Another problem is the strain that a person is subjected to when a rocket leaves the ground. Anything that is not moving tends to resist movement. As the rocket leaves the ground, it pushes upward, and the person is pushed back against the chair. During this thrust, gravity exerts a force on the body equal to nine times its normal force.

Once out of the earth's gravity an astronaut is affected by still another problem—weightlessness. Here, if a pencil drops, it doesn't fall. If a glass of water is turned upside down, the water will not fall out. All of us who are used to gravity expect things to have weightless conditions. Recent long flights have shown that the body needs special exercise in a spaceship.

　　  l. As a rocket leaves the ground, an astronaut experiences a force called——.

　　　　 A. gravity　　　　　　　　   B. a push

  　　   C. downward force　　　　    D. strain

　　 2. The force of gravity an astronaut is subjected to when the rocket leaves the ground is equal to

　　　　 A. 7 times its downward force　　　　　　  B. 5 times its normal force

　　　　 C. 3 times its upward force　　　　　　　　 D. 9 times its normal force

　　 3. Which of the following is not true according to the passage?

　　　　 A. Everything is down to the earth by magnetic force.

　　　　 B. Recent long flights have shown that the body needs special exercise in spaceship.

　　　　 C. A rocket must go at most 2, 500 miles an hour to take anyone beyond the gravity of the earth into space.

　　　　 D. There are many problems connected with space travel.

　　 4. Which is the best title for this passage?

　　　　 A. Weightlessness. Gravity and Strain Are All too Difficult to Understand

　　　　 B. Gravity is the Enemy of Space Travel

　　　　 C. Space Travel Problems

　　　　 D. People Cannot Leave the Earth Because of the Air

There are many problems connected with space travel. The first and greatest of them is gravity (地球引力). If you let your pencil drop to the floor, you can see gravity in action. Everything is down to the earth by magnetic force. The weight of something is another way of describing the amount of force exerted on it by gravity. A rocket must go at least 2, 500 miles an hour to take anyone beyond the gravity of the earth into space.

Another problem is the strain that a person is subjected to when a rocket leaves the ground. Anything that is not moving tends to resist movement. As the rocket leaves the ground, it pushes upward, and the person is pushed back against the chair. During this thrust, gravity exerts a force on the body equal to nine times its normal force.

Once out of the earth's gravity an astronaut is affected by still another problem—weightlessness. Here, if a pencil drops, it doesn't fall. If a glass of water is turned upside down, the water will not fall out. All of us who are used to gravity expect things to have weightless conditions. Recent long flights have shown that the body needs special exercise in a spaceship.

　　  l. As a rocket leaves the ground, an astronaut experiences a force called——.

　　　　 A. gravity　　　　　　　　   B. a push

  　　   C. downward force　　　　    D. strain

　　 2. The force of gravity an astronaut is subjected to when the rocket leaves the ground is equal to

　　　　 A. 7 times its downward force　　　　　　  B. 5 times its normal force

　　　　 C. 3 times its upward force　　　　　　　　 D. 9 times its normal force

　　 3. Which of the following is not true according to the passage?

　　　　 A. Everything is down to the earth by magnetic force.

　　　　 B. Recent long flights have shown that the body needs special exercise in spaceship.

　　　　 C. A rocket must go at most 2, 500 miles an hour to take anyone beyond the gravity of the earth into space.

　　　　 D. There are many problems connected with space travel.

　　 4. Which is the best title for this passage?

　　　　 A. Weightlessness. Gravity and Strain Are All too Difficult to Understand

　　　　 B. Gravity is the Enemy of Space Travel

　　　　 C. Space Travel Problems

　　　　 D. People Cannot Leave the Earth Because of the Air