2. 外语
  3. Suppose we were in a spaceship in free fall, where objects are weightless, and wanted to know a small solid object’s mass. We co

Suppose we were in a spaceship in free fall, where objects are weightless, and wanted to know a small solid object’s mass. We co

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问题 Suppose we were in a spaceship in free fall, where objects are weightless, and wanted to know a small solid object’s mass. We could not simply balance that object against another of known weight, as we would on Earth. The unknown mass could be determined, however, by placing the object on a spring scale and swinging the scale in a circle at the end of a string. The scale would measure the tension in the string, which would depend on both the speed of revolution and the mass of the object. The tension would be greater, the greater the mass or the greater the speed of revolution. From the measured tension and speed of whirling, we could determine the object’s mass.
Astronomers use an analogous procedure to "weigh" double-star systems. The speed with which the two stars in a double-star system circle one another depends on the gravitational force between them, which holds the system together. This attractive force, analogous to the tension in the string, is proportional to the stars’ combined mass, according to Newton’s law of gravitation. By observing the time required for the stars to circle each other (the period) and measuring the distance between them, we can deduce the restraining force, and hence the masses.
It can be inferred from the passage that the two procedures described in the passage have which of the following in common?
选项 A、They have been applied in practice.
B、They rely on the use of a device that measures tension.
C、Their purpose is to determine an unknown mass.
D、They can only be applied to small solid objects.
E、They involve attraction between objects of similar mass.
答案C
解析 Inference
The procedures described in the passage are introduced by the suggestion in the first paragraph that someone in a spaceship who wanted to determine a solid object’s mass could do so in a particular way. The second paragraph uses the word weigh in quotes to refer to a similar procedure for determining the mass of a double-star system.
A The language of the first paragraph is hypothetical: we could do particular things. Thus, there is no way to determine from the passage whether that procedure has been applied in practice.
B The first procedure relies on a spring scale, which measures tension, but the second procedure measures time and distance to determine restraining force.
C Correct. Both procedures determine mass: the first procedure can determine the mass of a small solid object on a spaceship in free fall, and the second can determine the mass of a double-star system.
D The first procedure would, according to the passage, be applied to a small solid object, but the second weighs double-star systems, which are clearly not small objects.
E The second procedure involves attraction between two stars, which could be of similar mass, in the same system, but the first procedure involves measuring tension in a string and speed of whirling, not attraction between objects.
The correct answer is C.
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