Section outline

  • General

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  • 重力

    Lesson Objectives
    ::经验教训目标

    • Define gravity.
      ::定义重力。
    • Explain how gravity affects the motion of objects.
      ::解释引力如何影响物体的运动。

    Lesson Vocabulary
    ::词汇表课程

    • gravity
      ::重力
    • orbit
      ::轨道轨道轨道轨道轨道轨道

    Introduction
    ::导言

    Why does the Moon orbit the Earth? Why does the Earth and the other planets orbit the Sun? Why are we pulled toward the ground? The answer to all these questions is  gravity . Gravity is the reason for many things we take for granted. Many of the things we do in our daily life would not be possible without gravity. For that matter, the universe would look very different.  
    ::为什么月球环绕地球?为什么地球和其他行星环绕太阳?为什么我们被拉向地面?所有这些问题的答案都是引力。重力是我们认为理所当然的许多事情的原因。没有引力,我们日常生活中的许多事情是不可能做到的。对于这一点,宇宙将看起来非常不同。

    The universe came into existence a long, long time ago. When it was young an invisible force did caused something to happen that was spectacular. The force responsible was gravity. Gravity caused dust and gas particles to pull together to form all the objects in our solar system (see  Figure  ). Gravity created the smallest moon to our largest sun. It created everything we see in our solar system. It has also caused all the other objects in our solar system to form. In fact, it caused all objects in every galaxy to form. Without gravity, there would be no us. 
    ::宇宙诞生了很久很久以前。 当它年轻时, 无形的力量确实造成了一件非常壮观的事情。 责任力是引力。 引力导致尘埃和气体粒子聚集在一起形成太阳系中的所有物体(见图 ) 。 引力将最小的月亮形成为我们最大的太阳。 它创造了我们在太阳系中看到的一切。 它也导致太阳系中所有其他物体形成。 事实上, 它导致每个星系中的所有物体形成。 没有引力,就没有我们了。

    lesson content
     
    Gravity helped to form our solar system and all the other solar systems in the universe.

    Defining Gravity
    ::重力定义

    So what is gravity? A typical definition of  gravity  is that it is a force. It causes an attraction between two masses. According to this definition, anything that has mass exerts a force. Any object exerts gravity on other objects. It does not matter how small it is, it has gravity. The more matter an object has, the more gravity it has. Your pencil has a tiny bit of gravity, but far too little to notice. What about a planet? It would have a lot of gravity. 
    ::重力是什么? 典型的重力定义是它是一种力量。 它在两个质量之间引起一种吸引。 根据这个定义, 任何有质量的东西都会施加一种力量。 任何物体都会对其他物体施加重力。 不论它有多小, 它都有重力。 物体越是重要, 它的重力就越大。 您的铅笔有一点点重力, 却很少能注意到。 关于一个行星, 它会有很大的重力。

    An object's gravity exerts a pull on other objects. Friction only occurs between objects that are touching. Gravity can act between objects that are not touching. In fact, gravity can act over very long distances. Where else can you feel gravity? 
    ::对象的引力对其它对象拉动。 摩擦只能在触碰的对象之间发生。 重力可以在未触碰的物体之间发生作用。 事实上, 引力可以在很远的距离内发生作用。 您还能感觉到引力吗 ?

    Earth’s Gravity
    ::地球的重力

    You are already very familiar with Earth’s gravity. It constantly pulls you toward the center of the planet. Gravity keeps you and everything else stuck to the surface. If not for gravity, we would be flung into space due to Earth's rotation on its axis. 
    ::您已经非常熟悉地球的引力。 它不断将您拉向行星的中心。 重力将您和所有其它东西困在地表上。 如果不是因为重力,我们就会因为地球在轴心上的旋转而被抛入太空。

    Gravity also pulls objects above the surface, from meteors to skydivers, down to the ground. Gravity between Earth and the Moon and between Earth and artificial satellites keeps all these objects circling around Earth. Gravity also keeps Earth moving around the sun. 
    ::地球与月球之间的重力以及地球与人造卫星之间的重力使所有这些物体环绕地球。 重力也使地球绕太阳移动。

