Free Fall

Core Sentence / 核心句

An object is in free fall whenever gravity is the only significant force, whether it moves upward or downward.

中文理解：自由落体不等于“从静止向下掉”；竖直上抛在离手后同样是 free fall。

Near Earth’s surface and neglecting air resistance:

a = g, g \approx 9.8 m/s^{2}

若 upward positive，则 $a = - g$ 。

最高点 $v_{y} = 0$ ，但 $a_{y} = - g$ 。 $g$ 的大小为正，分量符号由坐标决定。