignoring air resistance, which of the following statements is correct about its motion?
This lesson is cut from the topic's note, not rewritten. 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. The questions are from Shishya's validated question bank.
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