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GRAVITATION Comprehensive Physics Notes | Class 9 Science Mastering the Forces of the Universe Il. The Universal Force Understanding why things fall and planets orbit. Sir Isaac Newton Sir Isaac Newton was an English mathematician and physicist who formulated the laws of motion and universal gravitation. Legend has it that seeing an apple fall inspired his theory that the same force affecting the apple also governs the Moon's orbit. | Universal Law of Gravitation Every object in the universe attracts every other object with a force proportional to the product of their masses and SUFEN inversely proportional to the square of the distance B between them. A : d (c) bupen.com.np Where G is the Universal Gravitational Constant. | The Gravitational Constant (G) Universal Constant The value of G was found by Henry Cavendish using a sensitive torsion balance. It remains constant throughout the universe, unlike g. x 10771 N m? kg? It represents the force between two masses of 1kg each kept at a distance of 1m. ll. Free Fall and Motion Dynamics of objects under Earth's gravity. | Variation of Acceleration 'g' The Earth is not a perfect sphere (Oblate Spheroid). This causes variations in g. Value of g Equator (Lower g) Mid-Latitudes Poles (Higher g) Note: g also decreases with altitude and depth from Earth's surface. | Equations of Motion (Gravity) For objects falling vertically, we replace ‘a’ with 'g' in the standard equations: rv) Equation 1:v =u+ gt @ Equation 2:s = ut + jst" @ Equation 3:v" =u" + 2gs rv) Sign Convention: Take gas positive when moving downwards (with gravity) and negative when moving upwards (against gravity). | Weight Comparison: Earth vs. Moon Relative Gravity GB Weight on Moon (~16.7%) 1 on Moon Weight on Earth (100%) Mass of Moon is small, so it exerts less gravitational pull. lil. Fluids and Pressure Thrust, Buoyancy, and Archimedes' Principle. | Buoyancy a ads in CEE The Upthrust Sinking Floating The upward force exerted by a An object sinks if its density is An object floats if its density is fluid on an object immersed in it greater than the density of the less than the density of the fluid. is called buoyancy or upthrust. fluid. The weight exceeds Upthrust balances the weight. upthrust. | Archimedes' Principle The Statement "When a body is immersed fully or partially in a fluid, it experiences an upward force that is equal to the weight of the fluid displaced by it." Applications: e Designing ships and submarines. e Lactometers (purity of milk). e Hydrometers (density of liquids). | Important Numericals & Formulae Problem Type Formula to Use Key Example Value Gravitational Force F=Gx(M-m/ d?) G=67/% 168" Weight on Moon Wm = 1/6 X We 10 kg = 98 N (Earth) = 16.3 N (Moon) Pressure P = Force / Area 1 Pa=1 N/m? Equation of Motion v=utgt Take g = 9.8 m/s? (Earth) Sample: A ball thrown up takes 3s to reach max height. Calculate initial velocity. (Ans: 29.4 m/s) Questions & Summary Short Questions Long Questions © Define Universal Law of Gravitation. @ State Archimedes' Principle and its 3 applications. rv) Why does a sharp knife cut easily? rv) Explain why objects float or sink in a liquid. rv) Difference between 'G' and 'g’. @ Derive the value of 'g' on Earth's surface. Class 9 Science - Physics Unit Study Guide 
