CBSE Class 9 || Physics || Gravitation || Animation || in English @digitalguruji3147

Raising an object sets in motion its inevitable descent due to gravity's pull. A ball thrown upward clearly demonstrates this, as it always returns downward under Earth's attraction. Variations in gravitational strength across locations explain why an object's weight can differ, revealing key aspects of gravitational phenomena.

The lesson establishes the definition of gravitational force and explains the universal law that governs interactions between masses. It clarifies the dynamics of free fall while distinguishing weight from mass. It further examines the roles of thrust and pressure, and elucidates Archimedes’ principle in explaining buoyancy.

Gravity is the invisible force that unifies all matter by attracting objects toward each other. It naturally draws everything downward toward the Earth, ensuring a constant pull that shapes our everyday experience. This force operates even without direct contact, emphasizing its universal and inherent nature. Such gravitational interactions weave the fundamental fabric underlying the cosmic dance of the universe.

Centripetal force is the inward pull that channels objects into continuous curved trajectories, always directing them toward the center. Gravitational attraction between the Earth and the Sun generates this force, maintaining the Earth's uniform circular orbit. This principle explains how an object’s inertia is balanced by a consistent central force, ensuring stable circular motion.

Newton’s universal law of gravitation establishes that every object attracts every other with a force that grows with the product of their masses and diminishes with the square of their separation. The relationship is defined by a constant that measures gravitational pull in newtons, capturing the interplay between mass and distance. This principle underpins our understanding of how celestial bodies interact across the cosmos.

The gravitational constant is derived by rearranging the law of gravitation to form G = F · r²/(m₁ · m₂), quantifying the attractive force between two unit masses at a given separation. Its SI unit is newton meter squared per kilogram squared, with a value of approximately 6.67 x 10⁻¹¹. This constant is essential in explaining how gravity binds the Earth, shapes the moon’s orbit around the sun, and drives the tides influenced by both the moon and the sun.

Objects near Earth accelerate uniformly at 9.8 m/s² regardless of mass. Earth's gravitational pull produces this free-fall motion, described by the force equation F = mg. Gravitational interactions are also expressed through an inverse-square law linking mass and distance.

Mass represents the finite quantity of matter in a body that remains unchanged no matter where it is measured, reflecting its inherent inertia. It is an immutable attribute that can never be zero. Weight, on the other hand, is the gravitational force exerted on the mass, varying with the local acceleration due to gravity. Hence, while mass stays constant, weight fluctuates and may even vanish in environments with negligible gravitational pull.

The equations of motion define free falling behavior, where v = u + gt describes the increase in velocity, s = ut + ½gt² calculates displacement, and v² = u² + 2gs links velocity to distance. Gravity accelerates a falling body at +9.8 m/s², while the same force acts as -9.8 m/s² when an object is thrown upward. Additionally, the gravitational pull on the moon reduces an object's weight to one-sixth of its Earthly measure.

Thrust is defined as the force acting perpendicular to a surface, calculated by multiplying pressure by the area, with the unit of newtons. Pressure, on the other hand, is the force distributed per unit area, measured in newtons per square meter. The magnitude of pressure changes depending on the distribution of force; a larger area results in lower pressure, while a smaller area increases the pressure.

Weight distribution is intentionally used in school backpacks by incorporating design elements that widen the contact area, reducing shoulder pressure. A needle's pointed tip applies force on a small area to generate enough pressure to pierce fabric efficiently. When walking, a person's weight is concentrated over a smaller area, thereby increasing ground pressure compared to when standing, where the weight is spread over a larger surface.

When an object is immersed in a liquid, it experiences an upward force known as buoyancy, buoyant force, or upthrust. The magnitude of this force depends on the volume of the object submerged and the density of the liquid. These principles explain why objects float or sink, emphasizing the key factors in fluid mechanics.

Archimedes' Principle states that an object, whether wholly or partially immersed in a liquid, experiences an upward buoyant force equal to the weight of the liquid it displaces. This principle underlies the design of various devices such as submarines, lacro meters, and hydrometers. The concept of relative density, defined as the ratio of a substance's density to the density of water, provides a unitless measure to compare material properties in fluids.

Newton's law of gravitation enables the precise calculation of the mass of Earth, the sun, the moon, and other planets. Every planetary body, including Earth, is encircled by its own gravitational field that exerts a universal attractive force. Gravitation is recognized as one of the four fundamental forces of nature, alongside electromagnetism and the strong and weak nuclear forces.

The force of attraction between objects depends on the product of their masses and decreases as the square of the distance between them increases. Although gravity is fundamentally weak, it becomes significant with substantial masses and less effective at higher altitudes. An object's weight, which is its mass multiplied by gravitational acceleration, varies by location, exemplified by the Moon's one-sixth weight compared to Earth. Immersion in a fluid introduces a buoyant force that counteracts gravitational pull.