A:Thank you
B:No, it's nothing.
The teacher asked us to ____ ____ some chalk.
如图所示,人工用300N的力将重500N的物体匀速提升2m,共用了10秒,若在此过程中(忽略绳重和摩擦),求:
(1)绳子自由端移动的距离;
(2)动滑轮的重;
(3)工人做工的功率;
(4)滑轮组的机械效率.
如果2a-12=1-2a,则( )
(1.25+12.5)×0.8=( )
计算:(﹣2)×12÷-13x3.