physic物理问题英文3you step into an elevator at the 10th floor and

According to Newton's second law of motion,the acceleration a of the body is caused by the resultant force of the two forces acting on the body: the downward gravity G of the body and the upward supporting force N by the elevator floor. If the mass of the body is said to be m,then we have
G - N = ma,(The downward direction is supposed to be positive)
Thus N = G - ma,
If the body gets a positive acceleration,as the elevator starts moving down from rest, it is easy to find that
N < G, the elevator exerts a force N less than the gravity of the body,so according to the third law of motion(action-reaction law), you exerts a less force N'=N on the floor of the elevator, therfore you feel suddenly ligthter than normal.

楼上的兄弟，好强悍，可以用英文来解释这道题，他解释的完全正确，呵呵，我现在用汉语说一遍。
你在电梯上站着时，受两个力的作用，一个是向下的万有引力，一个是向上的电梯地板给你的弹力，又因为你是向下加速的，所以，这两个力的合力应该是向下的，这样才可能有向下的加速度，所以，向下的万有引力大于向上的地板的弹力。...