點解PE=KE??

其實原本的方程式是 P.E.(1) + K.E.(1) + Wf = P.E.(2) + K.E.(2)
原本的object 的動能+位能 再加上 work done, work done 可以正可以負(睇你的問題) = 結果object 的動能+位能!!!!
跟據你的問題在一個斜面中, 一個波由上至下術動,有摩擦力!!! 這一條問題個波的p.e. = mgh,而它的k.e. 是mv^2/2 + Iw^2/2....我估你沒有學到一樣野轉動的時候有多d的k.e. 多出來的Iw^2/2。現在我將你的問題改一改,唔用波,只用一個立體正方形,佢只是在斜面上滑行!一開始我們放個立體正方形,放在斜面上,因為mgsin@ > f(k), 所以它會向下加速! 假設放下的高度是h. 到地上的speed是 v. 咁我們就setup 這條equation. mgh + m(0)^2/2 + work done of f = mg(0) +mv^2/2. 我們向下是(+) work done of f 就會是'摩擦係數'*N*s, N是[email protected]是距離!!! f(k) 是向上所以它是負數!!! equation 就會變成mgh-mg(0) = mv^2/2- m(0)^2/2 + work done of f.
mgh-mg(0) 就是 ↓PE, mv^2/2- m(0)^2/2 就是↑KE.
由下至上就是用同一個方法!! 你試下做啦!

since energy is conserve so no energy will be loss in the system.

that's is
loss P.E = Gain in K.E + Work Done against friction OR
loss K.E = Gain in P.E + Work Done against friction

And
a body will have greater P.E...when it is in higher state...
P.E is depends on the high of the object and the mass of the object (P.E = mgh)
a body will have greater K.E when it has higher speed...K.E = 1/2mv^2

So in your case
since the ball higher state,so it wil have higher P.E..
when it is slide down from the wedge...the ball will change P.E into K.E
and because some energy is loss due to friction
so the we can written as
loss P.E = Gain in K.E + Work Done against friction

姐係因為你你個ball slide down 時有friction 所以我地必需張呢力都計埋係個system入面