Gravitation: Difference between revisions

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Since the time of the [[Greek_philosophy|Greek]] philosopher [[Aristotle]] in the [[4th century BC]], there have been many attempts to understand and explain gravity. Aristotle believed that there was no [[effect]] without a [[cause]], and therefore no [[Motion (physics)|motion]] without a [[force]]. He hypothesized that everything tried to move towards their proper place in the crystalline [[sphere]]s of the heavens, and that [[physical bodies]] fell toward the center of the [[Earth]] in proportion to their [[weight]]. Another example of an attempted explanation is that of the [[India|Indian]] [[astronomer]] [[Brahmagupta]] who, in 628 AD, wrote that "bodies fall towards the earth as it is in the nature of the earth to attract bodies, just as it is in the nature of water to flow".  
Since the time of the [[Greek_philosophy|Greek]] philosopher [[Aristotle]] in the [[4th century BC]], there have been many attempts to understand and explain gravity. Aristotle believed that there was no [[effect]] without a [[cause]], and therefore no [[Motion (physics)|motion]] without a [[force]]. He hypothesized that everything tried to move towards their proper place in the crystalline [[sphere]]s of the heavens, and that [[physical bodies]] fell toward the center of the [[Earth]] in proportion to their [[weight]]. Another example of an attempted explanation is that of the [[India|Indian]] [[astronomer]] [[Brahmagupta]] who, in 628 AD, wrote that "bodies fall towards the earth as it is in the nature of the earth to attract bodies, just as it is in the nature of water to flow".  


In 1687, the English physicist and mathematician [[Sir Isaac Newton]] published the famous ''[[Principia]]'', which hypothesizes the [[inverse-square law]] of universal gravitation.  In his own words, "I deduced that the forces which keep the planets in their orbs must be reciprocally as the squares of their distances from the centers about which they revolve; and them answer pretty nearly." Most modern non-relativistic gravitational calculations are based on Newton's work.
In 1687, the English physicist and mathematician Sir [[Isaac Newton]] published the famous ''[[Principia]]'', which presented the [[inverse-square law]] of universal gravitation.  In his own words he described the ideas that came to him in 1665, as a twenty-two year old student:<ref>R. S. Westfall, ''Never at Reʃt, A biography of Isaac Newton'', Cambridge University Press (1980), p. 143</ref>
 
<cite>
And the same year I began to think  of gravity extending to y<sup>e</sup> orb of the Moon & (having found out how to estimate the force with w<sup>ch</sup> [a] globe revolving within a sphere presses the surface of the sphere) from Keplers rule</cite> ([[Johannes Kepler|Kepler]]'s third law) <cite> of the periodical times of the Planets being in sesquialterate proportion of their distances from the center of their Orbs, I deduced that the forces w<sup>ch</sup> keep the Planets in their Orbs must [be] reciprocally as the squares of their distances from the centers about w<sup>ch</sup>  they revolve; & thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the earth, & found them answer pretty nearly.  
</cite>
 
Most modern non-relativistic gravitational calculations are based on Newton's work.
Between 1909 and 1915 [[Albert Einstein]] worked on his general theory of relativity,
Between 1909 and 1915 [[Albert Einstein]] worked on his general theory of relativity,

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In physics, gravitation or gravity is the tendency of objects with mass to accelerate toward each other, or, in other words, two masses attract each other. Gravitation is one of the four fundamental interactions in nature, the other three being the electromagnetic force, the weak nuclear force, and the strong nuclear force. Gravitation is the weakest of these interactions, but acts over great distances and is always attractive. In classical mechanics, gravitation is given by Newton's gravitational force, which is an inverse-square law. In general relativity, gravitation arises out of spacetime being curved by the presence of mass, and is not a force. In quantum gravity theories, either the graviton is the postulated carrier of the gravitational force[1], or time-space itself is envisioned as discrete in nature, or both.


Classically, the gravitational attraction of the earth endows objects with weight and causes them to fall to the ground when dropped. Moreover, gravitation is the reason for the very existence of the earth, the sun, and other celestial bodies; without it matter would not have coalesced into these bodies and life as we know it would not exist. Gravitation is also responsible for keeping the earth and the other planets in their orbits around the sun, the moon in its orbit around the earth, for the formation of tides, and for various other natural phenomena that we observe.

