What Is the Force of Gravity?
If you throw a ball up, it will come down again. What makes it come down? The ball comes down because it is pulled or attracted towards the Earth. The Earth exerts a force of attraction on all objects. Objects that are nearer to the Earth are attracted to it with a greater force than those that are further away. This force of attraction is known as the force of gravity. The gravitational force acting on an object at the Earth’s surface is called the weight of the object.
All the heavenly bodies in space like the moon, the planets and the stars also exert an attractive force on objects. The bigger and heavier a body is, the greater is its force of gravity. Thus, since the moon is a smaller body than Earth, the force it exerts on an object at its surface is less than that exerted by the Earth on the same object on the Earth’s surface. In fact, the moon’s gravitational force is only one-sixth that of the Earth. This means that an object weighing 120 kilograms on Earth will only weigh 20 kilograms on the moon. Therefore on the moon you could lift weights which are six times heavier than the heaviest weight that you can lift on Earth.
The Earth’s gravitational force or pull keeps us and everything else on Earth from floating away to space. To get out into space and travel to the moon or other planets we have to overcome the Earth’s gravitational pull.
Entry into Space
How can we overcome the Earth’s gravitational pull? Scientists have been working on this for a long time. It is only recently that they have been able to build machines powerful enough to get out of the Earth’s gravitational pull. Such machines are called space rockets. Their great speed and power help them to escape from the Earth’s gravitational pull and go into space.
Rockets
The powerful space rocket works along the same lines as a simple firework rocket. The firework rocket has a cylindrical body and a conical head. The body is packed with gunpowder which is the fuel. It is a mixture of chemicals that will burn rapidly to form hot gases.
At the base or foot of the rocket there is an opening or nozzle. A fuse hangs out like a tail from the nozzle. A long stick attached along the body serves to direct the rocket before the fuse is lighted.
When the gunpowder burns, hot gases rush out of the nozzle. The hot gases continue to rush out as long as the gunpowder burns. When these gases shoot downwards through the nozzle the rocket is pushed upwards. This is called jet propulsion. The simple experiment, shown in the picture, will help you to understand jet propulsion.
参考答案：
什么是重力?
假如你把一个球往上抛，它还会掉下来。球为什么会掉下来呢?球掉下来是因为球被吸引或拉向地球。地球对所有物体都施加吸引力。离地球较近的物体所受的吸引力，比远离地球的物体所受的吸引力大。这种吸引力通常叫做重力。作用于地球表面上物体的引力，叫做物体的重量。
空间所有的天体，如月球﹑行星和星星，对物体也都有吸引力。物体越大﹑越重，其引力也就越大。既然月球比地球小，因此，月球对月球表面上一物体施加的力，将比地球对地球表面上同一物体施加的力要小。实际上，月球的引力只有地球引力的六分之一。这就是说，地球上一个重120公斤的物体，在月球上会只重20公斤。因此，在月球上你能举起的重量，将是你在地球上所能举起的最大重量的六倍。
地球的引力或拉力，使我们和地球上别的任何东西都不致漂浮到空间中。要飞向空间并到月球或其他行星去旅游，就必须克服地球的引力。
进入空间
怎么才能克服地球的引力呢?科学家们为此进行了长期的研究。直到最近，他们才能够制造出足以克服地球引力的强有力的机器。这种机器就称为宇宙火箭。宇宙火箭的巨大速度和强大的动力，帮助它挣脱地球的引力而进入空间。
强大的宇宙火箭的工作原理和简单的烟火火箭是一样的。烟火火箭有一圆柱形壳体和一锥形头部。壳体内充填黑色火药作为燃料。它是一种化学品的混合物，能迅速燃烧形成炽热的气体。
火箭的底部有一个通道，即喷口。从喷口伸出一根尾巴似的引信。沿壳体装着一根长棍，用以在引信点着以前确定火箭发射的方向。
当黑色火药燃烧时，炽热的气体，即燃气从喷口急速排出。只要黑色火药还在燃烧，燃气就不断急速排出。当这些燃气通过喷口往下喷出时，火箭就被向上推进。这就叫做喷气推进。图中所示的简单实验，可以帮助你了解喷气推进。
