阅读理解 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.
单选题 1.It can be inferred from the passage that the two procedures described in the passage have which of the following in common?
【正确答案】 C
【答案解析】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.
单选题 2.According to the passage, the tension in the string mentioned in lines 8-9 is analogous to which of the following aspects of a double-star system?
【正确答案】 B
【答案解析】Supporting idea
The second paragraph states that an attractive force is analogous to the tension in the string. This attractive force is identified in the previous sentence as the gravitational force between the two stars in a double-star system.
A The second paragraph states that the speed with which the stars circle each other depends on the gravitational force between them, but it is that force that is analogous to the tension in the string.
B Correct. The second paragraph clearly identifies the gravitational force between the two stars as the attractive force that is analogous to the tension in the spring scale's string.
C The amount of time it takes for the stars to circle one another is necessary for calculating the force that holds them together, but it is the force itself that is analogous to the string's tension.
D The distance between the stars must be measured if the attraction between them is to be determined, but the attraction, not the distance, is analogous to the string's tension.
E The combined mass of the two stars is what the procedure is designed to determine; it is analogous to the mass of the small solid object, as described in the first paragraph.
The correct answer is B.
单选题 3.Which of the following best describes the relationship between the first and the second paragraph of the passage?
【正确答案】 A
【答案解析】Evaluation
This question requires understanding that the second paragraph describes a somewhat difficult-to-understand procedure that the first paragraph illustrates in smaller, and simpler, terms.
A Correct. The first paragraph illustrates, hypothetically, a simple procedure for determining mass, and this illustration provides the grounds on which the passage explains the procedure of the second paragraph.
B The first paragraph describes a situation in hypothetical terms, but the second paragraph does not test that situation's plausibility. Instead, the second paragraph draws an analogy between the initial situation and another procedure.
C The first paragraph does not evaluate the usefulness of the procedure for determining a small solid object's mass while in a spaceship in freefall; it simply describes how that procedure would work.
D The second paragraph provides no evidence; it describes a procedure analogous to what is described in the first paragraph.
E The second paragraph does not discuss the practical implications of the first paragraph's methodology but rather a procedure that is analogous to the hypothetical situation of the first paragraph.
The correct answer is A.
单选题 4.The author of the passage mentions observations regarding the period of a double-star system as being useful for determining
【正确答案】 D
【答案解析】Supporting idea
The author mentions the period of a double-star system in the final sentence of the second paragraph, defining it as the time required for stars to circle each other. Knowing this time, in combination with the distance between the stars, enables the determination of the restraining force between the stars.
A The final sentence of the second paragraph indicates that the period of a double-star system is measured independently of the distance between the two stars in the system.
B The passage is not concerned with how long it takes each star to rotate on its axis.
C The passage does not mention anyone's trying to determine the size of the orbit of a system's two stars. It does mention the related topic of distance between the stars but indicates that knowing such distance is required for measuring the stars' mass, not that it can be inferred from the period of the system.
D Correct. According to the passage, the restraining force, or gravitational attract on, between the two stars can be deduced based on the period and the distance between them.
E The passage does not mention the speed at which the star system moves through space.
The correct answer is D.