關於作用力和反作用力的物理問題

Before answering your questions, you need to be aware that action (作用力) and reaction (反作用力) act on different bodies. Hence, they cannot cancel each other out.

1) 因為身體施加於椅子的作用力和椅子所給的反作用力是相等的, 所以不論椅子如何破損, 椅子永遠坐不破.

The action is the force exerted on the chair. This force would break the chair if the chair was not strong enough. The reaction force acts on the body and which balances the body weight. Note that if the chair is broken, the reaction force might be smaller than the body weight, and the person would fall.

(2) 不論對牆施以多少力,它總是以同等的力把你推回去.因此該牆不論被施以多大的力,它總是推不倒.

The wall collapses because of the pushing force that you exert on it. The reaction force is experienced by you, not by the wall. Hence, the wall is only subjected to the action force.

(3) 由於作用力和反作用力相等,一個人跳起時因為跳起者對地板所施的作用力阻止反作用力把跳起者往上推.因此該個人永遠跳不起.

You have already said that the action force is exerted on the ground "跳起者對地板所施的作用力", NOTon the person. How come this force would prevent the person to jum up.

(4)當作用力和反作用力相等時,一個人能站在地板上.當作用力大於反作用力時,該個人會踩破地板.當作用力小於反作用力時,那個人能跳起.

No, this is not correct. A man stands firmly on the floor because his weight (which is given by the earth) is balanced by the "normal reaction" given by the floor. The "normal reaction" arises due to the force exerted on the floor by the man (this is the action force). Because the "weight" is constant, if the floor breaks, the "normal reaction" force given by the floor diminishes and the man falls.

If the man exerts an additional force (i.e. increasing the action force) onto the floor, the "normal reaction" will increase accordingly. Because "weight" is constant, the normal reaction force is now larger than the weight, the man thus jumps up.

第一,我肯定你上堂冇聽書
作用力同反作用力係施係不同物件上面,
A向B施加一道力,B會向A加一道力度相等但方向相方既力
所以只有一道力施向A,一道力施向B,佢地唔會抵消自己
所以你所講既情況全部唔成立

反作用力不是用來抵消作用力,而且兩者所施在的物件不同. 應該分開諗 1.椅子所給的反作用力是對於身體,而不是身體所施椅子的作用力 而你所說的椅子永遠坐不破是認為反作用力抵消了作用力, 當身體所施椅子的作用力大於椅子最多能受的力,椅子就會破. 2.同上 3.當反作用力少於weight, 人就不會跳起, 4. 作用力和反作用力的定義是 equal magnitude, opposite direction, acting on different object 踩破地板同坐破椅子一樣 如果反作用力大過weight 人就會跳起

2012-03-13 00:15:11 補充：
反作用力不是用來抵消作用力,而且兩者所施在的物件不同.
應該分開諗
1.椅子所給的反作用力是對於身體,而不是身體所施椅子的作用力
而你所說的椅子永遠坐不破是認為反作用力抵消了作用力,
當身體所施椅子的作用力大於椅子最多能受的力,椅子就會破.
2.同上
3.當反作用力少於weight, 人就不會跳起,
4. 作用力和反作用力的定義是
equal magnitude, opposite direction, acting on different object
踩破地板同坐破椅子一樣
如果反作用力大過weight 人就會跳起