The earth is not the only body that travels around the sun. With it are eight other planets, fellow members of the sun’s family.
Two of them, Mercury (水星) and Venus, are nearer while the other six, namely Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, also in their given order from the sun, are farther from the sun than the earth is. The farther they are, the longer trips they make around the sun. People noticed long ago that these traveling bodies moved around in the sky in definite paths. It is a force called gravity that holds them in their paths.
We know that every little bit of matter in the universe pulls upon every other bit of matter. The pull between two bodies is proportional (成比例的) to the product of their masses. Because the sun is so large the pull between the sun and the planets are thus great. If it were not for this pull, the planets would fly off into space. In the same way there exists a pull between the earth and the moon, which keeps the moon traveling in its orbit around our planet, the earth. Gravity holds you to its surface, and pulls back to it the ball which you throw into the air. Of course the ball also pulls on the larger earth but the earth is so much larger that the pull is not noticed.
Now remember that large bodies exert a greater pull than smaller ones which contain less material. But each object in the universe, no matter how small, pulls on all other objects to some degree.
小题1:There are ________ that travel around the sun in the sun’s family.
A．nine planets
B．eight planets
C．one star and ten planets
D．the earth and the sun小题2:Which two planets make the longest trips around the sun among all the planets in the solar system?
A．Mercury and Venus.
B．Neptune and Pluto.
C．Saturn and Uranus.
D．Mars and Jupiter.小题3:From the passage we can see__________
A．all the objects, big or small, must exert the same pull on one another
B．large objects exert the same pull on anybody as small objects
C．small objects exert the same on large ones
D．each object in the universe exerts a pull on all other objects小题4:From the sentence “The pull between two bodies is proportional to the product of their masses.” We can infer that the pull between__________
A．the sun and the moon is greater than between the sun and the earth
B．the earth and the moon is greater than that between the sun and the earth
C．the sun and the earth is greater than that between the earth and the moon
D．the sun and the earth is the same as that between the earth and the moon