History of gravitational theory

Since the time of the Greek philosopher Aristotle in the 4th century BC, there have been many attempts to understand and explain gravity. Aristotle believed that there was no effect without a cause, and therefore no motion without a force. He hypothesized that everything tried to move towards their proper place in the crystalline spheres of the heavens, and that physical bodies fell toward the center of the Earth in proportion to their weight. Another example of an attempted explanation is that of the Indian astronomer Brahmagupta who, in 628 AD, wrote that "bodies fall towards the earth as it is in the nature of the earth to attract bodies, just as it is in the nature of water to flow".

In 1687, the English physicist and mathematician Sir Isaac Newton published the famous Principia, which presented the inverse-square law of universal gravitation. In his own words he described the ideas that came to him in 1665, as a twenty-two year old student:[2]

And the same year I began to think of gravity extending to ye orb of the Moon & (having found out how to estimate the force with wch [a] globe revolving within a sphere presses the surface of the sphere) from Keplers rule (Kepler's third law) of the periodical times of the Planets being in sesquialterate proportion of their distances from the center of their Orbs, I deduced that the forces wch keep the Planets in their Orbs must [be] reciprocally as the squares of their distances from the centers about wch they revolve; & thereby compared the force requisite to keep the Moon in her Orb with the force of gravity at the surface of the earth, & found them answer pretty nearly.

Most modern non-relativistic gravitational calculations are based on Newton's work.

Between 1909 and 1915 Albert Einstein worked on his general theory of relativity, which furnishes a deep understanding of gravitation. Einstein's work culminated in the final form of his gravitational equations, presented on November 25, 1915.[3]

Newton's law of universal gravitation

In 1687 Newton published his work on the universal law of gravity in his book Philosophiae Naturalis Principia Mathematica ( Latin:Mathematical Principles of Natural Philosophy). Newton’s law of gravitation states that: every particle in the universe attracts every other particle with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. If the particles have masses m1 and m2 and are separated by a distance r (from their centers of gravity), the magnitude of this gravitational force is:

where:

F is the magnitude of the gravitational force between the two point masses
G is the gravitational constant
m1 is the mass of the first point mass
m2 is the mass of the second point mass
r is the distance between the two point masses

To see the change in gravity on earth based on the altitude (r in the above equation) based on real examples, you can use the eXtreme High Altitude Calculator

Gravitational potential

The above equation leads to the equation for the work done in moving a mass from infinity to a distance R, which is obtained by integrating the force of gravity:

and this is known as the gravitational potential energy of the two masses. Not that this energy is zero at infinity.

Using the Earth as an example, the work done in moving a mass from the Earth's surface to a distance h above the surface is given by:

where G is the universal gravitational constant, m is the object's mass, re is the Earth's radius, and me is the Earth's mass. Further

is the gravitational acceleration, g = 9.81 m/s2 on earth. Taking unit mass m = 1 in the last equation, we obtain the classical gravitational field U(h) of the Earth, with the zero of potential for h = 0 (at the surface of the earth). Note that for small heights h = x the gravitational potential field is linear in the height,

The corresponding force on a particle of mass m is (valid for small heights),

Integration, while using that the speed v of the mass at initial time is zero, gives

This means that, ignoring air resistance, an object falling freely near the Earth's surface increases in speed by 9.81 m/s (around 22 mph) for each second of its descent. Thus, an object starting from rest will attain a speed of 9.81 m/s after one second, 19.62 m/s after two seconds, and so on.

General relativity

For more information, see: Introduction to general relativity.

Newton's conception and quantification of gravitation held until the beginning of the 20th century, when the German-born physicist Albert Einstein proposed the general theory of relativity. In this theory Einstein proposed that inertial motion occurs when objects are in free-fall instead of when they are at rest with respect to a massive object such as the Earth (as is the case in classical mechanics). The problem is that in flat spacetimes such as those of classical mechanics and special relativity, there is no way that inertial observers can accelerate with respect to each other, as free-falling bodies can do as they each are accelerated towards the center of a massive object.

