题目

Fewerchildren spend time playing o, instead, they stay indoors playingcomputer games. 答案：【1】outdoors用如图所示的滑轮组将重G=12N的物体匀速提升20cm，所用的拉力F=5N，所做的有用功为________J．总功为________J，机械效率为________．（忽略摩擦阻力）．