如图（a）所示，“ ”型木块放在光滑水平地面上，木块的水平表面AB粗糙，与水平面夹角θ＝37°的表面BC光滑。木块右侧与竖直墙壁之间连接着一个力传感器，当力传感器受压时，其示数为正值；当力传感器被拉时，其示数为负值。一个可视为质点的滑块从C点由静止开始下滑，运动过程中，传感器记录到的力和时间的关系如图（b）所示。（已知sin37°＝0.6，cos37°＝0.8，取g＝10m/s²。）求：

    “Early to bed, early to rise（起身）makes people healthy, wealthy and wise（健康，富有，聪明）.” This is an old English saying. Children should have ten hours' sleep every night, or they can't do their work very well. They will not be wise. The body needs exercise. Walking, running, swimming and playing ball games are all exercise. Exercise keeps the body strong.