| The rocket engine, with its steady roar
like that of a waterfall or a thunderstorm, is an impressive symbol of the new
space age. Rocket engines have{{U}} (61) {{/U}}powerful enough to shoot
astronauts{{U}} (62) {{/U}}the earth's gravitational pull and land them
on the moon. We have now become space{{U}} (63) {{/U}}. Impressive and complex as it may appear, the rocket, which was{{U}} (64) {{/U}}in China over 800 years ago, is a relatively simple device. Fuel that is{{U}} (65) {{/U}}in the rocket engine changes{{U}} (66) {{/U}}gas. The hot and rapidly{{U}} (67) {{/U}}gas must escape, but it can do so only through an opening that{{U}} (68) {{/U}}backward. As the gas is{{U}} (69) {{/U}}with great force, it{{U}} (70) {{/U}}the rocket in the{{U}} (71) {{/U}}direction. Like the kick of a gun{{U}} (72) {{/U}}it is fired, it{{U}} (73) {{/U}}the laws of nature{{U}} (74) {{/U}}by Sir Isaac Newton when he discovered that"{{U}} (75) {{/U}}every action, there is an equal and opposite reaction." |