问题 选择题

如图是物体做直线运动的速度-时间图象,下列说法正确的是(  )

A.从t=2s到t=4s 物体处于静止状态

B.从t=0s到t=2s 物体加速度的大小为1m/s2

C.从t=4s到t=6s物体做反方向的匀减速直线运动

D.从t=0s 到t=6s 物体的位移为零

答案

A、由图知,t=2s到t=4s内物体的速度为2m/s,表示物体做匀速直线运动.故A错误.

B、从t=0s到t=2s 物体加速度的大小为a=

△v
△t
=
2
2
m/s2
=1m/s2.故B正确.

C、从t=4s到t=6s物体做正方向的匀减速直线运动.故C错误.

D、从t=0s 到t=6s 物体的位移等于图象梯形的“面积”,显然不等于零.故D错误.

故选B

单项选择题

With only about 1,000 pandas left in the world, China is desperately trying to clone the animal and save the endangered species. That’s a move similar to what a Texas A&M University researcher has been undertaking for the past five years in a project called "Noah’s Ark".

Dr. Duane Kraemer, a professor in Texas A&M’s College of Veterinary Medicine and a pioneer in embryo transfer work and related procedures, says he salutes the Chinese effort and "I wish them all the best success possible. It’s a worthwhile project, certainly not an easy one, and it’s very much like what we’re attempting here at Texas A&M—to save animals from extinction. "

Noah’s Ark is aimed at collecting eggs, embryos, semen and DNA of endangered animals and storing them in liquid nitrogen. If certain species should become extinct, Kraemer says there would be enough of the basic building blocks to reintroduce the species in the future.

It is estimated that as many as 2,000 species of mammals, birds and reptiles will become extinct over the next 100 years. The panda, native only to China, is in danger of becoming extinct in the next 25 years.

This week, Chinese scientists said they grew an embryo by introducing cells from a dead female panda into the egg cells of a Japanese white rabbit. They are now trying to implant the embryo into a host animal.

The entire procedure could take from three to five years to complete.

"The nuclear transfer of one species to another is not easy, and the lack of available panda eggs could be a major problem," Kraemer believes. "They will probably have to do several hundred transfers to result in one pregnancy. It takes a long time and it’s difficult, but this could be groundbreaking science if it works. They are certainly not putting any live pandas at risk, so it is worth the effort , "adds Kraemer, who is one of the leaders of the Missyplicity Project at Texas A&M, the first-ever attempt at cloning a dog.

"They are trying to do something that’s never been done, and this is very similar to our work in Noah’s Ark. We’re both trying to save animals that face extinction. I certainly applaud their effort and there’s a lot we can learn from what they are attempting to do. It’s a research that is very much needed.

The word "groundbreaking"(Paragraph 7) can be interpreted as()

A. essentially new

B. pioneering

C. evolutionary

D. epoch-making

单项选择题