问题 选择题

从合肥开往南京、上海的动车组开始运行,动车组的最大优点是列车的运行速度快.提高列车运行速度的一个关键技术问题是提高机车发动机的功率.动车组机车的额定功率是普通机车的27倍,已知匀速运动时,列车所受阻力与速度的平方成正比,即Ff=kv2,则动车组运行的最大速度是普通列车的(  )

A.1倍

B.2倍

C.3倍

D.9倍

答案

当牵引力等于阻力时,速度达到最大值,vmax=

P
f

而Ff=kv2,所以vmax=

3
P
k

所以当动车组机车的额定功率是普通机车的27倍时,动车组运行的最大速度是普通列车的3倍,故C正确.

故选C

单项选择题
阅读理解

阅读理解。

     To wash yourself, you take a bath (洗澡) or a shower. Animals need to keep clean, too. How do animals

clean themselves?

     Cats lick (舔) themselves to keep their fur clean. Have you ever watched a pet cat clean itself? Elephants

take baths much as you do. But instead of jumping into the tub (浴缸), they walk into a river or lake.

     Pigs also like to roll around in water. This keeps them clean and cool. If there is no clean water, they will

roll in mud (泥) on a hot day. They do this to cool off. Pigs do not really like mud. They are happier in a nice,

clean pond (水池).

     Bats have a funny way to keep clean. They lick their thumbs to clean their ears!

     Guess how polar bears clean themselves. They use snow, of course!

     Some animals take baths in dust instead of water! The wombat (毛鼻袋熊) is an Australian animal with

lots of fur. To get clean, it lies down. Then it covers itself with sand! Birds clean themselves in many ways.

Sometimes they wash in water. That's why some people put birdbaths in their yard. At other times birds take

dust baths, just like wombats.

     Birds also use their beaks (喙) to keep their feathers clean. They use their beaks the way you use a comb.

     Sometimes a bird has itchy bugs (发痒的虫子) in its feathers. Birds may use ants to help them clean off

the bugs. A bird will lie down on an ant nest. Then the ants will crawl on the bird. The ants make a kind of

bug spray (喷雾) on the feathers;. Then the itchy bugs die!

     Birds take baths in something else besides dust and ants! Some birds take "smoke baths" They sit on

chimneys. They wave their wings in the smoke!

1. This passage mainly tells us _____. [ ]

A. why some people put birdbaths in their yard

B. why birds allow ants to crawl on their body

C. how animals cool off on hot days

D. how some animals keep clean

2. What is a pig's favorite way to get clean? [ ]

A. Licking itself.

B. Lying down on ants.

C. Rolling in nice, thick mud.

D. Taking a bath in clean water.

3. Which of the following animals use dust to clean themselves? [ ]

A. Cats.

B. Bats.

C. Wombats.

D. Ants.