Gravitation |Physics |general-science-17

Gravitation

Gravitation Gravitation is a natural phenomenon by which objects with mass attract one another. In everyday life, gravitation is most commonly thought of as the agency which lends weight to objects with mass. Gravitation compels dispersed matter to coalesce, thus it accounts for the very existence of the Earth, the Sun, and most of the macroscopic objects in the universe. It is responsible for keeping the Earth and the other planets in their orbits around the Sun; for keeping the Moon in its orbit around the Earth, for the formation of tides; for convection (by which fluid flow occurs under the influence of a temperature gradient and gravity); for heating the interiors of forming stars and planets to very high temperatures; and for various other phenomena that we observe. Modern physics describes gravitation using the general theory of relativity, in which gravitation is a consequence of the curvature of space time which governs the motion of inertial objects.

Universal Law of Gravitation Every object in the universe attracts every other object with a force which is proportional to the product of their masses and inversely proportional to the square of the distance between them. IMPORTANCE OF THE UNIVERSAL LAW OF GRAVITATION The universal law of gravitation successfully explained several phenomena which were believed to be unconnected: (i) the force that binds us to the earth (ii) the motion of the moon around the earth (iii) the motion of planets around the Sun (iv) the tides due to the moon and the Sun.

Thurst & Pressure Thurst: Thrust is a reaction force described quantitatively by Newton's Second and Third Laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a proportional but opposite force on that system. Pressure: Pressure is the force per unit area applied to an object in a direction perpendicular to the surface. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure. Pressure = Thrust/ Area The SI unit of pressure is N/m2 or N m–2.

WHY OBJECTS FLOAT OR SINK WHEN PLACED ON THE SURFACE OF WATER? The nail sinks. The force due to the gravitational attraction of the earth on the iron nail pulls it downwards. There is an up thrust of water on the nail, which pushes it upwards. But the downward force acting on the nail is greater than the up thrust of water on the nail. So it sinks. The cork floats while the nail sinks. This happens because of the difference in their densities. The density of a substance is defined as the mass per unit volume. The density of cork is less than the density of water. This means that the up thrust of water on the cork is greater than the weight of the cork. So it floats. The density of an iron nail is more than the density of water. This means that the up thrust of water on the iron nail is less than the weight of the nail. So it sinks. Therefore, objects of density less than that of a liquid float on the liquid. The objects of density greater than that of a liquid sink in the liquid. Archimedes’ Principle Archimedes' principle, states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The principle applies to both floating and submerged bodies and to all fluids, i.e., liquids and gases. It explains not only the buoyancy of ships and other vessels in water but also the rise of a balloon in the air and the apparent loss of weight of objects underwater. In determining whether a given body will float in a given fluid, both weight and volume must be considered; that is, the relative density, or weight per unit of volume, of the body compared to the fluid determines the buoyant force. If the body is less dense than the fluid, it will float or, in the case of a balloon, it will rise. If the body is denser than the fluid, it will sink. Relative density also determines the proportion of a floating body that will be submerged in a fluid. If the body is two thirds as dense as the fluid, then two thirds of its volume will be submerged, displacing in the process a volume of fluid whose weight is equal to the entire weight of the body. In the case of a submerged body, the apparent weight of the body is equal to its weight in air less the weight of an equal volume of fluid. Relative Density = Density of substance / Density of water

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