Gravitation
8 Nov 2025

CLASS 9 SCIENCE Chapter- Gravitation | NOTES

INTRO- GRAVITATION –Gravitation is a natural phenomenon by which every object in the universe attracts every other object with a force that acts along the line joining their centres. -This force of mutual attraction between two masses is called the gravitational force. -Everybody that has mass exerts an attractive force on every other body in the universe. -The magnitude of this force depends on: #The greater the masses, the stronger the attraction; the larger the distance, the weaker the force. -The idea of gravitation was explained by Sir Isaac Newton in 1687. (He discovered that the same force that causes an apple to fall to the ground also holds the Moon in orbit around the Earth and the planets in orbit around the Sun. This inspired him to create the Universal Law of Gravitation, which mathematically defines the force.) *Nature of the Gravitational Force    Property                                                        Description Type of Force Non-contact (acts without physical contact) Direction Always acts along the line joining the centres of two bodies Nature Always attractive, never repulsive Range Infinite (though becomes very weak with distance) Strength Weak compared to other fundamental forces (e.g., electromagnetic) *Difference Between Gravitation and Gravity Basis Gravitation Gravity Definition The universal force of attraction between any two bodies in the universe. The gravitational force specifically exerted by the Earth on objects near its surface. Scope Universal – acts between any two masses. Local – acts only near the Earth. Example Attraction between the Sun and Earth. Attraction between Earth and an apple. Newton’s Universal Law of Gravitation Newton’s Universal law of 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. Let two objects of masses m1 and m2 be separated by a distance . Then, according to Newton’s law, To remove proportionality, a constant G (the universal gravitational constant) is introduced:                                                                   Where, G = 6.67×10-11 Nm2/kg2 *Relation between Newton’s third law of motion and Newton’s law of gravitation -Newton’s third law of motion: According to Newton’s third law of motion, “Every object exerts equal and opposite force on other object but in opposite direction.” –Newton’s law of gravitation: According to Newton’s law of gravitation, “Every mass in the universe attracts every other mass.” In case of freely falling stone and earth, stone is attracted towards earth means earth attracts the stone. But according to Newton’s third law of motion, the stone should also attract the earth.It is true that stone also attracts the earth with the same force F = m × a but due to very less mass of the stone, the acceleration (a) in its velocity is 9.8 m/s2 and acceleration (a) of earth towards stone is 1.65×10-24 m/s2 which is negligible and we cannot feel it. *Importance of Universal Law of Gravitation FREE FALL OF AN OBJECT AND ACCELERATION DUE TO GRAVITY Free fall is the motion of a body when it falls towards the Earth under the influence of gravitational force only, with no other force (like air resistance) acting on it. When an object is thrown upward, it reaches certain height, then it starts falling down towards earth. It is because the earth’s gravitational force exerts on it. This fall under the influence of earth is called ‘free fall of an object’. During this free fall direction do not change but velocity continuously changes which is called acceleration due to gravity. It is denoted by ‘g’. Its unit is same as acceleration m/s2. *Derivation for Acceleration Due to Gravity From Newton’s law of gravitation: According to Newton’s second law of motion: For a freely falling body: Equating both forces: Cancelling m from both sides: Formula: *The Value of acceleration due to gravity (g) where, *Difference Between Gravitational constant (G) and Acceleration due to gravity (g) Basis of Difference Universal Gravitational Constant (G) Acceleration Due to Gravity (g) 1. Definition It is a constant that measures the strength of the gravitational force between any two bodies in the universe. It is the acceleration produced in a body when it falls freely under the influence of Earth’s gravity. 2. Formula 3. Nature It is a universal constant, same everywhere in the universe. It varies from place to place (depends on height, depth, and latitude). 4. Value G= 6.67×10-11  5. Depends On It does not depend on mass, distance, or location. It depends on the mass and radius of the Earth. 6. Type of Quantity Scalar quantity (has only magnitude). Vector quantity (has magnitude and direction — toward Earth’s center). 7. Symbol G g 8. Unit N·m²/kg² m/s² 9. Discovery Discovered by Sir Isaac Newton. Measured experimentally on Earth. 10. Variation Remains constant throughout the universe. Decreases with altitude and depth; increases at poles. *Equations of Motion for a Freely Falling Body The same kinematic equations apply, replacing a by g: #For Object Falling Downward #For Object Thrown Upward *Factors Affecting the Value of g MASS AND WEIGHT *MASS *WEIGHT *Difference Between Mass and Weight Basis Mass Weight Definition Amount of matter contained in a body. Force with which Earth attracts a body towards its center. Formula m=w/g W = mg Nature Scalar quantity. Vector quantity (acts downward). Unit (SI) Kilogram (kg). Newton (N). Value Constant everywhere. Varies from place to place (depends on (g)). Measuring Instrument Beam balance. Spring balance. Zero Condition Never zero (matter cannot vanish). Becomes zero where (g = 0) (e.g., space). On Moon Same as on Earth. 1/6 of weight on Earth. THRUST AND PRESSURE When a force acts perpendicularly on a surface, it creates an effect known as thrust.The effect of thrust per unit area on a surface is called pressure. *Thrust *Pressure *Examples of Pressure BUYONCY The upward force exerted by a liquid (or fluid) on an object when it is partially or completely immersed in it is called buoyant force, and the property of a fluid to exert this upward force is called buoyancy. It…

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class 9 science structure of atom notes
4 Oct 2025

