问题
实验题
(5分)在”测定动滑轮机械效率”的实验中，小明用如图所示的动滑轮提升钩码，改变钩码的数量，正确操作，实验数据如下：
(1)实验时，用手__ 拉动弹簧测力计，使挂在动滑轮下的钩码缓缓上升．
(2)第①次实验时所做的有用功为__ _J，总功为___ __J，动滑轮的机械效率为__ _．
(3)由表中实验数据分析可知，同一动滑轮，所提升物重增大，机械效率将__ (选填”增大”、”减小”或”不变”)
答案
（1）匀速；（2）0.2；0.28；71.4%；（3）增大．
题目分析：：（1）在实验时，应用手匀速拉动弹簧测力计，使挂在动滑轮下的钩码缓缓上升．
（2）有用功：W有=Gh=1N×0.2m=0.2J，
总功：W总=FS=0.7×0.4m=0.28J，
动滑轮的机械：η=(W有/ W总 )×100%="(0.2J/" 0.28J) ×100%≈71.4%
（4）根据表格中数据可知，动滑轮效率：
η2=(W有 /W总 )×100%=(G2h2 /F2S2 )×100%=((2N×0.2m)/( 1.2N×0.4m) )×100%≈83.3%
η3=(W有 /W总) ×100%=(G3h3/ F3S3 )×100%=((3N×0.2m)/( 1.7N×0.4m)) ×100%≈88.2%
根据三次的机械效率及物重的变化可得同一动滑轮，所提升物重增大，机械效率将增大．
une 28th, 1902. Both his parents enjoyed singing and playing the piano. His grandparents loved opera and took their grandson to many productions. Richard attended many Broadway (百老汇) shows as a child. Richard began playing the piano by the age of three. At the age of fifteen, he decided that he would work in the musical theater. In the same year, he wrote the music for a stage show presented by a local group of young people. Then, he wrote music for a production of the students at Columbia University.