To deal with this difficulty, Einstein proposed that spacetime is curved by the presence of matter, and that free-falling objects are following the geodesics of the spacetime. More specifically, Einstein discovered the field equations of general relativity, which relate the presence of matter and the curvature of spacetime. The Einstein field equations are a set of 10 simultaneous, non-linear, differential equations whose solutions give the components of the metric tensor of spacetime. This metric tensor allows to calculate not only angles and distances between space-time intervals (segments) measured with the coordinates against which the spacetime manifold is being mapped but also the affine-connection from which the curvature is obtained, thereby describing the spacetime's geometrical structure. Notable solutions of the Einstein field equations include:

General relativity has enjoyed much success because of how its predictions have been regularly confirmed. For example:


Applications

A vast number of mechanical contrivances depend in some way on gravity for their operation. For example, a height difference can provide a useful pressure differential in a liquid, as in the case of an intravenous drip or a water tower. The gravitational potential energy of water can be used to generate hydroelectricity as well as to haul a tramcar up an incline, using a system of water tanks and pulleys; the Lynton and Lynmouth Cliff Railway [1] in Devon, England employs just such a system. Note, however, that gravity is used here to overcome gravity. Without gravity no force would be needed to pull a tram up a slope. Also, a weight hanging from a cable over a pulley provides a constant tension in the cable, including the part on the other side of the pulley to the weight.

Examples are numerous: For example molten lead, when poured into the top of a shot tower, will coalesce into a rain of spherical lead shot, first separating into droplets, forming molten spheres, and finally freezing solid, undergoing many of the same effects as meteoritic tektites, which will cool into spherical, or near-spherical shapes in free-fall. Also, a fractionation tower can be used to manufacture some materials by separating out the material components based on their specific gravity. Weight-driven clocks are powered by gravitational potential energy, and pendulum clocks depend on gravity to regulate time. Artificial satellites are an application of gravitation which was mathematically described in Newton's Principia.

Gravity is used in geophysical exploration to investigate density contrasts in the subsurface of the Earth. Sensitive gravimeters use a complicated spring and mass system (in most cases) to measure the strength of the "downward" component of the gravitational force at a point. Measuring many stations over an area reveals anomalies measured in mGal or microGal (1 gal is 1 cm/s^2. Average gravitational acceleration is about 981 gal, or 981,000 mGal.). After corrections for the obliqueness of the Earth, elevation, terrain, instrument drift, etc., these anomalies reveal areas of higher or lower density in the crust. This method is used extensively in mineral and petroleum exploration, as well as time-lapse groundwater modeling. The newest instruments are sensitive enough to read the gravitational pull of the operator standing over them.

Alternative theories

For more information, see: Alternatives to general relativity.

Historical alternative theories

Recent alternative theories

See also

Notes

  • Proposition 75, Theorem 35: p.956 - I.Bernard Cohen and Anne Whitman, translators: Isaac Newton, The Principia: Mathematical Principles of Natural Philosophy. Preceded by A Guide to Newton's Principia, by I. Bernard Cohen. University of California Press 1999 ISBN 0-520-08816-6 ISBN 0-520-08817-4
  • Max Born (1924), Einstein's Theory of Relativity (The 1962 Dover edition, page 348 lists a table documenting the observed and calculated values for the precession of the perihelion of Mercury, Venus, and Earth.)

References

  1. Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8. 
  2. R. S. Westfall, Never at Reʃt, A biography of Isaac Newton, Cambridge University Press (1980), p. 143
  3. A. Pais, Subtle is the Lord ... The science and life of Albert Einstein, Oxford University Press, Oxford (1982), p. 250.
  1. Halliday, David; Robert Resnick; Kenneth S. Krane (2001). Physics v. 1. New York: John Wiley & Sons. ISBN 0-471-32057-9. 
  2. Serway, Raymond A.; Jewett, John W. (2004). Physics for Scientists and Engineers, 6th ed.. Brooks/Cole. ISBN 0-534-40842-7. 
  3. Tipler, Paul (2004). Physics for Scientists and Engineers: Mechanics, Oscillations and Waves, Thermodynamics, 5th ed.. W. H. Freeman. ISBN 0-7167-0809-4. 
  4. Jefimenko, Oleg D., "Causality, electromagnetic induction, and gravitation : a different approach to the theory of electromagnetic and gravitational fields". Star City [West Virginia] : Electret Scientific Co., c1992. ISBN 0-917406-09-5
  5. Heaviside, Oliver, "A gravitational and electromagnetic analogy". The Electrician, 1893.

External links