Alevel物理|万有引力（Gravity）和场强（g）的区别,提到万有引力，不得不先提牛顿万有引力定律。
Newton’s Law of Gravitation
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Gravity is an attractive force that acts between all masses. It is the masses themselves that cause the force to exist. The force that acts between two masses, m1 and m2, those centres are separated by a distance of r is given by:
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任何两个有mass的物体之间都存在万有引力。
简单来说，就是两个具有质量的物体，他们之间的万有引力和各自的mass成正比，和距离的平方成反比。
这是遵循平方反比定律（inverse-square law）的。
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When one of the masses is of planetary size, M, the force between it and a test mass, m, whose centres are separated by a distance of r is given by:
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当其中物体具有比较大的mass，在它周围就会存在Gravitational field，而我们在field中放置一个test mass，m，那么他们之间的吸引力就可以写成：
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The minus sign means that the force is attractive, the force is in the opposite direction to the distance from the mass (displacement). This will become clearer when we look at the electric force
Negative = Attractive
Positive = Repulsive
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我们会注意到公式中有一个负号，代表的含义是力的方向，是吸引力。
地球周围的引力场
因此我们地球周围的Gravitational field如下图所示：
Gravitational Field Strength, g
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The gravitational field strength at a point is a vector quantity and is defined as:
The force per unit mass acting on a small mass placed at that point in the field
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万有引力场强的定义是单位质量的物体在field中受到的力。
用g表示，是一个有方向的物理量（Vector）
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If we use our equation for the gravitational force at a distance r and substitute this in for F we get:
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化简：
F和g的区别
1
Gravitational force（或者叫Gravitational attraction， Gravity）是一个力，与两个物体的mass成正比，与距离的平方成反比。
2
Gravitational field strength，g指的是单位mass受到力的大小，是Field本身的性质，只与field中心M和离它的距离有关。
3
二者都是Vector，有方向的物理量。
