题目

阅读理解The food,by its very nature, may be harmful for us; it may change while stored; orvarious chemicals may be added. It may grow in soil contaminated (污染) with lead or arsenic, or it may be wateredhigh in arsenic. Grains, fruit and vegetables are sprayed with chemicals, andthese may combine to be far more harmful than one chemical alone. Fish may becaught in areas of sea contaminated with mercury (汞)and arsenic. Meat may contain antibiotic residues. Furthermore,there are additives we are concerned about. What are added to food to maintainor improve the safety, freshness, taste, （13分）如图所示，一轻绳绕过无摩擦的两个轻质小定滑轮O1、O2和质量mB=m的小球连接，另一端与套在光滑直杆上质量mA=m的小物块连接，已知直杆两端固定，与两定滑轮在同一竖直平面内，与水平面的夹角θ=60°，直杆上C点与两定滑轮均在同一高度，C点到定滑轮O1的距离为L，重力加速度为g，设直杆足够长，小球运动过程中不会与其他物体相碰．最初用手沿光滑直杆方向拉住小物块在如图所示的位置静止，现将小物块从C点由静止释放，试求：（1）手沿杆的拉力要多大才能使小滑块在如图位置静止？；（2）小物块能下滑的最大距离；（3）小物块在下滑距离为L时的速度大小．（结果均可用根号表示）