如图所示，有一条渡船正在渡河，河宽为300 m，渡船在静水中的速度是v1＝3 m/s，水的流速是v2＝1 m/s，求下列条件渡船过河的时间．
(1)以最短的时间过河；
(2)以最短的位移过河．
(1)100 s (2)106.1 s
(1)当渡船的船头方向垂直于河岸时，即船在静水中的速度v1方向垂直于河岸时，过河时间最短，则
tmin＝
＝
s＝100 s.
(2)因为v1＝3 m/s，v2＝1 m/s，则v1＞v2，故当渡船合速度方向垂直河岸时，过河位移最短，此时合速度如图所示，则渡河时间为
t＝
＝
＝
s＝
75
s≈106.1 s.
Directions: There are 10 blanks in the following passage. For each numbered blank, there are 4 choices marked A, B, C and D.
| It is difficult to imagine what life would be like without memory. The meanings of thousands of everyday perceptions, the bases (51) the decisions we make, and the roots of our habits and skills are to be (52) in our past experiences, which are brought into the present (53) memory. Memory can be defined as the capacity to keep (54) available for later use. It includes not only "remembering" things like arithmetic or historical facts, but also any change in the way an animal typically behaves. Memory is (55) when a rat gives up eating grain because he has sniffed something suspicious in the grain pile. Memory is also involved when a six-year-old child learns to swing a baseball bat. Memory (56) not only in humans and animals but also in some physical objects and machines. Computers, for example, contain devices for storing data for later use. It is interesting to compare the memory-storage capacity of a computer (57) that of a human being. The in stant-access memory of a large computer may hold up to 100 000 "words" —ready for (58) use. An average American teenager probably recognizes the meanings of about 100 000 words of English. However, this is but a fraction of the total (59) of information which the teenager has stored. Consider, for example, the number of facts and places that the teenager can recognize on sight. The use of words is the basis of the advanced problem-solving intelligence of human beings. A large part of a person’s memory is in terms of words and (60) of words. |
A．kept
B．found
C．sought
D．stored