關於gas pressure

(1) 有個實驗話放張card紙係個裝滿水的玻璃杯上面,,then倒轉佢,,d水都唔fall out,,因為有air pressure balances the weight of water
當我地將玻璃杯例轉既時候, 由於地心吸力, D水就會流晒出黎,
但係點解我地加一張卡紙比佢, D水就唔會流出黎呢
要做到D水唔流出黎, 我地就要有一個相同大細, 不過相反方向既力比佢,
而果個力F, 就係你所講既air pressure

圖片參考：http://i.imgur.com/eo5Ea.png

空氣就好似一條彈弓咁樣,
當你拉長或壓縮佢既時候, 佢就會比一個力, 將佢自己變返原來既大小
當你倒轉個杯既時候, D水會向下流出黎,
水會將卡紙同杯之間既空間封住, 咁就冇空氣入到去喇
當少部份水流出個杯, 由於冇空氣入到去, 原來入面既空氣就會被拉長,
咁就會有一個力拉返D水入黎
當依個力同水既重量一樣既時候, D水就唔會再流出黎喇

圖片參考：http://i.imgur.com/5TkFO.png

咁點解冇卡紙既時候, 水會流出黎呢?
因為冇左卡紙, 空氣就可以走入去,
原來杯入面既被拉長既空氣(低壓既空氣)就會因為新加入既空氣變返正常氣壓
冇左果個力F, 水就可以繼續流出黎喇

不過atmospheric pressure咁大,,點解佢唔會勁到整凹埋張紙???

圖片參考：http://i.imgur.com/cXkT4.png

上面係一張卡紙既放大圖, 係卡紙入面有好多空間, 都係充滿住空氣
雖然卡紙外有氣壓打算整凹張紙, 但係紙入面同樣有氣壓想整凸張卡紙
兩個力相同, 所以張紙只會維持原狀
不過, 雖然氣壓係好大, 但係張紙既硬度足以承受依一個力, 咁就唔會凹喇


(2)又有個實驗話倒轉個空杯插落水度,,d水都唔會入去個杯入面
點解會咁???又點解打斜插就ok??

因為空氣既密度比水既密度低, 所以空氣係冇其他力比佢既時候係會向上升既
空氣就唔會又杯入面向下降再浮出水面喇
空氣佔有空間既, 咁杯內既空間完全被空氣佔有住既時間,
水又點入去呢?

不過只打斜個杯, 空氣就出到黎, 而水亦有空間比佢入去喇

Q1: 不過atmospheric pressure咁大,,點解佢唔會勁到整凹埋張紙???

It is because water is almost incompressible. The air pressure acts on the card and the pressure is transmitted to the water inside the cup. Due to the incompressibility of water, its volume could not be reduced. Hence, the card would not be pushed inward towards the water.

Q2: 又有個實驗話倒轉個空杯插落水度,,d水都唔會入去個杯入面
點解會咁???又點解打斜插就ok??
In fact, there is some water that goes into the cup because the water pressure outside the moth of the cup is greater than atmospheric pressure.. The water would then compress the air and increases the air pressure inside the cup. A stage is then reached when the increased air pressure equals to the water pressure, no more water could enter into the cup.

When the cup is tilted, the increase in air pressure in the cup forces the air itself to go out. Thus giving space for the water to go into the cup.