SECTION 2 Optional Translation (30 points)
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 the 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 bum 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 bums, hot gases or exhaust gases rush out of the nozzle. The hot gases continue to rush out as long as the gunpowder bums. 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.