mgh 10分

冇錯, 如果本身個物體冇速度, 我地要先比一個netforce (F-W), 比物體, 令其有一個速度u.
F*h= workdone by hand = change in KE + change in PE
change in PE 一定是mgh, 所以
Fh - mgh = change in KE
(F-W)h = change in KE
(F-W)h = Netforce * h = workdone by netforce = change in KE
所以, 使加的力大過重力, 會令物體加速, 但只要物體高左h, 佢的PE 一定多左mgh
只要h夠細, PE 一定是 mgh, 這和我們施加的力無關
相反, 如果物體由高處下跌h米, 物體一定少了mgh 的PE, 這些PE轉變為其KE, 令其加速, 這過程不需要我們加力, 但PE一樣是mgh, 所以mgh和我們的力無關．
不明白的話我寄email 給我, [email protected]

potential energy is a relative amount. if you choose the level in the desk, the pe is 0.

if you choose the level on the floor, eg 2m, the pe is 10x 10x2=200J relative to the floor.

ok?

if you want the object to rise, of course you must give a force larger than 100N,

if the object is on the desk, the downward force is 100N, by action and reaction, the

upward force provided by the desk is also 100N. so it is at rest.

when you try to lift the object, the download force is also 100N, the upward force is

provided by you now, not by the desk.

when the object doesn't touch the table, and you give 100N upload, the object will only

be at rest. when you give , say, 110N, then a net force of 10N upload is exerted to the

object. so it rise.ok?