问题 问答题

一辆电动玩具车A点由静止开始以加速度a1在地面上做匀加速直线运动,经过一段时间后立即做匀减速运动,最后停在C点.每隔0.2秒钟通过速度传感器测量物体的瞬时速度,下表给出了部分测量数据.求:

t ( s )0.00.20.41.21.4
v( m/s )0.01.02.01.10.7
(1)小车加速运动的加速度a1的大小

(2)AC的距离

(3)t=0.6s 时的瞬时速度 v.

答案

( 1)设加速过程加速度为a1,减速过程加速度为a2

由加速度的定义式:a1=

△v
△t
=5m/s2

    加速度为a2的大小 a2=

△v
△t
=2m/s2

 (2)设经t秒车达到最大速度为vm

从t时刻到1.4s列速度时间关系式:

v=vm-a2(1.4-t)

解得:t=0.5s; vm=2.5m/s

所以总位移:xAC=

v2m
2a1
+
v2m
2a2
=2.19m

(3)由速度时间关系式得:

v=vm-a2(0.6-0.5)=2.3m/s 

答:小车加速运动的加速度a1的大小5m/s2;AC的距离为2.19m,t=0.6s 时的瞬时速度 2.3m/s

单项选择题

Randy Kraus was paralyzed. His left side was useless. But his right hand was (1) enough to lift a bucket to his forehead. Once, he’d been a police officer and owned a private-eye agency. Once, he’d been p and able. Now, he felt he was nothing.

His (2) started with Parkinson’s disease, but it didn’t (3) there. In July 2002, the 60 year old Kraus went into the hospital for an operation to control the shaking. (4) , during the operation, he had a stroke. He was paralyzed. The (5) man, who loved golf, could think, but couldn’t (6) . Kraus wanted the doctors to (7) it to him straight. "You may never walk again, " they told him. "Maybe you won’t (8) be able to talk. "

Once at home, he (9) he couldn’t lift a fork or take a drink by himself. Physical treatment was so painful and slow. What did he have to (10) for So now Kraus held the gun against his head. (11) the cold metal on his skin, he began to consider not his pain, but the pain he (12) cause his wife, daughters and grandchildren. He didn’t pull the trigger.

"You are where you are", his exercise physiologist, Andrew Garud told him. "The pace would be slow; the pain would be (13) . But as long as you are (14) , you have the ability to get better. " After three months of (15) with Garud, Kraus wanted to see if he could (16) . He could. Then he took three steps, sat down and cried like a baby. One step (17) to another. Next he managed a short walk. It was the hardest (18) of Kraus’s life.

Garud kept saying he could (19) more. Now, Kraus can brush his teeth, shave himself and get around the house with a walker. Little success only the paralyzed can (20) understand.

16().

A.walk

B.stand

C.shake

D.fall

判断题