Dictionary
(physics) the force of attraction between all masses in the universe especially the attraction of the earth's mass for bodies near its surface "the more remote the body the less the gravity" "the gravitation between two bodies is proportional to the product of their masses and inversely proportional to the square of the distance between them" "gravitation cannot be held responsible for people falling in love"--Albert Einstein a manner that is serious and solemn a solemn and dignified feeling
|
Wikipedia
mergefrom Gravitational constant For other senses of the words gravity and gravitation, see gravity (disambiguation).''Gravity is the force of attraction between massive particles. Weight is determined by the mass of an object and its location in a gravitational field. While a great deal is known about the properties of gravity, the ultimate cause of the gravitational force remains an open question. General relativity is the most successful theory of gravitation to date. It postulates that stress-energy tensormass and energy curvaturecurve space-time, resulting in the phenomenon known as gravity. The effect of the bending of spacetime is often misunderstood as most people seem to prefer to think of a falling object as accelerating when the facts do not support that assumption. Ask any skydiver if he feels any acceleration (other than from wind resistance).Gravity, simply put, is acceleration. F=ma means that there must be a force that causes a mass to accelerate. For a rocket ship, that is the rocket motor. For the earth, that is the compression of the mass between something on the surface of the earth and the earth's center of mass. The acceleration is in relation to spacetime in that the weight you feel is your resistance to deviating from your path in spacetime. The same holds true in the rocket ship except that a rocket motor supplies the force to accelerate you from your spacetime path. There is no difference between weight you feel because of gravity or the rocket.
Newton's law of universal gravitation - Newton's law of universal gravitation states the following::Every object in the Universe attracts every other object with a force directed along the line_(mathematics)line of centers of mass for the two objects. This force is proportional to the product of their masses and inversely proportional to the square (algebra)square of the separation between the centers of mass of the two objects.Given that the force is along the line through the two masses, the law can be stated symbolically as follows.:where:F'' is the magnitude of the (repulsive) gravitational force between two objectsG'' is the gravitational constant, that is approximately : ''G'' = 6.67 × !10−11? N m2 !kg-2m'' 1? is the mass of first !objectm''2repulsive force F is always negative, and this means that the net attractive force is positive. The minus sign is used to hold the same value meaning as in the Coulomb's Law, where a positive force as result means repulsion between two Electric_chargecharges.Thus gravity is proportional to the mass of each object, but has an inverse square relationship with the distance between the centres of each mass.Strictly speaking, this law applies only to point-like objects. If the objects have spatial extent, the force has to be calculated by integralintegrating the force (in vector form, see below) over the extents of the two bodies. It can be shown that for an object with a spherically-symmetric distribution of mass, the integral gives the same gravitational attraction on masses outside it as if the object were a point mass.#f n_11 This law of universal gravitation was originally formulated by Isaac Newton in his work, the Philosophiae Naturalis Principia MathematicaPrincipia Mathematica (1687). The history of gravitation as a physical concept is considered in more detail #Historybelow.
Vector form - Newton's law of universal gravitation can be written as a vector (spatial)vector equation to account for the direction of the gravitational force as well as its magnitude. In this formulation, quantities in bold represent vectors.: or where 'F'''12 is the force on object 1 due to object 2G'' is the gravitational !constantm''1? and ''m''2 are the masses of the objects 1 and 2:r21 = r2 − r1 is the distance between objects 2 and 1: is the unit vector from object 2 to 1 It can be seen, that the vector form of the equation is the same as the scalar form, except for the vector value of F and the unit vector. Also, it can be seen that F12 = − F!21.Grav itational? acceleration is given by the same formula except for one of the factors m::
Gravitational field - The gravitational field is a vector field that describes the gravitational force an object of given mass experiences in any given place in space. It is a generalization of the vector form, which becomes particularly useful if more than 2 objects are involved (such as a rocket between the Earth and the Moon). For 2 objects (e.g. object 1 is a rocket, object 2 the Earth), we simply write instead of and instead of and define the gravitational field as::so that we can write::This formulation is independent of the objects causing the field. The field has units of force divided by mass; in SI, this is !N·kg−1.
Problems with Newton's theory - Although Newton's formulation of gravitation is quite accurate for most practical purposes, it has a few problems:
Theoretical concerns - There is no prospect of identifying the mediator of gravity. Newton himself felt the inexplicable ''action at a distance (physics)action at a distance'' to be unsatisfactory (see "#Newton's reservationsNewton's reservations" below).Newton's theory requires that gravitational force is transmitted instantaneously. Given classical assumptions of the nature of space and time, this is necessary to preserve the conservation of angular momentum observed by Johannes Kepler. However, it is in direct conflict with Einstein's theory of special relativity which places an upper limit—the speed of light in vacuum—on the velocity at which signals can be transmitted.
