题干

降落伞在下落一定时间后的运动是匀速的.若在有风的天气,风可以使跳伞员以3m/s 的速度沿水平方向向东运动,且无风时跳伞员竖直匀速下落的速度为4m/s,有风时跳伞员落地时速度的大小为(   )

A:3m/s

B:4m/s

C:5m/s

D:7m/s

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答案(点此获取答案解析)

C

同类题1

任务型阅读

A. A general sleep rule.

B. The importance of sleep.

C. A funny sleeping example.

D. Different levels of sleep.

E. The time we need for sleep.

F. Different states of sleep.

____

    Sleep, as we know, is important to us because it helps restore tired organs and tissues in our body. But how much sleep do we actually need?

____

    For most of us, eight hours seems to be about the right amount. Yet we know that there are a great many people who get along perfectly with less sleep and some who may even need more. A great deal depends on the way we live. But a good general rule to follow is to sleep as long as we have to in order to feel happy and be able to work at our best when we are awake.

____

    There are actually different levels of sleep. There is a deep sleep and a shallow sleep. In a shallow sleep our body does not get the same kind of rest as it gets in a deep sleep, so that after eight hours of a shallow sleep we may still feel tired. But a short deep sleep can be very restful.

____

    Alexander the Great was able to get a deep sleep whenever he needed it. Once, during the night before an important battle, he remained awake longer than anyone else. Then he wrapped himself in a cloak and lay down on the earth. He slept so deeply that his generals had to wake him three times to give command to attack!

____

    Normally when we go to sleep, our “ sleep center” blocks off nerves so that both our brain and our body go to sleep. One prevents us form wanting to do anything and the other makes our internal organs and limbs go to sleep. But someone will fall asleep (brain sleep) and keep on marching, because his body is not asleep!

同类题4

某生物兴趣小组开展探究实验,课题是“培养液中酵母菌种群数量与时间的变化关系”.

实验材料、用具:菌种和无菌培养液、试管、血球计数板 (计数室为1mm×1mm方格,由400个小格组成)、滴管、无菌水、台盼蓝染液(对活细胞无染色效果)、显微镜等.

酵母菌的显微计数方法:

①血球计数板:是带有微小方格刻度的玻璃片,用于在显微镜下对微生物的计数.

②将含有酵母菌的培养液滴在计数板上,计数一个方格内的酵母菌数量,再以此为根据,估算试管中的酵母菌总数.连续观察7d,并记录每天的数值.

根据以上叙述回答下列问题:

(1)本实验没有另设对照实验,原因是____ .该实验____(填“需要”或“不需要”) 重复实验,原因是____ .

(2)在吸取培养液计数前,要轻轻震荡几次试管,原因是____ .如果一个小方格内酵母菌过多,难以数清,应当采取的措施是____ 

(3)某同学第5天在使用血球计数板计数时做法如下:

①振荡摇匀试管,取1mL培养液并适当稀释(稀释样液的无菌水中加入了几滴台盼蓝染液).

②先将____ 放在计数室上,用吸管吸取稀释后的培养液滴于其边缘,让培养液自行渗入,多余培养液用滤纸(吸水纸)吸去,制作好临时装片.

③显微镜下观察计数:在观察计数时只计____ (填“被”或“不被”)染成蓝色的酵母菌.在用血球计数板对某一稀释10倍样品进行计数时,发现在一个小方格内(盖玻片下的培养液厚度为0.1mm)酵母菌平均数为5,据此估算10mL培养液中有酵母菌____ 个.