Gravitation
Gravitation is the force of attraction between any two masses, and it keeps us on the Earth and the Moon in orbit. This Class 9 chapter covers the universal law of gravitation, acceleration due to gravity, the difference between mass and weight, and then moves to thrust, pressure, buoyancy, Archimedes' principle and relative density — explaining why objects float or sink.
Learning objectives
- State the universal law of gravitation.
- Distinguish mass from weight and explain free fall.
- Define thrust and pressure and use P = F/A.
- Explain buoyancy and Archimedes' principle.
- Define and calculate relative density.
Key concepts
Universal law of gravitation
Every object 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 them: F = G·m₁·m₂/r², where G is the universal gravitational constant.
Mass, weight and free fall
Mass is the amount of matter in a body (constant everywhere); weight is the force of gravity on it, W = mg, which varies with g. Near the Earth's surface g ≈ 9.8 m/s². An object falling only under gravity is in free fall.
Thrust and pressure
Thrust is the force acting perpendicular to a surface; pressure is thrust per unit area, P = F/A, measured in pascals (Pa). The same force gives more pressure on a smaller area, which is why sharp tools cut better.
Buoyancy, Archimedes' principle and relative density
Buoyancy is the upward force exerted by a fluid on an object in it. Archimedes' principle states that this upthrust equals the weight of fluid displaced. Relative density is the ratio of a substance's density to that of water; objects with relative density less than 1 float.
Important formulas
Universal law
F = G·m₁·m₂ ÷ r²
Weight
W = mg (g ≈ 9.8 m/s²)
Pressure
P = F ÷ A (pascal)
Relative density
= density of substance ÷ density of water
Key definitions
- Gravitation
- The force of attraction between any two objects having mass.
- Weight
- The force with which gravity pulls a body, W = mg.
- Pressure
- The thrust (force) acting per unit area on a surface.
- Buoyancy
- The upward force exerted by a fluid on an object placed in it.
Solved examples
Q1. Find the weight of a 10 kg object on Earth. (g = 9.8 m/s².)
Solution: W = mg = 10 × 9.8 = 98 N.
Q2. A force of 100 N acts on an area of 2 m². Find the pressure.
Solution: P = F/A = 100/2 = 50 Pa.
Q3. Why does an iron nail sink but a ship made of iron floats?
Solution: The nail's relative density is greater than 1, so it sinks. A ship is hollow with a large volume, so it displaces enough water for the upthrust to balance its weight, making it float (relative density of the whole ship is less than 1).
Common mistakes to avoid
- Treating mass and weight as the same — weight changes with g, mass does not.
- Forgetting the inverse-square (1/r²) in the law of gravitation.
- Confusing thrust (force) with pressure (force per area).
- Thinking heavy objects always sink — floating depends on relative density.
Gravitation — MCQ Quiz
11 questions with instant feedback. Use number keys 1–4 to answer.
The force of attraction between any two masses is:
Practice questions
Short answer
What is the difference between mass and weight?
Mass is the amount of matter (constant); weight is the gravitational force on it (W = mg), which varies with g.
State Archimedes' principle.
A body immersed in a fluid experiences an upward thrust equal to the weight of the fluid it displaces.
Why do school bags have wide straps?
Wide straps spread the force over a larger area, reducing the pressure on the shoulders.
Long answer
State the universal law of gravitation and define the gravitational constant G.
Every object attracts every other object with a force F = Gm₁m₂/r², directly proportional to the product of their masses and inversely proportional to the square of the distance between them. G is the universal gravitational constant — the force between two unit masses placed one unit distance apart — with value about 6.67 × 10⁻¹¹ N·m²/kg².
Explain why an object weighs less on the Moon than on the Earth.
Weight is W = mg, and g on the Moon is about one-sixth of that on the Earth because the Moon has much less mass and a smaller radius. The object's mass stays the same, but since g is smaller, its weight on the Moon is about one-sixth of its weight on the Earth.
HOTS (Higher Order Thinking)
Why is it easier to swim in sea water than in river water?
Sea water is denser, so it exerts a greater upthrust (buoyant force) on the swimmer for the same volume displaced, making it easier to stay afloat.
A camel walks easily on sand but a man in shoes sinks. Why?
The camel's broad feet spread its weight over a larger area, lowering the pressure on the sand, whereas narrow shoes concentrate the force over a small area, increasing pressure and causing sinking.
Quick revision
Revision notes
- Universal law: F = Gm₁m₂/r² (inverse-square).
- Mass is constant; weight W = mg varies with g (≈ 9.8 m/s²).
- Pressure P = F/A (pascal); thrust is the perpendicular force.
- Archimedes: upthrust = weight of fluid displaced; RD < 1 floats.
Key takeaways
- Don't confuse mass and weight.
- Smaller area → higher pressure for the same force.
- Floating depends on relative density, not just heaviness.
Frequently asked questions
What is the universal law of gravitation?
Every mass attracts every other mass with a force proportional to the product of the masses and inversely proportional to the square of the distance between them.
What is the difference between mass and weight?
Mass is the quantity of matter (same everywhere); weight is the gravitational force on it (W = mg) and changes with location.
What does Archimedes' principle state?
An object in a fluid is pushed up by a force equal to the weight of the fluid it displaces.