it will do that.

它的运动轨迹就是如此
What I'm now telling you is,

我现在要告诉你们的是
if I take a complex body

如果我扔出一个结构复杂的物体
made of 20,000 parts,

它由两万个零件组成
all connected to each other pushing and pulling,

所有的连接部位都相互拉扯或挤压
if you fling that crazy thing in the air,

如果你将这个不可思议的东西扔向空中
it'll do all kinds of gyrations and jiggling as it moves around.

在它运动的过程中会摇晃和旋转
But if you found its center of mass,

但如果你能找到它的质心
the center of mass will follow simply a parabola,

质心的运动轨迹只会是抛物线
because the external force on it is just M·g.

因为作用在物体上的外力只有 M·g
So, it'll be M·R two dots equals m·g

用等式来表示就是 M·R 两点等于 m·g
and that's just motion with constant acceleration

该运动的加速度保持恒定