问题 选择题

一质点自原点开始在x轴上运动,初速度v0>0,加速度a>0。当a值减小至零的过程中,则质点的

A.速度不断减小,位移逐渐增大

B.速度和位移都只能逐渐增大到某个定值

C.速度逐渐增大,位移趋近于某个定值

D.速度趋近于某个定值,位移逐渐增大

答案

答案:D

题目分析:一质点做直线运动,加速度方向与速度方向相同,表明物体做加速运动,加速度的大小开始减小,但方向不变,由于加速度还是与速度同方向,故物体还是加速,加 速度的大小开始减小,表明速度增加的慢了,但速度还是在增加,速度方向不变,位移还在增加,当加速度趋向于0时,速度趋近某个定值做匀速运动,位移一直增 大.,D正确。

点评:本题关键是加速度的大小和方向的物理意义要理解清楚,加速度与速度同方向表示加速,反向表示减速;加速度的大小表示速度变化的快慢,与速度的大小无关

解答题
阅读理解

Can trees talk? Yes — but not in words. Scientists have reason to believe that trees do

communicate with each other. Not long ago, researchers learned some surprising things. First a willow tree attacked in the woods by caterpillars(毛毛虫)changed the chemistry of its leaves and made them taste so terrible that they got tired of the leaves and stopped eating them. Then even more astonishing, the tree sent out a special vapor—a signal causing its neighbors to change the chemistry of their own leaves and make them less tasty.

Communication, of course, does not need to be in words. We can talk each other by smiling, raising our shoulders and moving our hands. We know that birds and animals use a whole vocabulary of songs, sounds, and movements. Bees dance their signals, flying in certain patterns that tell other bees where to find nectar for honey. So why shouldn’t trees have ways of sending messages?

1.It can be inferred from the passage that caterpillars do not feed on leaves that          .

A.have an unpleasant taste

B.are lying on the ground

C.have an unfamiliar shape

D.bees don’t like

2.According to the passage, the willow tree was able to communicate with each other by       .

A.waving its branches              B.giving off a special vapor

C.dropping its leaves               D.changing the color of its trunk

3.According to the passage, bees communicate by        .

A.touching one another             B.smiling one another

C.making special movement         D.making unusual sound

4.The author believes that the incident described in the passage         .

A.cannot be taken seriously          B.seems completely reasonable

C.should no longer be permitted      D.must be checked more thorough