问题 计算题

跳伞运动员做低空跳伞表演,他在离地面323 m高处,由静止开始在竖直方向做自由落体运动。一段时间后,立即打开降落伞,以12.5 m/s2的平均加速度匀减速下降,为了运动员的安全,要求运动员落地速度最大不得超过5 m/s。(g=10m/s2

(1)求运动员展开伞时,离地面高度至少为多少?着地时相当于从多高处自由落下?

(2)求运动员在空中的最短时间是多少?

答案

解:设下落时的高度为,展开伞时的速度,此时距地面的高度为,落地时速度,匀减速下降时的加速度为

(1)

代入数据可得:

(2)自由落体过程:

匀减速过程:

总时间:

判断题
阅读理解

My mother through her fine example instilled(慢慢灌输) within me the desire to help others. She taught me to notice those whom others ignore.

The railroad tracks were within a mile of our house. Beggars frequently would knock at the doors in our neighborhood in summer asking for food. They were often turned away. Others simply ignored the knocks.

My mum always went to the door smiling. She would invite them to stay and have something to eat. She would have me open up the card table and carry a chair out onto the covered side porch(走廊). There was a nice view into the flower gardens, and it was cool even on the hottest days.

She would bring a red and white tablecloth and set the table as carefully as she did for any company. She would pour their coffee and pile their plates with food and dessert. Usually she sent a packed lunch with them when they were ready to go, too. Sometimes she would take the newspapers to those who asked about work.

She listened to those who wanted to talk. She never asked what would lead them to be in this difficult situation. It did not matter. She saw only the need. Some thought she did too much for them. She would say, “In the depression(萧条时期) when my father was searching around for any work to feed us, thoughtful people fed him. I am just repaying their kindness. I cannot offer them money, but I can see they do not leave hungry and discouraged,” My mum was quite a lady.

36. The writer’s house was frequently visited by beggars because___________.

A. they seemed to be very rich        B. his mother was known to be generous

C. they lived near a railway station     D. there was a railway near their house

37. How did the writer’s mother treat beggars?

A. She was on guard against them.     B. She looked down upon them.

C. She respected them equally.        D. She took pride in them.

38. The writer’s mother sometimes took out newspapers_________.

A. for the beggars to kill the time       B. to teach the beggars how to read

C. to help the beggars who asked about work  

D. to help the beggars to find their families

39. Which of the following can make an explanation for the mother’s attitude to beggars?

A. Her father was once helped by others.

B. Her father liked to help beggars.

C. She was once a beggar herself.

D. Her mother taught her to help beggars.