[Phy] 急!唔該有冇人可以話啤我聽principle of moment(s)是甚麼來的?

Z-LOUIS

如題.
我上過wiki查, 但始終唔係太明白
請知道的人解釋一下plz.

超夢夢的夢境

如題.
我上過wiki查, 但始終唔係太明白
請知道的人解釋一下plz.
Z-LOUIS 發表於 16/5/2009 10:04 PM

                               
登錄/註冊後可看大圖

其實樓主想要甚麼程度的答案...

先解釋 Moment 是甚麼吧
用 scalar 的說法是這樣
Let X be the axis to be considered
  Moment about X due to a force F
=smallest distance between X and the line of action * F
(line of action 就是把 vector F 無限延長那條線)
明白嗎?不明白也沒關係,因為將會把問題簡化
moment 跟 torque 其實是一樣的
所以 quote wiki,
Torque is the tendency of a force to rotate an object about an axis

然後是正題
"The sum of torques due to several forces applied to a single point is equal to the torque due to the resultant force"
這裡的 sum 都是 vector sum,所以如果是3D,用 scalar 解釋會很複雜

簡化問題
由3D變成2D
Consider A as a point on the plane
Then take X as the line passing through A perpendicular to the plane

  Moment about point A
=distance between A and the line of action * F
而不難想像 rotation 的方向是順/逆時針
Denote 其中一個方向是正,另一個為負
之後 sum of torques 就用可以直接加

resultant force 是 vector sum,不過怎樣加起來樓主應該明白吧