    The Earth and Moon orbit each other. This Earth-Moon system orbits the Sun in a regular path ( Figure  ).  Gravity  is the force of attraction between all objects. Gravity keeps the Earth and Moon in their orbits. Earth’s gravity pulls the Moon toward Earth’s center. Without gravity, the Moon would continue moving in a straight line. It would fly off into space.
    ::地球和月球环绕着地球和月球。这个地球月球系统在正常路径(图 ) 中绕着太阳运行。 重力是所有物体之间的吸引力。 重力将地球和月球置于它们的轨道中。 地球的重力将月球引向地球的中心。 没有引力,月球将继续沿着直线移动。 它会飞向太空。

    The moon moves around Earth. It makes a circular path called an  orbit . Why doesn’t Earth’s gravity cause the Moon to crash into Earth? The reason is simple. The moon is moving really fast. It is moving fast enough to counter the force of Earth’s gravity. As a result, the moon keeps orbiting Earth. It never crashes into it.  
    ::月球环绕地球。它环绕地球,环绕轨道。为什么地球的引力不导致月球坠入地球?原因很简单。月球移动速度非常快,足以抵消地球引力。结果,月球一直绕着地球运行,从不撞入地球。

    lesson content
     
    The Moon orbits the Earth, and the Earth-Moon system orbits the Sun.

     

    Gravity and Weight
    ::重重和重重

    Weight measures the force of gravity pulling on an object. Because weight measures force, scientist use  newtons (N)  to describe it. On Earth, a mass of 1 kilogram has a weight of about 10 newtons. On the moon, gravity is less. As a result, the same mass would weigh less. Weight is measured with a scale, like the spring scale in  Figure  . The scale measures the force with which gravity pulls an object downward.
    ::量度重力拉动物体的强度。 由于重量量力, 科学家使用牛顿( N) 来描述它。 在地球上, 1公斤的重量约为 10 牛顿。 在月球上, 重力小于 10 牛顿。 因此, 重力会小于 同样的重量。 像图中的春季尺度一样, 用一个比例尺测量重量。 比例尺测量引力拉下物体的强度 。

    lesson content
     
    A scale measures the pull of gravity on an object.

    Law of Gravity
    ::重力定律

    People have known about gravity for thousands of years. After all, they constantly feel its effects. They knew that things fall toward the ground. People did not truly know why it happened. Sir Isaac Newton spent a lot of time studying gravity. He developed his law of gravity in the late 1600s. It explained how it affected all objects in the same way. Newton is pictured in  Figure  .
    ::人们知道引力已有数千年。 毕竟, 他们不断感受到它的影响。 他们知道它会落到地上。 人们并不知道它为什么会发生。 艾萨克·牛顿爵士花了很多时间研究引力。 他在1600年代后期制定了重力法则。 它解释了它是如何以同样的方式影响所有物体的。 牛顿在图中被描绘了。

    lesson content
     
    Sir Isaac Newton discovered that gravity is universal.

    Newton’s Law of Universal Gravitation (Advanced Topic)
    ::牛顿的普遍引力法(高级专题)

    Newton was the first one to suggest that gravity is universal. That means gravity affects all objects in the universe. That’s why his law of gravity is called the  law of universal gravitation . Universal gravitation means that all objects are affected by gravity in the same way. This is the reason the apple falling from the tree and the Moon being held in orbit is the same. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. In fact, there is gravity between you and every mass around you. Even tiny molecules of gas are attracted to one another by the force of gravity.
    ::牛顿是第一个提出引力是普遍性的。这意味着引力影响宇宙中的所有物体。这就是为什么他的引力定律被称为宇宙引力定律的原因。宇宙引力指所有物体都受到同样的重力影响。这就是苹果从树上掉下来而月球却在轨道上的原因。宇宙引力也意味着当地球对你们拉动时,你就会对地球拉动。事实上,你和你的周围的每个质量都有引力。即使是气体的微小分子也会受到重力的吸引。

    Newton’s law had a huge impact on how people thought about the universe. It explains the motion of objects not only on Earth but in outer space as well.
    ::牛顿的法律对人们如何看待宇宙产生了巨大影响。 它解释了物体不仅在地球上,而且在外层空间的动态。

