题目

如图是建筑工地常用的一种“深穴打夯机”，电动机带动两个滚轮匀速转动将夯杆从深坑提上来，当夯杆底端刚到达坑口时，两个滚轮突然彼此分开，将夯杆释放，随后，夯杆只在重力作用下运动，落回深坑，夯实坑底，且不反弹．然后两个滚轮再次压紧，夯杆再次被提到坑口，如此周而复始．己知两个滚轮边缘的速度恒为v=4m/s，滚轮对夯杆的正压力FN=2×104N，滚轮与夯杆间的动摩擦因数μ=0.3，夯杆质量m=1×l03kg ，坑深h=6.4m，假定在打夯的过程中坑的深Justin’s bedroom was so full of flat bicycle tires, bent tennis rackets, deflated basketballs, and games with missing pieces that you could hardly get in the door.  His parents pleaded with him to clean out his room.“What use is a fish tank with a hole in the bottom?” his father asked.  But Justin simply smiled and repeated his motto, “Never throw anything out, you never know when it might come in handy.”When Justin was away from home, he always carried his blue backpack — a smaller bag of his bedroom — a place to store the many objects that he collected.  It was so worn and stretched that it hardly resembled a backpack anymore.Justin had earned a reputation for figuring things out and getting people out of otherwise hopeless situations.  Many of his classmates and neighbors sought him out when they needed help with a problem.  On the first day of school, his friend Kenny, came looking for Justin.“Do you think you have something in your backpack that could help me remember my locker combination (箱子密码)?”  he asked.  “I lost the piece of paper it was written on.  I have a science class in two minutes and if I’m late on the first day it’ll make me look bad for the rest of the year.”  Kenny looked really worried.“Relax,” Justin said, taking his backpack off and opened it.  “Remember how you borrowed my notebook in my home to write the combination down?  Well, I know how I can recover what you wrote.”He took the notebook and a soft lead pencil out of his backpack.  The page that Kenny had written on had left faint marks on another page in the notebook.  Justin held the pencil on its side and rubbed it lightly over the marks.  Slowly but surely the numbers of the locker combination appeared in white.“That’ s amazing!”  Kenny said, “I owe you one.”  And he dashed off to open his locker.It was just another day in the life of the boy whose motto was “Never throw anything out; you never know when it might come in handy.”小题1:Why is Justin’s room in such a mess?A．He always forgets to clean it. B．He never throws anything out.C．He has no time to clean it.D．He shares the room with his brother.小题2:In what way is Justin’s blue backpack used?A．He uses it as a place to store collected objects.B．He uses it to carry his books and sports equipment.C．His parents tell him to clean it all the time.D．He carries it to remind others of his ability.小题3:The underlined phrase “pleaded with” means              .A．ignoredB．askedC．pushedD．admired小题4:How does Kenny feel toward Justin?A．AnnoyedB．DisinterestedC．GratefulD．Angry