问题 问答题

如图所示,A、B两物体相距x=7m,物体A在水平拉力和摩擦力作用下,正以v1=4m/s的速度向右匀速运动.而物体B此时的速度v2=10m/s,由于摩擦力作用向右匀减速运动,加速度a=-2m/s2,求:

(1)B物体还能运动多少时间.

(2)物体A追上B所用的时间.

答案

(1)物体B做匀减速直线运动,设速度减为零的时间为t1

由速度公式:vB=v2+at,则有:t1=

0-v2
a
=5s

所以B物体还能运动5s时间.

(2)在t1=5s的时间内,做匀减速的物体B位移为:

xB1=v2t1+

1
2
a
t21
=10×5-
1
2
×2×52m
=25m,

做匀速运动物体A的位移为:

xA1=v1t=4×5m=20m,

由于xA1<xB1+x,故在B物体停止之前,物体A未追上物体B;

5s后,物体B静止不动,故物体A追上物体B的总时间为:

t=

xA1+S
vA
=
25+7
4
s
=8s

答:(1)B物体还能运动5秒时间.

(2)物体A追上B所用的时间为8秒.

完形填空

My mother used to ask me what the most important part of the body was. Through the years I would guess that what I thought was the correct answer.

When I was young, I thought sound was very important to us, so I said, “My ears, Mum.” She said, “No. Many people are deaf. But you keep thinking about it and I will ask you again soon.”

Several years passed before she asked me again. Since making my first try, I had contemplated (仔细考虑)the correct answer. So this time I told her, “Mum, sight is very important to everybody, so it must be our eyes.” She looked at me and told me, “You are learning fast, but the answer is not right because there are many people who are blind.”

Over the years, Mother kept asking me the same question and her answer was always, “No, but you are getting cleverer every year, my child.”

Then last year, my grandpa died. Everybody was hurt. Everybody was crying. My mum looked at me when it was our turn to say our final goodbye to Grandpa. She asked me, “Do you know the most important body part yet, my dear?”

I was surprised when she asked me this now. I always thought this was a game between her and me. She saw the confusion(迷惑)on my face and told me, “This question is very important. It shows how you have really lived your life.” I saw her eyes filled with tears(眼泪). She said, “My dear, the most important body part is your shoulder.”

I asked, “Is it because it holds up your head?”

She answered, “No, it is because it can hold the head of a friend or loved one when they cry. Everybody needs a shoulder to cry on sometimes in life, my dear. I only hope that you have enough love and friends that you will have a shoulder to cry on when you need it.”

Then and there I knew the most important part of the body is not a selfish one. It is the care to the pain(痛苦) of others.

小题1: What did his mother use to ask him?________________________________________________

小题2: Did he think sound was very important to them when he was young? ______________________

小题3: When did his grandpa die? ________________________________________________________

小题4:Why is shoulder the most important part of the body? __________________________________

小题5:What did his mother want to tell him in the end? ______________________________________

多项选择题