单选题
The Earth has a force that pulls thing toward itself. We
call this force gravity (地心引力). This is something we live with all the time, and
we take it for granted and hardly ever think about it. But it is a most
important factor (因素) in rocket (火箭) operation and must overcome (克服) if we are
to get anywhere in space, or off the ground at all. Take the
throwing of a ball as an example. The harder the ball is thrown, the faster and
higher it will go. What is the secret? Its speed. If we could throw the ball
hard enough it would go up and up forever and never come down. The speed at
which it would have to be thrown do this is known as escape speed. Of course, we
cannot throw a ball hard enough because the speed required to escape completely
from the Earth's gravity is seven miles per second, or over twenty-five thousand
miles per hour. Once a spacecraft has reached escape speed, no
further power is needed. A rocket aimed at the Moon, for instance, will "coast"
(滑行) the rest of the way because the Earth's gravity cannot then pull it back,
and there is no air resistance (阻力) in space to slow it down. This "coasting" is
known as "free fall" Free fall is an important feature (特点) of space travel: it
would be impossible to carry enough fuel to provide powered flight all the
time.
单选题
As there is always the Earth's gravitational (地心力的) pull, people
______.
A. accept it without questioning it
C. seldom think about how to use it
B. never notice its presence
D. often try to escape from it
【正确答案】
A
【答案解析】[解析] 细节题。题干意为“由于地球引力的一直存在，人们______。”从文章第一段第三句话“This is something we live with all the time, and we take it for granted and hardly ever think about it.”可知，人们把它视为理所当然，从来也没有去想过它。所以本题的正确答案为A。
单选题
What is the most important factor for a rocket to escape from the
Earth?
单选题
How fast will a rocket go to escape completely from the Earth's
gravity?
A. Seventy miles per second
B. Seven thousand miles per hour
C. Over twenty-five thousand miles per second
D. Over twenty-five thousand miles per hour
【正确答案】
D
【答案解析】[解析] 细节题。题干意为“火箭要完全逃脱地球引力需要达到什么速度?”从文章第二段最后一句话“Of course... twenty-five thousand miles per hour.”可知，要完全逃脱地球引力速度需要达到“over twenty-five thousand miles per hour”，所以本题的正确答案为D。
单选题
Why is it not necessary to provide powered flight in space all the
time?
A. Few spacecraft complete their flight operations.
B. No further power is needed after the escape speed is reached.
C. The spacecraft will be attracted by the Moon.
D. "Free fall" takes the place of the Earth's gravitational pull.
【正确答案】
B
【答案解析】[解析] 细节题。题干意为“为什么在宇宙中不需要一直耗能的飞行?”从文章最后一段第一句话“Once a spacecraft has reached escape speed, no further power is needed.”可知，宇宙飞船一旦达到逃逸速度，就不再需要更多的能量了。由此可见，本题的正确答案应为B。