Class 9th- GRAVITATION (Prashant Kirad)

Introduction

  • Class: 9th

  • Topic: Gravitation

  • Presenter: Prashant Bhaiya

Overview of Gravitation

  • Covers key topics:

    • Universal Law of Gravitation

    • Free Fall

    • Value of g

    • Mass and Weight

    • Weight on the Moon

    • Thrust and Pressure

    • Buoyancy

    • Archimedes’ Principle

Historical Context

  • In 1666, Isaac Newton observed an apple fall, leading him to the concept of gravity.

  • Recognized gravity as a universal force acting on objects towards Earth and keeping planets in orbit.

Centripetal Force

  • Definition: Force that acts on an object moving in a circular path, directing it towards the center.

  • Example: Earth's gravity provides centripetal force that maintains the Moon's orbit.

  • Key aspect of circular motion is constant change in direction and velocity.

Definition of Gravitation

  • Gravitation: The force of attraction between any two bodies.

  • Examples:

    • Gravity causes a ball to fall to the ground.

    • Keeps planets in orbit around the Sun.

    • Causes disturbance in the motion of objects rolling downhill.

Newton’s Law of Gravitation

  • States that every object attracts every other object with a force:

    • Directly proportional to the product of their masses.

    • Inversely proportional to the square of the distance between them.

Gravitational Force Case Studies

  • Earth is affected by the Sun's gravity but does not fall into it due to its orbital velocity.

  • Equal and opposite forces exist between a stone and the Earth, yet only the stone visibly moves downward due to Earth's greater mass.

  • The force of gravitation doubles if the distance between two objects is halved.

Calculating Gravitational Force

  • Example Calculation:

    • If masses are 80 kg and 200 kg separated by 6m, apply Newton’s Law.

Importance of Universal Law of Gravitation

  • Fundamental to understanding:

    • Binding force keeping us on Earth.

    • The Moon's motion around Earth.

    • Earth’s motion around the Sun.

    • Relation to ocean tides influenced by the Moon.

Acceleration Due to Gravity (g)

  • Formula: g = G * (M/R²)

  • Defined variables:

    • G = gravitational constant (6.673 × 10⁻¹¹ Nm²/kg²)

    • M = mass of Earth (6 × 10²⁴ kg)

    • R = radius of Earth (6.37 × 10⁶ m)

  • Final calculated value: g ≈ 9.8 m/s²

Variation of g

  • Altitude: g decreases with greater height from Earth's surface.

  • Depth: g decreases deeper into the Earth due to mass distribution.

  • Latitude: g is highest at the poles (due to shape of Earth) and lowest at the equator.

  • Question on quicker drop at poles vs. equator due to variations in g.

Mass vs. Weight

  1. Mass

    • Defined as the amount of matter, SI unit: kilogram (kg)

    • Scalar quantity, constant regardless of location.

  2. Weight

    • Measure of gravitational force on an object, SI unit: Newton (N)

    • Vector quantity, varies with acceleration due to gravity.

    • On the Moon, weight is 1/6 of weight on Earth.

Differences between Mass and Weight

  • Mass:

    • Scalar, constant everywhere, measured in kg.

    • Does not change based on location.

  • Weight:

    • Vector, varies with gravity, measured in N.

    • Can be zero in free-fall or at Earth's center.

Weight on the Moon

  • Weight calculation using the universal law of gravitation.

  • Comparative calculation of weight on Moon vs. Earth, yielding approximately 1/6 of Earth’s weight.

Free Fall Definition

  • Motion of an object where gravity is the only force acting upon it, leading to acceleration downwards.

Motion Equations

  • Forms of motions affected by gravity:

    • First Equation: v = u + g*t

    • Second Equation: s = ut + 0.5gt²

    • Third Equation: v² = u² + 2gs

Characterization of Pressure

  • Definition: Thrust acting on a unit area, and its effect varies by contact area.

  • SI Unit: Pascal (Pa) or N/m².

Thrust and its Effects

  • Thrust's impact based on area of contact, illustrated in various scenarios (e.g., knife cutting, carrying heavy objects).

  • Thrust on loose sand vs. lying down: Larger contact area leads to less pressure.

Archimedes’ Principle

  • States the buoyant force on an object is equal to the weight of the fluid it displaces.

  • Applications in submarines, hot-air balloons and ships discussed.

Conclusion

  • Essential understanding of gravitation is foundational for recognizing its relevance across diverse physical phenomena.