地心引力

维基百科，自由的百科全书

地球表面的重力加速度被表示为符号g，近似地等于 9.8 m/s²（米每秒的平方）或 32 英尺每秒的平方。這表示，當忽略空氣阻力時，物件在地球表面上自由下跌的加速度為 9.8 m/s（即約22mph）。 換言之，一半空的靜止物件在一秒後的速度為9.8 m/s，兩秒後為19.6 m/s，如此類推。地球本身也受到下落物體的等值的吸引力，也就是說地球朝著下落物體加速移動，但是因為地球本身質量遠大於下落物質量，所以地球的加速度相對來說非常小。

非重力加速度和重力加速度有相似的單位。非重力加速度通常用於加速物體上如飛機或是賽車，常以g的倍數來表示。用於重力單位時，g常被誤認為重量單位毫克g。

準確的重力加速度值依地球上不同地點而異。在地球上標準重力加速度定義為9.80665 m/s²。這個值常被表示為 g_n、g_e (但這兩個符號有時也表示地球赤道的重力加速度值, 9.78033 m/s²)、 g₀、gee,或g (此符號有時表示區域重力加速度)。重力加速度的單位是和每秒平方反比或是使用cgs制中的重力梯度單位eotvoses。

When measuring g with precision, it is important to distinguish between the actual strength of gravity and the apparent strength of gravity. Local variations in the actual strength of the Earth's gravitational field arise because the earth is not a perfect sphere and is not of uniform density. The main deviation from sphericity is the earth's equatorial bulge, which causes gravity to be weaker at the equator than the poles. The local topography (such as the presence of mountains) and geology (the density of rocks in the vicinity) also influence the gravitional field to a small extent.

Other forces acting on an object may augment or oppose the earth's actual gravitational field, causing variations in the apparent force of gravity (see also Apparent weight.) One example is the centrifugal force caused by the earth's rotation, which imparts an upwards force opposing gravity and diminishing its apparent effect. This effect is stronger at lower latitudes (i.e. nearer the equator), reducing to zero at the poles. Another example is buoyancy: even in air, objects experience a small supporting force which reduces the apparent strength of gravity. Finally, the gravitational effects of the Moon and the Sun (also the cause of the tides) also have a small effect on apparent gravity, depending on their relative positions; typical variations are 2 µm/s² (0.2 mGal) over the course of a day.

In combination, the equatorial bulge and the effects of centrifugal force mean that sea-level gravitational acceleration increases from about 9.780 m/s² at the equator to about 9.832 m/s² at the poles, so an object will weigh about 0.5% more at the poles than at the equator [1]. See Gee for further information.

Gravity also decreases with altitude (since greater altitude means greater distance from the earth's centre). All other things being equal, an increase in altitude from sea level to the top of Mount Everest (8,850 metres) causes a weight decrease of about 0.28%. It is a common misconception that astronauts in orbit are weightless because they have flown high enough to "escape" the earth's gravity. In fact, at an altitude of 250 miles (roughly the height that the space shuttle flies) gravity is still nearly 90% as strong as at the earth's surface, and weightlessness actually occurs because orbiting objects are in free-fall.

If the earth was of perfectly uniform composition then, during a descent to the centre of the earth, gravity would decrease linearly with distance, reaching zero at the centre. In reality, the gravitational field peaks within the Earth at the core-mantle boundary where it has a value of 10.7 m/s².

[编辑] 比較地球、太陽、月球及其他星球的引力

下表列出 gravitational accelerations (in multiples of g) at the surface of the Sun, the Earth's moon, and each of the planets in the solar system. The "surface" is taken to mean the cloud tops of the gas giants (Jupiter, Saturn, Uranus and Neptune). It is usually specified as the location where the pressure is equal to a certain value (normally 75 kPa?). For the Sun, the "surface" is taken to mean the photosphere.

太陽	27.9
水星	0.37
金星	0.88
地球	1.00 (定義為標準)
月球	0.16
火星	0.38
木星	2.64
土星	1.15
天王星	0.93
海王星	1.22
冥王星	0.06

For spherical bodies, surface gravity in m/s² is 2.8 × 10⁻¹⁰ times the radius in metres times the average density in kg/m³ (kilograms per cubic metre).

When flying from Earth to Mars, climbing against the field of the Earth at the start is 100 000 times heavier than climbing against the force of the sun for the rest of the flight.

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