physic物理问题英文3

来源：百度知道 编辑：UC知道 时间：2024/06/06 23:31:38
you step into an elevator at the 10th floor and press the 2nd-floor button. the elevator accelerates downward briefly as it picks up speed. there are two forces acting on you:the floor pushing up on you and gravity pulling down. compare these two forces while the elevator is accelerating downward. explain your answer in terms of Newton's second law of motion.

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。