Read the following passage and mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions
Archimedes’s Principle is a law of physics that states that when an object is totally or partially immersed in a fluid, it experiences an upthrust equal to the weight of the fluid displaced. The principle is most frequently applied to the behaviour of objects in water, and helps to explain floating and sinking, and why objects seem lighter in water. It also applies to balloons.
The key word in the principle is “upthrust”, which refers to the force acting upward to reduce the apparent weight of the object when it is under water. If, for example, a metal block with a volume of 100 cm3 is dipped in water, it displaces an equal volume of water, which has a weight of approximately 1 N (3.5 oz). The block therefore seems to weigh about 1 N less.
An object will float if its average density is less than that of water. If it is totally submerged, the weight of the water it displaces (and hence the upthrust on it) is greater than its own weight, and it is forced upward and out of water, until the weight if water displaced by submerged part is exactly equal to the weight of the floating object. Thus a block of wood with a density six tenths that of water will float with six tenths of its volume under water, since at that point the weight of fluid displaced is the same as the blocks’s own weight. If a dense material is made into a suitable shape, it will float because of Archimedes’s principle. A ship floats, whereas a block of iron of the same mass sinks.
It is also because of Archimedes’s principle that ships float lower in the water when they are heavily loaded (more water must be displaced to give the necessary upthrust). In addition, they cannot be so heavily loaded if they are to sail in fresh water as they can if they are to sail in the sea, since fresh water is less dense than sea water, and so more water must be displaced to give the necessary upthrust. This means the ship is lower in the water, which can be dangerous in rough weather.
From “Archimedes’s Principle”, MicrosoftÒ Student 2008[DVD]. Microsoft Corporation, 2007.
Ships cannot be so heavily loaded if they want to sail in fresh water as they sail in the sea, because ____
A. fresh water is ‘lighter’ than sea water
B. there’s too much salt in sea water
C. sea water is ‘saltier’ than fresh water
D. fresh water is more polluted
Đáp án A
A. fresh water is ‘lighter’ than sea water: ý nghĩa của cả câu này là “Tàu thuyền không thể được chất nặng khi chúng đi trên sông như khi chúng đi trên biển, bởi vì nước sông thì nhẹ hơn nước biển”.
Chúng ta có thể tìm thấy ý này trong câu hai của đoạn bốn: “In addition, they cannot be so heavily loaded….to give the necesssary upthrust”