CBSE Class 9 Science Notes – Chapter 9 : Gravitation

Gravitation

Force between two objects or bodies which attract each other due to their masses

Effects of Gravity

Free fall

• When an object falls down towards the earth under the gravitational force only.
• Velocity of free falling body changes and is said to be acceleration due to gravity.
• Acceleration due to gravity, ‘g’. Unit is m/s². Value of g = 9.8 m/s².

Universal Law of Gravitation

• It states that every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance only.
• F∝(Mm/d²)

F =(GMm/d²) [ ∵ G = Universal gravitational constant]

• SI unit of G = Nm²/Kg²
• Value of G = 6.673 × 10^–11 Nm²/Kg²

Relation Between g And G

• g =(GM/R²)

(where G = gravitational constant, discovered by Cavendish

M = Mass of the earth

R = Radius of the earth)

g =(6.7×10^-11Nm²Kg^–2 ×6 × 10²⁴Kg/(6.4 × 10^–6 m)²)

• As the radium of the earth increases from the poles to the equator, the value of g becomes greater at the poles than at the equator.

Mass And Weight

Mass

• Mass of an object is the measure of its inertia. (Scaler quantity)
• It remains the same everywhere in the Universe.
• SI Unit : Kg  

Weight

• Weight is the force of attraction of the earth on the object (vector quantity)
• Weight of the body changes form place to place
• W = mg
• SI Unit: Newton (N)

Thrust

• Net force applied in a particular direction.
• Depends on the area on which it acts.
• SI Unit: Newton

Archimedes’ Principle

When a body is immersed fully or partially in a field, it experiences an upward force that is equal to the weight of the fluid displaced by it.

Applications:

• Used in designing ships and submarines
• Used in lactomoters to find purity of milk.
• Used in hydrometer to determine density of liquid.