题目材料
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.
Which of the following best describes the relationship between the first and the second paragraph of the passage?
- AThe first paragraph provides an illustration useful for understanding a procedure described in the second paragraph.
- BThe first paragraph describes a hypothetical situation whose plausibility is tested in the second paragraph.
- CThe first paragraph evaluates the usefulness of a procedure whose application is described further in the second paragraph.
- DThe second paragraph provides evidence to support a claim made in the first paragraph.
- EThe second paragraph analyzes the practical implications of a methodology proposed in the first paragraph.
显示答案
正确答案: A