题干

做“探究力的平行四边形定则”的实验,在水平放置的木板上铺一张白纸,把橡皮条的一端固定在木板的A点,橡皮条的另一端拴上两细绳套,如图所示,两个弹簧测力计分别钩住细绳套,互成角度拉橡皮条使之伸长,到达某一位置O时需记下____、____,描下____,再用一个弹簧测力计钩住细绳套把橡皮条拉长,使结点到达位置____,再记下____.

上一题 下一题 0.0难度 选择题 更新时间:2017-03-03 11:44:40

答案(点此获取答案解析)

两弹簧测力计的读数,两细绳的方向,结点位置O,O点,弹簧测力计的读数和细绳的方向

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!