急問!!! centre of gravity!!

首先 centre of gravity 既defination 係"the gravitational force on a body effectively acts at a single point , called the centre of gravity of the body"

實際上every individual elements within the body 都會under gravitational force ,但最重要係"effectively"呢個字. 就係話如果gravitational forces acting on those individual elements are switched off and gravitational force acting on the centre of gravity is switched on , 個motion of the body and the effects of those forces will not change.

不是 weight 聚晒係個centre 到..

而係在計算中，需要簡化計算，例如一個想計算一個四方盒子的運動，則不能把四方盒子拆解成無數的粒子，每個獨立計算吧！

因此在計算中，要找出一個’點’來代表該四方盒子，而這個點就是'centre of gravity' (或者正確一點，是centre of mass，而係平均gravity 的情況下'centre of mass' 與'centre of gravity'是在同一點上。

This follows the definition of centre of gravity. The centre of gravity is the point in the object AS IF all the weight 0f the object is concentrated at that point.
The concept behind is that if the object is construed as composed with a large number of small objects. There exist a point on the large object that the moments of all these small objects would balance out. This point is just called the centre of gravity of the large object.