4、如图所示,甲、乙、丙三个相同的杠杆,所挂的物体受到的重力均为G,它们分别在方向如图所示的力F1、F2、F3作用下处于平衡状态,那么 A.F1=F2=F3 B

4、如图所示,甲、乙、丙三个相同的杠杆,所挂的物体受到的重力均为G,它们分别在方向如图所示的力F1、F2、F3作用下处于平衡状态,那么 A.F1=F2=F3 B

题型:不详难度:来源:
4、如图所示,甲、乙、丙三个相同的杠杆,所挂的物体受到的重力均为G,它们分别在方向如图所示的力F1、F2、F3作用下处于平衡状态,那么
A.F1=F2=F3 B.F1<F2<F3C.F1>F2>F3 D.F2>F3>F1

答案
D
解析

试题分析:三个图中,阻力和阻力臂都相同,因此只需判断动力臂的大小即可判断出拉力的大小关系.设杠杆的长为S,由图和杠杆的平衡条件知:
甲:
乙:
丙:
联立上述三式,易求得:F1<F3<F2
故选D.
点评:在相同的作用点上,不同的力的方向对应着不同的力臂,这一点需要注意.
举一反三
在图中,A装置是     滑轮(选填“定”或“动”).若用30N的拉力将旗帜匀速提升10m,则拉力所做的功为     J.
题型:不详难度:| 查看答案
如图,杆秤是一种测量工具。使用时,将待测物体挂在秤钩A上,用手拎住秤钮B或C(相当于支点),秤砣D在秤杆E上移动,当杆秤水平平衡时就可以在秤杆上读出读数。

⑴从科学的角度来看,杆秤应该是一种测量物理量      的工具。
⑵小明买5斤花生,但回家测量发现只有4.5斤。如果小贩在秤盘上做手脚,则小贩是
    _秤盘的质量;如果小贩在秤砣上做手脚,则小贩是__     ___秤砣的质量。(以上两空选填增大或减小)
题型:不详难度:| 查看答案
在学习了支点、动力、阻力概念后,甲、乙两小组继续研究动力对杠杆平衡的影响。他们将一个玩具“海宝”固定在杠杆一端的B点作为阻力,且保持阻力的大小、方向、作用点都不变,在杠杆的另一端用力使杠杆在水平位置平衡,并用测力计测出动力的大小。实验中,甲小组每次都保持动力在竖直方向、只改变动力作用点的位置,分别如图(a)所示;乙小组每次都保持动力作用点的位置不变、改变动力的方向(“动力的方向”用OA连线跟动力方向的夹角θ表示,且0°<θ≤90°),分别如图(b)所示;表一、表二是两小组同学记录的实验数据。

表一   甲小组
表二   乙小组
实验
序号
支点到动力作用点的距离 S (厘米)
动力的方向(夹角θ)
动力的大小(牛)
实验
序号
支点到动力作用点的距离 S (厘米)
动力的方向
(夹角θ)
动力的大小(牛)
1
5
90°
6
4
15
30°
4
2
10
90°
3
5
15
45°
2.8
3
15
90°
2
6
15
90°
2
(1)分析比较表一中第    列和第     列的实验数据可知:作用在杠杆一端的阻力不变时,要使杠杆平衡,在动力方向不变(θ=90°)的情况下,支点到动力作用点的距离S越大,动力越小。
(2)分析比较表二中第三列与第四列的实验数据可知:作用在杠杆一端的阻力不变时,要使杠杆平衡,在动力作用点不变的情况下, ______   _____。
(3)进一步分析图(b)所示的实验现象及表二中的实验数据,可以发现,作用在杠杆一端的阻力不变时,要使杠杆平衡,当动力作用点不变,动力的方向改变以后,动力的大小与____________    ____有关,且成反比关系。
(4)进一步综合分析表一与表二中的实验数据及相关条件,可得出的初步结论是:作用在杠杆一端的阻力不变时,影响杠杆平衡的因素是__    _和__    ___。
题型:不详难度:| 查看答案
使用滑轮组肯定不能做到的是
A.既省力又改变力的方向
B.既省力又省距离
C.省力但不能改变力的方向
D.不省力但可以改变力的方向

题型:不详难度:| 查看答案
如图所示,杠杆处于平衡状态,则F的力臂是
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               
A.OAB.OCC.ODD.FD

题型:不详难度:| 查看答案
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