问题 选择题

放在粗糙水平面上的物体,第一次受水平力F1的作用,第二次受与水平面成θ角的斜向上的拉力F2的作用,这两次使物体以相同的加速度发生相同的位移,在两次情况下,力F1和F2对物体做功分别为W1和W2,则(  )

A.F1>F2,W1>W2

B.无法比较F1和F2的大小,但W1>W2

C.无法比较W1和W2的大小,但F1>F2

D.力和功都无法比较大小

答案

由题,两次物体的初速度、加速度、位移都相同,则物体获得的速度相同.

根据动能定理得:

第一次:W1-μmgx=

1
2
mv2

第二次:W2-μ(mg-F2sinθ)x=

1
2
mv2

所以W1>W2

根据牛顿第二定律得:

a=

F1-μmg
m
=
F1
m
-μg

a=

F2cosθ-μ(mg-F2sinθ)
m
=
F2cosθ
m
-μg+
μF2sinθ
m

所以有:

F1
m
=
F2cosθ
m
-
μF2sinθ
m

F1=F2(cosθ-μsinθ)

由于不知道θ的大小,所以无法比较F1和F2的大小

故选B

单项选择题

After winning a big game, athletes are often asked how they feel. Usually athletes say how (1) it is to win this big game. On the contrary, the fact is that losing a big game, or in my case, all the games, can be even harder.
From the spectators’ point of view, last year’s basketball (2) for my high school team was nothing (3) of an embarrassment. And (4) 0 and 20 is certainly nothing to be proud of, that season had a bigger impact on me than any other. As a team captain, I knew it probably wouldn’t be the (5) year, but did I ever think we would lose every game Of course not. Since six of our top players had graduated, it was clear that we were a (n) (6) team who would struggle. The struggle began earlier than expected, (7) , as our team center was suspended (禁赛) for the season, and two key members decided to (8) for some reason. At this point, dropping out probably passed through every player’s mind, but in the end, we all stuck if out, (9) to work still harder.
Then there came a time when even our (10) and given up on us. Personally, I felt like it was (11) worthwhile giving my all. I thought, if even the coach doesn’t believe in us, why should I But just as my hope began to (12) , a teammate called a meeting. He said," Nobody thinks we are going to win, but, as teammates and friends, we (13) it to each other to go all out every game. "
It was that moment that (14) me how to be a leader. It hit me that I certainly didn’t (15) the role. As a leader you can never quit (16) the team who looks up to you. It is one (17) to be named captain, feel great and go through the motions, but it is quite another to be a real captain and make sure everyone realizes their full (18) all the time.
I am sure it is great to go through without losing and bringing home awards. But in all honesty, I (19) the fact that my team lost every game last year. It may not help me to become a better basketball player, but it already has made me a better leader, and (20)

A.act
B.deserve
C.play
D.take

单项选择题