en.wikipedia.org/wiki/Gravity#Newton.27s_reservations
Weight is determined by the mass of an object and its location in a gravitational field. While a great deal is known abou t the properties of gravity, the ultimate cause of the gravitational for ce remains an open question.
suspension brid ge to modern gravity theory with Newton's laws. Newton's theory is much simpler than general relativity, and used very often, so it is usually t aught first.
is the mass of second object r is the distance between the objects Thus gravity is proportional to the mass of each object, but has an inver se square relationship with the distance between the centres of each mas s Strictly speaking, this law applies only to point-like objects.
vector field that describes the gravitat ional force an object of given mass experiences in any given place in sp ace. It is a generalization of the vector form, which becomes particularly use ful if more than 2 objects are involved (such as a rocket between the ea rth and the moon).
and define the gravitational field \mathbf g(\mathbf r) as: \mathbf g(\mathbf r) = G {m_2 \over r^2} \, \mathbf{\hat{r}} so that we can write: \mathbf{F}( \mathbf r) = m \mathbf g(\mathbf r) This formulation is independent of the objects causing the field.
inertial motion follow can " deviate" or change direction over time. This deviation appears to us as an acceleration towards massive objects, which Newton characterized as b eing gravity.
The geodesics of the Schwarzschild solution describe the observed behavior of objects being acted on gravitationally, including t he anomalous perihelion precession of Mercury and the bending of light a s it passes the Sun.
The Gravity Field and Steady-State Ocean Circulation Explorer projec t (GOCE) will measure high-accuracy gravity gradients and provide a glob al model of the Earth's gravity field and of the geoid.
Mountains and other geological features cause subtle variations in the Ea rth's gravitational field; the magnitude of the variation per unit dista nce is measured in inverse seconds squared or in eotvoses.
A larger variation in gravity occurs when we move from the equator to the poles. Gravity increases as the density towards the poles increases. Th is is due to the effects of the rotation of the earth (both centrifugal force and the flattening due to this rotation, resulting in the poles be ing closer to the center of mass of the Earth), and is also related to t he fact that the Earth's density changes from the surface of the planet to its centre.
edit Gravity, and the acceleration of objects near the Earth The acceleration due to the apparent "force of gravity" that "attracts" o bjects to the surface of the Earth is not quite the same as the accelera tion that is measured for a free-falling body at the surface of the Eart h (in a frame at rest on the surface). This is because of the rotation o f the Earth, which leads (except at the poles) to a centrifugal force wh ich slightly lessens the acceleration observed.
This factor is inde pendent of distance, because both interactions are inversely proportiona l to the square of the distance. Therefore on an atomic scale mutual gra vity is negligible.
Planck units: the charge of a proton is 0085, while the mass is only 8 10^-20. From that point of view, the gravitational for ce is not small as such, but because masses are small.
The small magnet is able to overwhe lm the gravitational interaction of the entire Earth. Similarly, when do ing a chin-up, the electromagnetic interaction within your muscle cells is able to overcome the force induced by Earth on your entire body.
quantum gravity is currently an important topic of research amongst physicists. General relativity is essentially a geom etric theory of gravity. Quantum mechanics relies on interactions betwee n particles, but general relativity requires no exchange of particles in its explanation of gravity.
quantum mechanics, but this promise has yet to be realized. It is notable that in general relativity gravitational radiation (which u nder the rules of quantum mechanics must be composed of gravitons) is on ly created in situations where the curvature of spacetime is oscillating , such as for co-orbiting objects.
edit Experimental tests of theories Today General Relativity is accepted as the standard description of gravi tational phenomena. F or weak gravitational fields and bodies moving at slow speeds at small d istances, Einstein's General Relativity gives almost exactly the same pr edictions as Newton's law of gravitation.
cosmological constant of approximately 70% of the critical density. Even to this day, scientists try to challenge General Relativity with mor e and more precise direct experiments. The goal of these tests is to she d light on the yet unknown relationship between Gravity and Quantum Mech anics.
Space probes are used to either make very sensitive measurem ents over large distances, or to bring the instruments into an environme nt that is much more controlled than it could be on Earth.
Speed of gravity: Einstein's theory of relativity predicts that the speed of gravity (defined as the speed at which changes in location of a mass are propagated to other masses) should be consistent with the spee d of light.
Kopeikin experiment produced meas urements of the speed of gravity which matched this prediction. However, this experiment has not yet been widely peer-reviewed, and is facing cr iticism from those who claim that Fomalont-Kopeikin did nothing more tha n measure the speed of light in a convoluted manner.
space probes differ very slightly from what would be expected according to known effects (gravita tional or otherwise). The possibility of new physics has not been ruled out, despite very thorough investigation in search of a more prosaic exp lanation.
edit History Although the law of universal gravitation was first clearly and rigorousl y formulated by Isaac Newton, the phenomenon was observed and recorded b y others.
Ptolemy had a vague conception of a force tending to ward the center of the Earth which not only kept bodies upon its surface , but in some way upheld the order of the universe.
Johannes Kepler inferred that the planets move in their orbits under some influence or force exerted by the Sun; but the laws of motion were not then sufficien tly developed, nor were Kepler's ideas of force sufficiently clear, to m ake a precise statement of the nature of the force.
Aristotle, that objects with different ma ss fall at different rates. To Aristotle, it simply made sense that obje cts of different mass would fall at different rates, and that was enough for him. Galileo, however, actually tried dropping objects of different mass at the same time. Aside from differences due to friction from the air, Galileo observed that all masses accelerate the same.
When dealing with objects near the surface of a planet, the change i n r divided by the initial r is so small that the acceleration due to gr avity appears to be perfectly constant.
Galileo didn't have Newton's equations, though, so his insight into gravity's proportionality to mass was invaluable, and possi bly even affected Newton's formulation on how gravity works.
edit 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 nev er, 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 forc es acting on bodies, but in the case of gravity, he was unable to experi mentally identify the motion that produces the force of gravity. Moreove r, he refused to even offer a hypothesis as to the cause of this force o n grounds that to do so was contrary to sound science. He lamented the fact that "philosophers have hitherto attempted the searc h of nature in vain" for the source of the gravitational force, as he wa s convinced "by many reasons" that there were "causes hitherto unknown" that were fundamental to all the "phenomena of nature". These fundamenta l phenomena are still under investigation and, though hypot...
|
http://csua.com/feed/