page_title: Gravity
section_title: Earth's gravity
hierarchical_section_title: Gravity / Specifics / Earth's gravity
caption_attribution_description: English: An initially-stationary object which is allowed to fall freely under gravity drops a distance which is proportional to the square of the elapsed time. This image, spanning half a second, was captured with a stroboscopic flash at 20 flashes per second. During the first 1/20th of a second the ball drops one unit of distance (here, a unit is about 12 mm); by 2/20ths it has dropped at total of 4 units; by 3/20ths, 9 units and so on. To take the picture, the ball - about the size of a tennis ball - was suspended by a short length of black thread and was released as the shutter was opened and the flash triggered. The shutter remained open for the whole of the half-second period, during which time the flash fired multiple times to capture the ball at 1/20 second intervals. Français&#160;: Un objet en chute libre depuis une position fixe parcours une distance proportionnelle au carré du temps écoulé. Cette photo a été prise sur une période de 0,5 seconde avec un flash stroboscopique réglé sur 20 impulsions par seconde. La balle, de la taille d'une balle de tennis, était retenue pas un fil noir sectionne au moment ou l'exposition a commence et ou le flash produisit sa première impulsion lumineuse. Română: Un obiect staţionar lăsat în cădere liberă în câmpul gravitaţional al Pământului. Legea de mişcare este o funcţie de gradul al doilea, poziţia la fiecare moment fiind egală cu pătratul timpului scurs de la începutul căderii. Imaginea a fost expusă timp de 500 de milisecunde, cu un flash stroboscopic, declanşat cu o frecvenţă de 20 de flash-uri pe secundă. În primele 50&#160;ms, mingea cade pe o unitate de distanţă (unitatea de distanţă este de aproximativ 12&#160;mm&#8212;distanţa parcursă în 50&#160;ms de un obiect lăsat liber în câmpul gravitaţional al Pământului de la o viteză iniţială de 0&#160;m/s); după ce s-au scurs 100&#160;ms, mingea a căzut 4 unităţi de distanţă; după 150 ms, mingea a parcurs 9 unităţi, şi aşa mai departe. Pentru realizarea imaginii, mingea (de dimensiunile unei mingi de tenis) a fost suspendată cu un fir negru şi a fost lăsată să cadă în momentul începerii expunerii şi odată cu primul bliţ. Aparatul a continuat să expună timp de o jumătate de secundă, timp în care bliţul s-a declanşat de mai multe ori, luminând mingea la intervale de 50&#160;ms
context_page_description: Gravity, or gravitation, is a natural phenomenon by which all things with mass or energy—including planets, stars, galaxies, and even light—are brought toward one another. On Earth, gravity gives weight to physical objects, and the Moon's gravity causes the ocean tides. The gravitational attraction of the original gaseous matter present in the Universe caused it to begin coalescing and forming stars and caused the stars to group together into galaxies, so gravity is responsible for many of the large-scale structures in the Universe. Gravity has an infinite range, although its effects become increasingly weaker as objects get further away.
  Gravity is most accurately described by the general theory of relativity, which describes gravity not as a force, but as a consequence of the curvature of spacetime caused by the uneven distribution of mass. The most extreme example of this curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole's event horizon.
context_section_description: Every planetary body (including the Earth) is surrounded by its own gravitational field, which can be conceptualized with Newtonian physics as exerting an attractive force on all objects. Assuming a spherically symmetrical planet, the strength of this field at any given point above the surface is proportional to the planetary body's mass and inversely proportional to the square of the distance from the center of the body.
  The strength of the gravitational field is numerically equal to the acceleration of objects under its influence. The rate of acceleration of falling objects near the Earth's surface varies very slightly depending on latitude, surface features such as mountains and ridges, and perhaps unusually high or low sub-surface densities. For purposes of weights and measures, a standard gravity value is defined by the International Bureau of Weights and Measures, under the International System of Units (SI).
  That value, denoted g, is g = 9.80665 m/s² (32.1740 ft/s²).
  The standard value of 9.80665 m/s² is the one originally adopted by the International Committee on Weights and Measures in 1901 for 45° latitude, even though it has been shown to be too high by about five parts in ten thousand. This value has persisted in meteorology and in some standard atmospheres as the value for 45° latitude even though it applies more precisely to latitude of 45°32'33".
  Assuming the standardized value for g and ignoring air resistance, this means that an object falling freely near the Earth's surface increases its velocity by 9.80665 m/s (32.1740 ft/s or 22 mph) for each second of its descent. Thus, an object starting from rest will attain a velocity of 9.80665 m/s (32.1740 ft/s) after one second, approximately 19.62 m/s (64.4 ft/s) after two seconds, and so on, adding 9.80665 m/s (32.1740 ft/s) to each resulting velocity. Also, again ignoring air resistance, any and all objects, when dropped from the same height, will hit the ground at the same time.
  According to Newton's 3rd Law, the Earth itself experiences a force equal in magnitude and opposite in direction to that which it exerts on a falling object.  This means that the Earth also accelerates towards the object until they collide.  Because the mass of the Earth is huge, however, the acceleration imparted to the Earth by this opposite force is negligible in comparison to the object's.  If the object does not bounce after it has collided with the Earth, each of them then exerts a repulsive contact force on the other which effectively balances the attractive force of gravity and prevents further acceleration.
  The force of gravity on Earth is the resultant (vector sum) of two forces: (a) The gravitational attraction in accordance with Newton's universal law of gravitation, and (b) the centrifugal force, which results from the choice of an earthbound, rotating frame of reference.  The force of gravity is weakest at the equator because of the centrifugal force caused by the Earth's rotation and because points on the equator are furthest from the center of the Earth.  The force of gravity varies with latitude and increases from about 9.780 m/s² at the Equator to about 9.832 m/s² at the poles.
caption: An initially-stationary object that is allowed to fall freely under gravity drops a distance that is proportional to the square of the elapsed time. This image spans half a second and was captured at 20 flashes per second.
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