中四物理力學,呢點不明!!有冇人識?

所謂一圖抵萬語

[img]http://i155.photobucket.com/albums/s317/byaiceki/test1.jpg[/img]


Fx Fy 其實是指一個力的分力。

Fx 是向x的力(你當成向水平方向的力吧)
Fy 是向y的力(你可當成向垂直方向的力)

看上圖的三角形，根據我們中一學的cos 同 sin 就得知

sinθ= Fy/F
Fy=Fsinθ


cosθ= Fx/F
Fx=Fsinθ


可是你會問啊，分力不是這個模樣的嗎？

[img]http://i155.photobucket.com/albums/s317/byaiceki/test2.jpg[/img]

啊，這個問題問得好，因為力是一個方向的而已，而你要計得是量，所以你可以任意移動它，把他拚成你愛看的三角形就行了

In general, F is a vector; which includes a magnitude and an angleθ. In order to solve the combination of vector (add or subtract), we can use 2 standard axes (X-axis & Y-axis) to present the magnitude of the vector.

Therefore, the X part is F cosθ & Y part is F sinθ. Then you can add all X-axis magnitude together (reverse is negative) and you can have a so-called resultant magnitude in X- & Y- axis.

Then F = (X^2 Y^2)^(0.5) and θ=tan^(-1) (Y/X)

Hope you can understand.