Disagreement with observation - Newton's theory does not fully explain the precession of the perihelion of the orbit of the planet Mercury (planet) Mercury. There is a 43 arcsecond per century discrepancy between the Newtonian prediction (resulting from the gravitational tugs of the other planets) and the observed precessionfn3 .The predicted deflection of light by gravity is only half as much as observations of this deflection, which were made after General Relativity was developed in 1915.The observed fact that gravitational and inertial masses are the same for all bodies is unexplained within Newton's system. General relativity takes this as a postulate. See equivalence principle.
Newton's reservations - It's important to understand that while Newton was able to formulate his law of gravity in his monumental work, he was deeply uncomfortable with the notion of "action at a distance" which his equations implied. He never, in his words, "assigned the cause of this power". In all other cases, he used the phenomenon of motion to explain the origin of various forces acting on bodies, but in the case of gravity, he was unable to experimentally identify the motion that produces the force of gravity. Moreover, he refused to even offer a hypothesis as to the cause of this force on grounds that to do so was contrary to sound science.He lamented the fact that "philosophers have hitherto attempted the search of nature in vain" for the source of the gravitational force, as he was convinced "by many reasons" that there were "causes hitherto unknown" that were fundamental to all the "phenomena of nature". These fundamental phenomena are still under investigation and, though hypotheses abound, the definitive answer is yet to be found. While it is true that Einstein's hypotheses are successful in explaining the effects of gravitational forces more precisely than Newton's in certain cases, he too never assigned the cause of this power, in his theories. It is said that in Einstein's equations, "matter tells space how to curve, and space tells matter how to move", but this new idea, completely foreign to the world of Newton, does not enable Einstein to assign the "cause of this power" to curve space any more than the Law of Universal Gravitation enabled Newton to assign its cause. In Newton's own words:I wish we could derive the rest of the phenomena of nature by the same kind of reasoning from mechanical principles; for I am induced by many reasons to suspect that they may all depend upon certain forces by which the particles of bodies, by some causes hitherto unknown, are either mutually impelled towards each other, and cohere in regular figures, or are repelled and recede from each other; which forces being unknown, philosophers have hitherto attempted the search of nature in vain.''If science is eventually able to discover the cause of the gravitational force, Newton's wish could eventually be fulfilled as well.It should be noted that here, the word "cause" is not being used in the same sense as "cause and effect" or "the defendant caused the victim to die". Rather, when Newton uses the word "cause," he (apparently) is referring to an "explanation". In other words, a phrase like "Newtonian gravity is the cause of planetary motion" means simply that Newtonian gravity explains the motion of the planets. See Causality and Causality (physics).
Einstein's theory of gravitation - Albert EinsteinEinstein's theory of gravitation answered the problems with Newton's theory noted above. In a revolutionary move, his theory of general relativity (1915) stated that the presence of mass, energy, and momentum causes spacetime to become curvaturecurved. Because of this curvature, the paths that objects in inertiainertial motion follow can "deviate" or change direction over time. This deviation appears to us as an acceleration towards massive objects, which Newton characterized as being gravity. In general relativity however, this acceleration or freefallfree fall is actually inertial motion. So objects in a gravitational field appear to fall at the same rate due to their being in inertial motion while the observer is the one being accelerated. (This identification of free fall and inertia is known as the Equivalence principle.)The relationship between the presence of mass/energy/momentum and the curvature of spacetime is given by the Einstein field equations. The actual shapes of spacetime are described by Exact solutions of Einstein's field equationssolutions of the Einstein field equations. In particular, the Schwarzschild solution (1916) describes the gravitational field around a spherically symmetric massive object. The geodesics of the Schwarzschild solution describe the observed behavior of objects being acted on gravitationally, including the anomalous perihelion precession of Mercury and the bending of light as it passes the Sun.Arthur Eddington found observational evidence for the bending of light passing the Sun as predicted by general relativity in 1919. Subsequent observations have confirmed Eddington's results, and observations of a pulsar which is occultationocculted by the Sun every year have permitted this confirmation to be done to a high degree of accuracy. There have also in the years since 1919 been numerous other tests of general relativity, all of which have confirmed Einstein's theory.
