archimede principle

Archimedes Principle states that the upthrust acting on an object equals the weight of liquid displaced by the object.
When the principle is applied to a floating object, because the upthrust just balances the weight of the object. The principle can be stated as: the weight of a floating object equals the weight of liquid displaced. This is the Principle of Floatation, a specialized form of Archimedes Principle.
The principle simply indicates that the buoyance of an object in a liquid can be calculated by finding the amount (weight) of the displaced liquid.
Archidedes first discovered that, when he was taking a bath in a bathtub, he noticed that the amount of water overflowing the tub was proportional (related consistently) to the amount of his body that was being immersed (covered by water). He then applied this phenomenon to check the purity of gold in a crown. This is how the principle was first formulated.
The following web-page gives a short brief of the event.
http://www.notablebiographies.com/An-Ba/Archimedes.html
Two famous uses in history:
1. To determine the purity of a gold crown given to King Hieron. Archimedes was assigned such task by the king and he applied this principle that he discovered to finsih the task.
2. To determine the weight of an elephant. The elephant to be weighted was put onto a boat. The depth of the boat beneath the water line was marked. Without the elephant on the boat, pieces of stone were put onto the boat until the water line coincided with the mark again. Then the stones were weighted piece by piece and from which the weight of the elephant, which equals the total weight of the stone pieces, could be found. This is the story known as "曹沖秤象".
Please refer to the following webpages for detail:
http://www.wss.edu.hk/article.php?article=13
http://tw.myblog.yahoo.com/jw!XkVb3ESQFRY3pjg55z7LTNfkBGiZ/article?mid=1011&prev=1014&next=1010

It states that if an object is put into the water in which a certain
volume of water is expelled, than the loss of weight due to density
(or the floating force) is equal to the mass of that volume of water.

the principle is discovered when archimede is bathing. He found that
his body slightly floats and the water of the volume same to the volume of his body under water is expelled out.

one famous use is that it can be used to make boats, to calculate
when boat floats. it is also useful in biology to calculate the volume
of an animal under water.

浮力定律

又稱阿基米德定律，由阿基米德發現。定律為：

物體在液體（氣體）中所受浮力，等於所排開的液體（氣體）的重力。根據浮力定律可以算出：當物體飄浮或懸浮時，浮力等於物體的重力；但當物體處於下沉狀態，浮力小於重力。

浮力 = 物體在液體中所減輕的重量 = 物體在空氣中重 - 物體在液體中的重

= 物體所排開液體的體積 * 液體密度 = 物體所排開的液體重

[編輯] 物體的浮沉

當物體浮時，其重力等於其浮力。 當物體沉時，其重力大於其浮力。

物體浮沉與密度的關係： 已知B=F=W=mg=pvg 當物體沉時，其W>B , 所以 (ρV)物體g>(ρV)液體g ，基於g為固定值，所以當二者V相等時（物體處於載浮載沉），可得ρ物體 > ρ液體，所以我們會以液體及物體密度來決定該物體的浮沉

[編輯] 狀態

* 當物體的密度小於浸泡液體或氣體的密度時且物體不在液體的表面的狀態稱為上浮。
* 當物體的密度小於浸泡液體或氣體的密度時且物體在液體的表面的狀態稱為飄浮。
* 當物體的密度等於浸泡液體或氣體的密度時的狀態稱為懸浮。
* 當物體的密度大於浸泡液體或氣體的密度時的狀態稱為下沉。

浮力兩大概念：

1. 浮力=液體密度*被排開液體的體積*質量與重量比值(9.8N/Kg)
2. 當物體浮在水面上，其浮力等於該物體之重量