题目

Take a look around your school and you’ll find all kinds of great inventions developed by some really amazing minds. Here are just a few of the people who have helped make schools great.James PillansThe blackboard and colored chalks (粉笔) This headmaster in Edinburgh, Scotland, was the first to use a blackboard and colored chalks in the 1800s. He used them to teach geography.Anna BreadinThe school deskThis designer designed the first school desk. Her design included storage (储存)space under the table area for books and a folding seat.John Lee LoveThe pencil sharpener (卷笔刀)The Ame把物体竖直地挂在劲度系数为1000N/m的弹簧下端，弹簧伸长2cm。如果把物体放在动摩擦因数为0.30的水平地面上，用同一根弹簧沿水平方向拉物体。当物体产生2m/s2的加速度时，弹簧伸长        cm。