题目内容：

    How Soccer Can Help Us Understand Physics

Sports provide a great way to understand some concepts(概念) in physics. Physics, after all is the study of matter, motion(运动), force, and energy. And since sports like soccer, swimming and cycling involve bodies moving through space, they  can help us understand how the principles of physics work.

Imagine that you're looking at a soccer hall on a grassy field. If you do nothing to the hall, it will stay motionless on the grass. If you kick the ball, it will roll along the grass before coming to rest again. Pretty simple , right?

For thousands of years, though, people thought that objects like this soccer ball come to rest because they have a natural tendency to stop. It took a famous physicist by the name of Sir Isaac Newton, who lived in the 1600s, to prove that this was not exactly correct.

Newton suggested that objects like the soccer ball have a natural tendency to keep moving. The only reason they stop, he believed is because an unbalanced force acts on them. By an unbalanced force, Newton meant the force applied to the soccer ball by its environment. When kicked the surface of the ball travels over the grass, creating friction(摩擦力).The taller the grass, and the rougher the surface of the ball, the more friction is created.  And the more friction that exists between the ball and the grass, the less it will travel after being kicked.

Now, imagine that there is no grass. Instead. the ball is resting on a frozen lake. When you kick the ball on the ice, the ball will go much farther than it would have on the grass. This is because ice provides a lot less friction than the grass.

Even so, ice does cause some friction. The ball's interaction with the frozen water crystals on the surface of the lake eventually causes it to come to rest again. But now imagine that instead of ice, the ball is in a place where there's no friction at all. The ball is floating in a vacuum.  If you remove friction entirely, kicking the soccer ball would cause it to keep going and going at the same speed, until some force caused it to slow down and stop. To paraphrase Sir Isaac Newton, a soccer ball on the grass will stay where it is unless acted on by a force. Similarly, once you kick the ball, it will remain in motion unless acted on by force. This, in so many words, is known as Newton's First Law of Motion.

1.According to the passage, once an object like a soccer ball is in motion, it has a natural tendency to          .

A.stop B.slow down C.keep moving D.change direction

2.What does the writer explain in the passage?

A.The idea of motion, using sports as examples.

B.The force of friction, using music as examples.

C.The sport of soccer, using examples of teams and players.

D.The importance of understanding the concepts in physics.

3.What does the underlined word" tendency" mean?

A.A very small chance of something happening.

B.A fifty-fifty chance of something happening.

C.The way something normally behaves or acts.

D.The fear of doing something or acting in a certain way.

4.The passage is mainly about             .

A.the motion of objects B.the life of Sir Isaac Newton

C.why we have to learn physics D.why we kick the ball on a grassy land