CLASS 9 SCIENCE (Chemistry) Chapter-4 STRUCTURE OF ATOMS

CHARGED PARTICLES IN MATTER The presence of charged particles in matter is suggested by the phenomena of static electricity and electricity conduction through certain substances.   Therefore, Atoms can be divided further into sub-atomic particles. *SUB-ATOMIC PARTICLES An atom is the smallest unit of an element that still retains its chemical properties. It consists of three fundamental sub-atomic particles: Electrons (e−): These are the negatively charged particles having very little mass. They orbit the nucleus. Protons (p+): These are the positively charged particles. They are located in the nucleus and have a mass of about one atomic mass unit. Neutrons (n): These are the particles that are neutral (have no charge). They are also found in the nucleus and have a mass roughly equivalent to that of a proton. EARLY MODELS OF ATOM *DALTON’S ATOMIC THEORY Dalton’s theory states that matter is made up of small indestructible atoms that combine in set ratios and rearrange in reactions without being created or destroyed. This theory suggested that Atom is indivisible – which could not be broken down into smaller particles. # But the discovery of Sub-Atomic Particles inside the atom disproved this principle of Dalton’s atomic theory. *DISCOVERY OF ELECTRONS (1897) -Given by: J.J. Thomson in 1897. -J.J. Thomson discovered the electron in 1897 through the cathode ray experiment, proving atoms are not indivisible. -Experiment Setup -Key Observations -Conclusion -Thomson’s Contribution -Characteristics of an Electron *DISCOVERY OF PROTON -Given by: E. Goldstein in 1886 –E. Goldstein discovered protons in 1886 through canal rays, identifying them as positively charged particles inside atoms. -Experiment Setup -Key Observations -Conclusion -Importance -characteristics of proton *DISCOVERY OF NEUTRON – Given by: James Chadwick -In 1932, James Chadwick discovered the neutron, a neutral particle in the nucleus with mass equal to a proton. -Experiment Key Observations -Conclusion -Importance -Characteristics of Neutron ATOMIC MODEL There are three Atomic Models on Arrangement of Sub-Atomic Particles. 1) Thomson’s Model of the Atom (1898) -After discovering the electron, J.J. Thomson proposed a model to explain the structure of the atom. -Thomson imagined the atom as a sphere of positive charge with electrons studded inside it, called the Plum Pudding Model. -Main Features -Limitations -Importance 2) Rutherford’s Model (Nuclear Model) -Rutherford performed the Gold Foil Experiment (α-particle scattering experiment) with Geiger and Marsden. -Aim: To test Thomson’s Plum Pudding Model. -Experiment Setup -Observations -Main Features of Rutherford’s Model -Limitations -Importance 3) Bohr’s Model -Niels Bohr improved Rutherford’s model. -Solved the stability problem and explained hydrogen spectrum. -Postulates of Bohr’s Model -Achievements –Limitations Atomic Number and Mass Number An atom is made up of subatomic particles: protons, neutrons, and electrons. The atomic number and mass number are two important terms used to describe the composition of an atom. *Atomic Number (Z) The atomic number, symbolized by Z, is the number of protons in the nucleus of an atom. Formula: Atomic Number(Z)=Number of Protons For a neutral atom: Number of Electrons=Number of Protons *Mass Number (A) The mass number, symbolized by A, is the total number of protons and neutrons in the nucleus of an atom. Since the mass of electrons is negligible, the mass number essentially represents the total mass of the atom’s nucleus. Formula: Mass Number(A)=Number of Protons + Number of Neutrons You can rearrange this formula to find the number of neutrons: Number of Neutrons = Mass Number(A) – Atomic Number(Z) *Isotope Notation Elements are often represented with their atomic and mass numbers. The notation is as follows: Where: Example: Carbon (C) A common isotope of carbon has an atomic number of 6 and a mass number of 12. This can be written as: Electron Distribution in Orbits After the discovery of protons, electrons, and neutrons, the next task was to understand how electrons are arranged within an atom. -Niels Bohr and Bury developed laws for the distribution of electrons in different shells (orbits/energy levels) surrounding the nucleus. *Rules for Electron Distribution 1. Naming of Shells 2. Maximum Number of Electrons in a Shell 3. Octet Rule (Stability Rule) 4. Filling of Electrons in Successive Shells *Importance of Electron Distribution VALENCY Valency is the combining capacity of an atom. It is the number of electrons an atom must gain, lose, or share to achieve a stable electronic configuration, typically having a full outermost shell (an octet of 8 electrons, or a duplet of 2 for elements like Helium). This stable state resembles the electronic configuration of a noble gas. -Atoms are stable when they have 8 electrons in their outermost shell (Octet Rule). -To become stable, atoms: *How to Determine Valency The valency of an element is determined by the number of electrons in its outermost shell, known as valence electrons. *Types of Valency Based on how atoms achieve stability, we can distinguish between two types of valency: *Noble Gases: Zero Valency Noble gases (Group 18 elements) like Helium, Neon, and Argon have a completely filled outermost shell (a stable octet or duplet). Because they are already stable, they do not need to gain, lose, or share electrons. #Valency of noble gases is zero. They are generally unreactive. *Importance of Valency ISOTOPES AND ISOBARS *Isotopes -Definition – Characteristics – Example: Chlorine has two isotopes, Chlorine-35 and Chlorine-37. – Uses of Isotopes *Isobars – Definition – Characteristics

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