The Earth has a force that pulls thing toward itself. We call this force gravity. 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 in rocket operation and must overcome 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 escape speed has been reached by a space
A. accept it without questioning it
B. never notice its presence
C. hardly think about how to use it
D. often try to escape from it
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