问题 多选题

下列关于加速度的说法中正确的是(  )

A.加速度表示运动速度的增加量

B.加速度的方向总是和速度方向相同

C.加速度表示速度变化的快慢

D.加速度在数值上等于单位时间内速度的变化量

答案

A、因为a=

△v
△t
,由此可知加速度是单位时间内速度的变化,而不是增加的速度,故A错误.

B、在a=

△v
△t
中,速度的变化△v与加速度a是矢量,它们两个的方向是相同的,即加速度的方向与速度变化的方向相同,故B错误.

C、加速度表示的是速度变化的快慢,所以加速度大表示速度变化快,加速度小表示速度变化慢,故C正确.

D、a=

△v
△t
,加速度在数值上等于单位时间内速度的变化量,故D正确.

故选CD.

阅读理解

阅读理解。

     GUATEMALA CITY(Reuters)-A fish that lives in mangrove swamps (红树沼泽) across the Americas

can live out of water for months at a time, similar to how animals adapted (适应) to land millions of years

ago, a new study shows.

     The Mangrove Rivulus, a type of small killifish, lives in small pools of water in a certain type of empty

nut or even old beer cans in the mangrove swamps of Belize, the United States and Brazil. When their living

place dries up, they live on the land in logs (圆木), said Scott Taylor, a researcher at the Brevard Endangered

Lands Program in Florida.

     The fish, whose scientific name is Rivulus marmoratus, can grow as large as three inches. They group

together in logs and breathe air through their skin until they can find water again.

     The new scientific discovery came after a trip to Belize.

     "We kicked over a log and the fish just came crowding out," Taylor told Reuters in neighboring Guatemala

by telephone. He said he will make his study on the fish known to the public in an American magazine early

next year.

     In lab tests, Taylor said he found the fish can live up to 66 days out of water without eating.

     Some other fish can live out of water for a short period of time. The walking catfish found in Southeast

Asia can stay on land for hours at a time, while lungfish found in Australia, Africa and South America can

live out of water, but only in an inactive state. But no other known fish can be out of water as long as the

Mangrove Rivulus and remain active, according to Patricia Wright, a biologist at Canada's University of

Guelph.

     Further studies of the fish may tell how animals changed over time.

     "These animals live in conditions similar to those that existed millions of years ago, when animals began

making the transition (过渡) form water onto land," Wright said.

1. The Mangrove Rivulus is a type of fish that ______.

A. likes eating nuts

B. prefers living in dry places

C. is the longest living fish on earth

D. can stay alive for two months out of water

2. Who will write up a report on mangrove Rivulus?

A. Patricia Wright.

B. Researchers in Guatemala.

C. Scientists from Belize.

D. Scott Taylor.

3. According to the text, lungfish can ______.

A. breathe through its skin

B. move freely on dry land

C. remain alive out of water

D. be as active on land as in water

4. What can we say about the discovery of Mangrove Rivulus?

A. It was made quite by accident.

B. It was based on a lab test of sea life.

C. It was supported by an American magazine.

D. It was helped by Patricia Wright.

填空题