题目

如图所示，在倾角α=60°的斜面上放一个质量m的物体，用k=100N/m的轻弹簧平行斜面吊着。发现物体放在PQ间任何位置都处于静止状态，测得AP=22cm，AQ=8cm，则物体与斜面间的最大静摩擦力等于多少？ 答案：【答案】Fm=7N【解析】【试题分析】由题，当物体位于P点时，将要向上运动，静摩擦力沿斜面向下达到最大．当物体位于Q点时，物体将向下运动，静摩擦力沿斜面向上达到最大．根据平衡条件和胡克定律列方程求解最大静摩擦力．设弹簧的原长为l0．在P点有：k（Lap-l0）=mgsin60°+Fmax ①在Q点有：k（Laq-l0My friend, Victor, is a zoo keeper in the National Zoo. His work is very interesting but it is hard. Sometimes it is dangerous, too. Every morning the zoo keepers have to get up at five to get food ready for the animals.Each kind of animal eats special food. For example, the lions must eat meat; the giraffes will eat grass and leaves; and the parrots must have fruit and nuts. Sometimes some animals won’t let the others eat their food. If this happens, each of them must be fed separately(分离的). Every morning the zoo keepers must clean each cage. This can be dangerous, especially(尤其) with lions and tigers. To make the work easier, the keepers always lock the animals in the sleeping compartments(隔间) while they are washing the cages. When the animals are sick, the zoo keepers must look after them. Each sick animal is kept in a separate cage and given medicine. Sometimes the zoo workers have to inject(注射) the sick animals, even the lions.All the keepers live near the zoo. Each of them has a small house. There he lives with his family. They do not have holidays on Sundays and public(公众的) holidays. Each of them has a day off during the week. Each keeper has a holiday on a different day from that of the other keepers.The most important job of the keepers is to look after the safety of the visitors. They must stop those people from doing dangerous things. 小题1:What kind of food may be for the animals like lions, tigers and bears?A．Nuts.B．Fruit.C．Meat.D．Grass and leaves.小题2:Some animals must eat food separately because                  .A．they eat special foodB．they often quarrel with each other while eatingC．they always make troubleD．they won’t let the others eat their food小题3:All the zoo keepers must work                  .A．seven days a weekB．on SaturdaysC．on special festivalsD．on weekdays小题4:Which of the following is WRONG?A．The zoo keepers have to get up at five to get the food ready for all the animals.B．The zoo keepers must clean all the cages every morning.C．The zoo keepers must look after the animals when they are sick.D．The zoo keepers live in a small house together near the zoo.小题5:What is the best title(标题) for the passage?A．The ZooB．The Zoo KeepersC．An Interesting JobD．A Dangerous Job