题目内容
阅读下列短文,从所给的四个选项中,选出最佳答案
A radar (雷达) set produces radio waves and then gives them out into space by means of antenna (天线) . When the radio waves get to an object, they will come back to the radar antenna. The radar set then changes the radio echoes (回波) into an image (图像) . And from this image we can find out the direction of the object and its distance from the set.
How can the radar show us the distance to the object then? Radio waves go out at a known speed about 300,000 kilometers every second. If the radio waves come back in 1/1,000 second, then they just have gone 300 kilometers. So the object must be half that distance, that is, 150 kilometers away.
(1) When the radio waves reach an object, they will _____.
[ ]
A.change their direction
B.return to their antenna
C.disappear completely
D.go through the object
(2) We can find out where the object is _____.
[ ]
A.by means of antenna
B.from the radio waves
C.from the picture the antenna receives
D.from the radio echo
(3) Which of the following orders shows rightly how a radar works?
[ ]
a.The operator determines the distance and the direction according to the image
b.Radio waves are sent out into space by radar
c.Antenna receives the radio echoes and changes them into an image
d.Image of object is shown on the screen
A.b-d-c-a B.b-c-d-a
C.b-c-a-d D.b-a-d-c
(4) Which of the following formula (公式) is correct to judge the distance of an object?
[ ]
D distance
T time for the radio waves to reach an object and come back
S speed of the radio waves
A.D=TS/2 B.D=TS
C.D=2T/S D.D=2S/T
(5) The reason why the object must be half the distance is that _____.
[ ]
A.the radio waves come back after they reach an object
B.the radio waves are divided into two halves
C.the radio waves will go a long way
D.the radio waves will not go a long way
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