题目

Shirley was a shy girl and she seldom answered questions in our class. She was afraid of making mistakes(犯错误). Sometimes she couldn' t finish her homework because she didn't want to make any mistakes. ①Nothing changed until Mr Smith，a new teacher, came to our class.One morning, Mr Smith asked Shirley to answer some questions. After a while, she was in tears because she gave the wrong answers. At that time, Mr Smith took out a small box.②盒子里装满了橡皮。 Look, Shirley, he said, standing next to her. I have something to show you. He took out the erasers and put them on the如图所示，某货场需将质量m1＝50kg的货物（可视为质点）从高处运送至地面，为避免货物与地面发生撞击，现利用光滑倾斜轨道SP、竖直面内弧形光滑轨道PQ，使货物由倾斜轨道顶端距底端高度h=1m处无初速度滑下．两轨道相切于P, 倾斜轨道与水平面夹角为θ=600, 弧形轨道半径R＝2m，末端切线水平．地面上紧靠轨道依次排放两块完全相同的木板A、B，长度均为l＝4m，质量均为m2＝50kg，木板上表面与弧形轨道末端Q相切．货物与木板间的动摩擦因数为μ1，木板与地面间的动摩擦因数μ2＝0．12．（不考虑货物与各轨道相接处能量损失,最大静摩擦力与滑动摩擦力大小相等，取g＝10m/s2）（1）求货物到达弧形轨道始、末端时对轨道的压力．（2）若货物滑上木板A时，木板不动，而滑上木板B时，木板B开始滑动，求μ1应满足的条件．（3）若μ1＝0．30，求货物滑上木板后与木板系统所能产生的热量．