當相同的硬幣各自沉到油和水的情況

兩個重力相等的鐵塊體積相等，又因為他們的密度比煤油和水大，所以會浸沒在液體中。放在水中的鐵塊所受浮力F浮1=V排p水g因為鐵塊的體積V鐵=V排所以F浮1=V鐵p水g放在煤油中的鐵塊所受的浮力F浮2=V排p油g因為鐵塊的體積V鐵=V排所以F浮2=V鐵p油g煤油的密度比水小所以，你懂的。

2013-10-17 13:51:12 補充：
比较密度大小就可以了。水的密度1000kg/m³；汽油密度750kg/m³；

(1000-750)/750=1/3即同样体积下水的浮力比汽油的浮力大1/3

2013-10-17 18:34:01 補充：
因水的浮力比油大些,所以水的承托力會油大些,故硬幣在油中會下沉快些。

By Archimedes Principle, upthrust = weight of liquid displaced = pVg
where p is the density of liquid
V is the volume of the coin
g is the acceleration due to gravity

Net upward force acting on the coin
= Vdg - pVg where d is the density of coin material
= Vg(d - p)

Because density of oil is smaller than that of water, the term (d-p) for oil is larger than that for water. Therefore, the upward force acting on the coin is larger in oil than in water. This leads to a lower downward acceleration of the coin in oil. As a result, the coin in oil falls slower than the coin in water does. The coin in water thus reaches the bottom of the vessel earlier than the one in oil.

2013-10-17 23:16:42 補充：
Sorry...I made a typing mistake.
The net DOWNWARD force = Vdg - pVg = Vg(d - p)
Hence, downward force in oil is larger than that in water. The coin in oil reaches the bottom first.

當然是油，油比水輕，油可以浮水面。

2013-10-17 19:43:09 補充：
中英各異，你最好要求再解釋，我只會說自己正確。