第二节书面表达Dear peter'm writing to invite you to attend a lecture about exercise and health inour schooAs you know there are many students who are overweight or short-sightedin our school. They devot
17.C由题意可知,在θ-30°时物块受到的静摩擦力达到最大,方向竖直向下。根据平衡条件有Fcb=mg+m,解得F=433m力F开始缓慢地铃时擦力减√3小,当摩擦力为零时,设F的方向与竖直方向夹入下c0=mg,0sa=4a>30°,0在30°~a时,摩擦力的方向向不,物块处于静止状态,当0在a~90°时,摩擦力的方向向上,最大静摩分Fn=Fsin,因 sin 6> sina,