Class 9 Science – Chapter 10: Gravitation (Detailed Notes)

Class 9 Science – Chapter 10: Gravitation (Detailed Notes)

1. Introduction to Gravitation

• Gravitation is the force of attraction between any two objects having mass.
• It is a universal force that acts everywhere in the universe.

2. Universal Law of Gravitation

• Proposed by Sir Isaac Newton.
• 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.
• Mathematically,
  F = G \frac{m_1 m_2}{d^2}
  where,
  F = gravitational force,
  m_1, m_2 = masses of the two objects,
  d = distance between their centers,
  G = universal gravitational constant.

3. Acceleration Due to Gravity (g)

• It is the acceleration produced in a body due to the gravitational force of the Earth.
• Its average value near Earth's surface is approximately 9.8 \, m/s^2.
• The value of g varies slightly with altitude and location on Earth.

4. Mass and Weight

• Mass: Amount of matter in an object. It is constant everywhere.
• Weight: Force exerted by gravity on an object. Weight = Mass × Acceleration due to gravity (W = mg).
• Weight varies with location, as g varies.

5. Free Fall

• When an object falls freely under the influence of gravity alone, it is said to be in free fall.
• In free fall, acceleration is equal to g and is constant.
• Air resistance is neglected in free fall.

6. Motion of Objects Under Gravity

• Objects fall with the same acceleration regardless of their masses (if air resistance is neglected).
• This was demonstrated by Galileo's famous experiment.

7. Satellites

• A satellite is an object that moves around a planet in a circular or elliptical orbit.
• Natural satellites include the Moon; artificial satellites are man-made objects launched into orbit.
• Satellites are in free fall but do not fall to Earth because they have horizontal velocity that keeps them in orbit.