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.
The author of the passage mentions observations regarding the period of a double-star system as being useful for determining
选项 A、the distance between the two stars in the system
B、the time it takes for each star to rotate on its axis
C、the size of the orbit the system’s two stars occupy
D、the degree of gravitational attraction between the system’s stars
E、the speed at which the star system moves through space
答案D
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.
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