    Factors That Influence the Strength of Gravity (Advanced Topic) 
    ::影响重力力量的因素(高级专题)

    Newton’s law also states that the strength of gravity between any two objects depends on two factors. It depends on the mass of the objects. It also depends on the distance between them.
    ::牛顿的法律还规定,两个物体之间的重力取决于两个因素。它取决于物体的质量。它也取决于它们之间的距离。

    • Objects with greater mass have a stronger force of gravity. For example, because Earth is so massive, you can feel its affects as it holds you to the ground. This is unlike the force your desk or pencil might exert on you as they are very small in comparison. That’s why you and the desk remain in place on the floor. You are not naturally pulled toward your desk.
      ::质量较大的物体具有较强的重力。例如,由于地球如此巨大,你可以感受到它的影响,因为它将你压在地上。这与你的办公桌或铅笔可能施加于你的力力不同,因为它们相比很小。这就是为什么你和办公桌留在地上的原因。你自然不会被拉到办公桌上。
    • Objects that are closer together have a stronger force of gravity. For example, the moon is closer to Earth than it is to the more massive sun. That is why the force of gravity is greater between the moon and Earth than between the moon and the sun. That’s why the moon circles around Earth rather than the sun. This is illustrated in  Figure  .
      ::更接近的物体具有更强大的引力。 比如, 月亮更接近地球, 而不是更大的太阳。 这就是为什么月亮和地球之间的引力大于月亮和太阳之间的引力。 这就是为什么月亮环绕地球而不是太阳的原因。 图中说明了这一点。

    All objects in the universe have a gravitational attraction to each other .( Figure  ) The strength of the force of gravity depends on two things. They are the mass of the objects and the distance between them. The greater the objects’ mass, the greater the force of attraction. As the distance between the objects increases, gravity's force on them decreases.
    ::宇宙中的所有物体都具有引力吸引力 .(图 ) 引力的强度取决于两件事。 它们就是物体的质量以及它们之间的距离。 物体的质量越大, 吸引力就越大。 随着物体之间的距离越长, 物体的引力就越小。

    lesson content
     
    The strength of the force of gravity between objects A and B depends on the mass of the objects and the distance (u) between them.

    Orbital Motion (Advanced Topic)
    ::轨道动力(高级专题)

    The moon moves around Earth in a circular path called an  orbit . Why doesn’t Earth’s gravity pull the moon down to the ground instead? The moon has enough forward velocity to partly counter the force of Earth’s gravity. It constantly falls toward Earth, but it stays far enough away from Earth so that it actually falls around the planet. As a result, the moon keeps orbiting Earth and never crashes into it. The diagram in  Figure   shows how this happens. 
    ::月球在环绕地球的轨道中移动。 为什么地球的引力不能将月球拖向地面? 月球有足够的前方速度来部分对抗地球引力。 它不断向地球倾斜,但距离地球足够远,因此它实际上会绕地球坠落。 结果,月球一直绕着地球运行,从未撞入地球。 图中的图表显示了这种情况是如何发生的。

    lesson content
     
    In this diagram, "v" represents the forward velocity of the moon, and "a" represents the acceleration due to gravity. The line encircling Earth shows the moon’s actual orbit, which results from the combination of "v" and "a."

    Lesson Summary
    ::经验教训摘要

    • Gravity is traditionally defined as a force of attraction between two masses. Weight measures the force of gravity and is expressed in newtons (N).
      ::重力传统上的定义是两种质量之间的吸引力,重量测量重力,以牛顿(N)表示。
    • According to Newton’s law of universal gravitation, gravity is a force of attraction between all objects in the universe. The strength of gravity depends on the masses of the objects and the distance between them. 
      ::根据牛顿宇宙引力法则,重力是宇宙中所有物体之间的一种吸引力。 重力的强度取决于物体的质量以及它们之间的距离。

    Lesson Review Questions
    ::经验回顾问题

    Recall
    ::回顾

    1. What is the traditional definition of gravity?
      ::重力的传统定义是什么?
    1. How is weight related to gravity?
      ::重量与重力有何关联?
    1. Summarize Newton’s law of universal gravitation. (Advanced Topic)
      ::总结牛顿的普遍引力法。 (高级专题)