Units of measurement and variations in gravity - ) will measure high-accuracy gravity gradients and provide a global model of the Earth's gravity field and of the geoid. (ESA image)]]Gravitational phenomena are measured in various units, depending on the purpose. The gravitational constant is measured in newton (unit)newtons times metre squared per kilogram squared. Gravitational acceleration, and acceleration in general, is measured in metre per second squaredmetres per second squared or in non-SI units such as galileo (unit)galileos, geegees, or footfeet per second squared.The #Comparative gravities of different planets and Earth's Moonacceleration due to gravity at the Earth's surface is approximately 9.8 m/s2, more precise values depending on the location. A standard value of the Earth's gravitational acceleration has been adopted, called ''geegn''. When the typical range of interesting values is from zero to tens of metres per second squared, as in aircraft, acceleration is often stated in multiples of ''gn''. When used as a measurement unit, the standard acceleration is often called "gee", as ''g'' can be mistaken for g, the gram symbol. For other purposes, measurements in millimetres or micrometres per second squared (mm/s² or µm/s²) or in multiples of milligals or milligalileos (1 mGal = 1/1000 Gal), a non-SI unit still common in some fields such as geophysics. A related unit is the eotvos (unit)eotvos, which is a centimeter-gram-second system of unitscgs unit of the gravitational gradient. Mountains and other geological features cause subtle variations in the Earth's gravitational field; the magnitude of the variation per unit distance is measured in inverse seconds squared or in eotvoses. Typical variations with time are 2 µm/s² (0.2 mGal) during a day, due to the tides, i.e. the gravity due to the Moon and the Sun.A larger variation in the effect of gravity occurs when we move from the equator to the poles. The effective force of gravity decreases as the distance from the equator decreases, due to the rotation of the Earth, and the resulting centrifugal force and flattening of the Earth. The centrifugal force causes an effective force 'up' which effectively counteracts gravity, while the flattening of the Earth causes the poles to be closer to the center of mass of the Earth. It is also related to the fact that the Earth's density changes from the surface of the planet to its centre.The sea-level gravitational acceleration is 9.780 m/s² at the equator and 9.832 m/s² at the poles, so an object will exert about 0.5% more force due to gravity at sea level at the poles than at sea level at the equator curious.astro.cornell.edu.
Comparison with electromagnetic force - The gravitational interaction of protons is approximately a factor 1036 weaker than the electromagnetismelectromagnetic repulsion. This factor is independent of distance, because both interactions are inversely proportional to the square of the distance. Therefore on an atomic scale mutual gravity is negligible. However, the main interaction between common objects and the Earth and between celestial bodies is gravity, because at this scale matter is electrically neutral: even if in both bodies there were a surplus or deficit of only one electron for every 1018 protons and neutrons this would already be enough to cancel gravity (or in the case of a surplus in one and a deficit in the other: double the interaction). However, the main interactions between the charged particles in cosmic plasma (that makes up over 99% of the universe by volume), are electromagnetic forces.In terms of Planck units: the charge of a proton is 0.085, while the mass is only sn8-20 . From that point of view, the gravitational force is not small as such, but because masses are small.The relative weakness of gravity can be demonstrated with a small magnet picking up pieces of iron. The small magnet is able to overwhelm the gravitational interaction of the entire Earth. Similarly, when doing a chin-up, the electromagnetic interaction within your muscle cells is able to overcome the force induced by Earth on your entire body. Gravity is small unless at least one of the two bodies is large or one body is very dense and the other is close by, but the small gravitational interaction exerted by bodies of ordinary size can fairly easily be detected through experiments such as the Torsion bar experimentCavendish torsion bar experiment. demonstrates gravitational field.]]'''
|
|
Websites
efi Sports Medicine - Total Gym
Online health equipment and home fitness equipment manufacturer directory to purchase home fitness and commercial fitness equipment, pilates equipment and Total Gym infomercial fitness equipment.
http://www.efisportsmedicine.com/
ratCong
Flash animations.
http://www.ratcong.com/
GeoPro
World Wide Geophysical Exploration
http://www.geopro.com/
The Grant Chronicles
Astrophysics Alternate explanations
http://www.grantchronicles.com/
Aero BUNGY Maxi-TRAMPOLINE.com BUNGEE TRAMPOLINE manufacturer 4in1 elastic Salto trampoline funball shootair, CLIMBING WALLS
BUNGY Maxi-TRAMPOLINE.com Bungee trampoline manufacturer, SALTO ELASTIC 4in1 mobile trailer, Worldwide Developer for attractions parks CLIMBING wall, Funball Shootair, space bikes, rodeo mechanic bull, sports games simulator
http://www.maxi-trampoline.com/
Leibniz Institute for Applied Geosciences
GGA Institute is an independent, non-university research institute. Its full name is 'Leibniz Institute for Applied Geosciences', The focus of our work is exploring structures and processes in the subsurface, and the development of methods and techniques required in geophysics, geohydraulics, geothermics, and geochronology. Research targets are directed to address the needs of the society and solve issues of future concern. The institute's activities are mainly in Germany. It cooperates with national and international partners.
http://www.gga-hannover.de/
Institute of Physics (IOP)
Job news, journals, author and referee services, conference diary.
http://www.iop.org/
Gravitygames.com
Information on Extreme Sports games, news, results, photos and videos, calendar. Includes online games.
http://www.gravitygames.com/
Curtin University of Technology, Home Page
Curtin University of Technology, located in Western Australia. Has courses in Business, Engineering, Science, Computing, Health Science, Humanities, Agriculture, and Mining.
http://www.curtin.edu.au/
|
|