题目

Research has long backed the therapeutic (治疗的) value of diary-keeping for teenage girls and boys. But according to a new study, when teenagers detail their sadness onto a blog, the therapeutic value is even greater. Blogging, it seems, can be good for you.     The study, published in the journal Psychological Services and conducted by Meyran Boniel-Nissim and Azy Barak, psychology professors at the University of Haifa, Israel, found the contact with an online community through a blog made it more effective in relieving (解除) the writer’s social anxiety than a private dia如图所示，在拉力作用下，物体在水平桌面上以速度向右匀速运动，绳子自由端内移动了．已知的重力为，桌面对的摩擦力为，机械效率为（滑轮及绳重不计）．求：（）使用滑轮组做的有用功和总功分别为多少．（）拉力的功率为多少． （）3600J;（）4500J；（3） 9000W． 【解析】有用功：W有＝fs＝600N×6m＝3600J；由η＝可得,W总＝＝4500J；则拉力的功率：P＝＝900W.