问题 填空题

一根细绳,一端固定于光滑水平面上的O点,另一端系一质量为2千克的小球,使小球在水平面上绕O点作半径为1米的匀速圆周运动,运动速度为5米/秒,则小球的加速度大小为______ m/s2,细绳的拉力大小为______N.

答案

已知圆周运动的半径r=1m,线速度v=5m/s,则小球的加速度大小为an=

v2
r
=25m/s2

小球在光滑水平面上做匀速圆周运动,由细绳的拉力提供小球的向心力,根据牛顿第二定律得

细绳的拉力F=man=50N

故答案为:25;50

阅读理解

Have you wondered why different animals or pests have their particular colors? Colors in them seem to be used mainly to protect themselves.

Birds, especially seagulls are very fond of locusts, but birds can’t easily catch locusts because locusts change their colors together with the change of the color of crops. When crops are green, locusts look green. But when crops are ripe, locusts take on exactly the same brown color as crops have. Some other pests with different colors from plants are usually easily found and eaten by their enemies. So they have to hide themselves in terror for lives and appear only at night.

If you study the animal life in any part of the world, you will find the main use of coloring is to protect themselves, bears, wolves and other beasts move quietly through forests. They are usually invisible to the eyes of hunters, because they have the color much like the barks(树皮) of trees.

An even more strange act remains to be noticed. A kind of fish living in seas can send out a kind of very black liquid when it faces danger. While the liquid is over, its enemies cannot find it, and it quickly swims away. Thus, it has existed up to now though it is not powerful at all.

小题1:This passage mainly talks about ______.

A.the change of color in locusts

B.the protective coloration of animals and pests

C.how a certain sea fish protects itself

D.animals or pests can dye themselves different colors小题2:Locusts are ______ but they aren’t easily wiped out(消灭)by their enemies because _______.

A.animals; they are powerful enough

B.beasts; they are dangerous to their enemies

C.pests; they take on the same colors as crops

D.birds; they fly very fast小题3:What does the underlined word “invisible” in Paragraph 3 mean?

A.can’t be seen

B.can be seen

C.can’t move

D.can’t change

判断题