phy,,乜都唔知呀,,help我呀

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