引用:
原帖由 ken759 於 2016/3/26 09:11 PM 發表 
做做下ex 發現對work done既concept唔多清晰
例如有個resistive force,咁W=Fs 個F應該要係負數?
但係如果轉做負數就會計唔到....
如果冇 resistive force(eg. friction),咁個F就係正數啦
兩個好似有矛盾...
Do you mean the forces in the same question? Or just about the general concepts?
Generally, in a case you're pushing a box forward on a rough surface, the "work done by you" means the force by you times the distance. Let's say F1 * s. But you may also consider the work done by the floor on the box that makes you difficult to push. Here let's denote it as F2 * s.
Now you see? Defining which force you're talking about is so important. In the above case, F1 is positive while F2 is negative.
