题目

阅读理解 Mrs Green’s telephone number was three-four-six-three, and the number of the cinema near her house was three-four-six-four, so people often made a mistake and telephoned her when they wanted the cinema.    One evening the telephone rang and Mrs Green answered it. A tired man said, “At what time does your last film begin?”    “I’m sorry”, she said Mrs Green, “but you have the wrong number. T his is not the cinema.”    “Oh, it began twenty minutes ago,” said the man.” That’s too bad. Goodbye.”    Mrs Green was very surprised ,so she told her h 如图所示，图线表示作用在做直线运动的物体上的合外力与物体运动距离的对应关系，物体开始时处于静止状态，则当物体在外力的作用下，运动30 m的过程中，合外力对物体做的功为________J．