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CBSE | Class 9 | science
Read the chapter in a clean notebook-style flow before moving into deeper practice.
Summary Note
Key concept recap
Gravitation is a natural phenomenon by which all objects with mass attract one another. It is responsible for the motion of planets around the Sun and the falling of objects towards Earth.
Newton realized that the force responsible for a falling apple and the motion of the moon is the same—gravitational force.
The gravitational force acts between any two objects in the universe, regardless of size or distance.
Summary Note
Key concept recap
Every object in the universe attracts every other object with a force proportional to the product of their masses and inversely proportional to the square of the distance between them.
Mathematically, F = G × (M × m)/d², where F is gravitational force, G is the gravitational constant, M and m are masses, and d is the distance between them.
This law explains the motion of celestial bodies and tides caused by the moon and the Sun.
Summary Note
Key concept recap
When objects fall solely under the effect of gravity, they are said to be in free fall.
The acceleration during free fall is called acceleration due to gravity, denoted by 'g'. Its value near Earth’s surface is approximately 9.8 m/s².
All objects fall with the same acceleration regardless of mass, neglecting air resistance.
Summary Note
Key concept recap
The motion of objects under gravity can be described using standard kinematic equations by replacing acceleration (a) with 'g':
v = u + gt, s = ut + ½gt², v² = u² + 2gs.
These equations help calculate velocity, distance, and time for free-falling objects or vertically projected bodies.
Summary Note
Key concept recap
Mass is the quantity of matter in an object and remains constant everywhere in the universe.
Weight is the force with which the Earth attracts an object and is calculated using W = mg.
Weight varies with location due to variation in 'g', but mass remains unchanged.
Summary Note
Key concept recap
The moon has less mass than the Earth, so the gravitational pull it exerts is weaker.
Weight of an object on the moon is 1/6th of its weight on Earth due to lower gravitational acceleration.
This is important in space missions and understanding gravity on other celestial bodies.
Summary Note
Key concept recap
Thrust is the force applied perpendicular to the surface, while pressure is thrust per unit area.
Pressure = Thrust / Area. Smaller the area, greater the pressure for the same force.
Applications include sharp-edged tools, wide tires, and foundations of buildings.
Summary Note
Key concept recap
Fluids exert an upward force called buoyancy on objects immersed in them.
Archimedes’ Principle states that an object immersed in a fluid experiences an upward force equal to the weight of the fluid displaced.
This explains why some objects float while others sink, depending on their density relative to the fluid.
Summary Note
Key concept recap
Gravitational Force – Attractive force between any two masses.
Universal Law of Gravitation – F = G(Mm)/d².
Free Fall – Motion of object under gravity alone.
Acceleration due to Gravity – g ≈ 9.8 m/s² on Earth.
Mass – Measure of matter, constant everywhere.
Weight – Force of gravity on an object (W = mg).
Thrust – Force applied perpendicular to a surface.
Pressure – Thrust per unit area, measured in pascal (Pa).
Buoyancy – Upward force on objects in fluid.
Archimedes’ Principle – Buoyant force equals weight of displaced fluid.
