Short Notes -1 – Chapter 9 – Gravitational -Science- Ncert-Class 9

Short Notes -1 – Chapter 9 – Gravitational -Science- Ncert-Class 9

9.1 Gravitation

Observation of Celestial Motion:

◆The chapter begins by noting that the moon orbits the earth, indicating the presence of a force acting between the two bodies.

Newton’s Insights:

◆It references the famous anecdote of Newton’s contemplation upon seeing an apple fall from a tree. This event sparked Newton’s curiosity about the nature of the force causing objects to fall towards the earth.

◆Newton wondered if the same force that causes an apple to fall also acts between the earth and the moon.

Newton’s Conjecture:

◆Newton conjectured that the force responsible for the moon’s orbit around the earth is the same force that causes objects to fall towards the earth.

◆He reasoned that since the moon doesn’t move off in a straight line but instead orbits the earth, it must be attracted by the earth’s gravitational force.

Observation of Moon’s Motion:

◆The text highlights that despite the moon constantly falling towards the earth due to gravity, it doesn’t appear to be falling from our perspective on earth.

◆This observation raises questions about how gravity works and how it affects celestial bodies.

9.1.Activity

Centripetal Force:

◆Before the thread holding a stone is released, the stone moves in a circular path with a certain speed, changing direction at every point.

◆The change in direction involves a change in velocity, which requires acceleration. The force causing this acceleration, directed towards the center of the circular path, is called the centripetal force.

◆Without the centripetal force, the stone would fly off along a tangent to the circular path.

Gravitational Force:

◆The motion of the moon around the earth is also due to the centripetal force, which is provided by the force of attraction of the earth.

◆Similarly, all planets in the solar system orbit the sun due to the gravitational force exerted by the sun.

◆According to Newton’s third law of motion, the apple attracts the earth, but due to the difference in mass (the earth’s mass being much larger than that of the apple), the acceleration of the earth towards the apple is negligible and not perceptible.

Universal Gravitation:

◆Newton concluded from these observations that there exists a force of attraction between all objects in the universe, not just between the earth and the moon or between the sun and the planets.

◆This force of attraction between objects is called the gravitational force.

◆Newton’s law of universal gravitation states that every object in the universe attracts every other object with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between their centres.

9.2 Free Fall

Definition of Free Fall:

◆Free fall refers to the motion of objects falling towards the Earth under the influence of gravitational force alone.

◆When objects fall freely towards the Earth, they experience a change in velocity due to the Earth’s gravitational pull.

Acceleration Due to Gravity:

◆The change in velocity of falling objects results in acceleration, which is caused by the Earth’s gravitational force.

◆This acceleration due to the Earth’s gravitational force is denoted by g and has the same unit as acceleration, meters per second squared (m/s² ).

◆The acceleration due to gravity, g, causes a change in the magnitude of the velocity of falling objects but does not change the direction of their motion.

 Calculation of Gravitational Force:

◆According to Newton’s second law of motion, force (F) is the product of mass (m) and acceleration.

◆Therefore, the magnitude of the gravitational force (F) acting on an object can be calculated as the product of its mass and the acceleration due to gravity (g), expressed as F=m x g.

Mathematical Representation:

◆If the mass of the stone in the activity is m, then the magnitude of the gravitational force acting on it (F) is m x g.

9.3 Mass

Mass and Inertia:

◆Mass is a fundamental property of matter that quantifies the amount of substance present in an object.

◆Inertia, the resistance of an object to changes in its state of motion, is directly related to its mass. Greater mass implies greater inertia.

Consistency of Mass:

◆The passage reinforces that an object’s mass remains constant regardless of its location. Whether the object is on Earth, the moon, or even in outer space, its mass remains the same.

◆This constancy of mass implies that an object’s inertia remains consistent across different environments.

9.4 Weight

Definition of Weight:

◆The weight of an object is the force with which it is attracted towards the Earth due to gravity.

◆Mathematically, weight (W) is determined by multiplying the mass (m) of the object by the acceleration due to gravity (g), expressed as W=m×g.

Representation of Weight:

◆Weight is represented by the force of attraction exerted by the Earth on an object, acting vertically downwards.

◆It possesses both magnitude and direction, with its magnitude being directly proportional to the mass of the object.

Unit of Weight:

The SI unit of weight is the newton (N), which is the same as that of force.

This unit reflects the force exerted by gravity on an object due to its mass.

Relation to Mass:

◆The value of gravitational acceleration (g) is constant at a given location.

Therefore, at a particular place, the weight of an object is directly proportional to its mass (m), denoted as W∝m.

◆This relationship allows us to use weight as a measure of mass at a given location, as mass remains constant everywhere while weight varies depending on the local gravitational acceleration.