题目

用一水平外力将木块压在竖直墙上，使木块保持静止不动，如图4-3-5所示，当水平外力增大时，则木块（    ） 图4-3-5A.对墙的压力增大，受的静摩擦力不变B.对墙的压力增大，受的静摩擦力增大C.对墙的压力不变，受的最大静摩擦力增大D.对墙的压力增大，受的最大静摩擦力不变 答案：解析：水平外力增大时，木块对墙的压力就增大，由于木块有重力，所以木块具有向下滑动的运动趋势，木块受到向上的静摩擦力.静摩擦力与重力达到平衡，使物体保持静止状态.压力虽然增大，但物体重力不变，向下的运动趋势不变，所以物体受到的静摩擦力不变.而最大静摩擦力跟正压力成正比，压  Most musicians agree that the best violins were made in Cremona,Italy, about 200 years ago. They even sound better than violins made today. Violin makers and scientists try to make instruments like the old Italian violins. But they aren't the same. Why are these old Italian violins so special? Many people think they have an answer.  Some people think it is the age of the violins. But there is a problem here. Not all old violins sound wonderful. Only those from Cremona are special. So age cannot be the answer.  Other people think the secret to those violins is the wood. The wood of the violin is very important. It must be from certain kinds of trees. It must not be too young or too old. Perhaps the violin makers of Cremona knew some?thing special about wood for violins.  But the kind of wood may not be so important. It may be more important to cut the wood in a special way. Wood for a violin must be cut very carefully. It has to be the right size and shape. The smallest difference will change the sound of the violin.   Musicians sometimes think that this is the secret of the Italians.  Size and shape may not be the answer either. Scientists make new violins that are exactly the same size and shape. But the new violins still do not sound as good as the old one. Some scientists think the secret may be the varnish (清漆) ，which covers the wood of the violin and makes it look shiny. It also helps the sound of the instrument. Since no one knows what the Italian violin makers used in their varnish,no one can make the same varnish today.There maynever be other violins like the violins of Cremona. And there are not many of the old violins left. So these old violins are becoming more and more precious.5. What would be the best title for the passage?   A. The secrets of Cremona violins   B. The history of Italian violins   C. Special musical instruments   D. How to make the best violins6. Other people think the Italian violin makers         .   A. were lucky   B. did not know much about the violins   C. used many kinds of wood   D. knew something special7. What is still unclear about Cremona violins according to the writer?   A. The shape. B. The size.   C. The wood. D. The varnish.8. What can we learn from this passage?   A. Modern things are always better than ancient ones.   B. Ancient things are always better than modern ones.   C. Once a cultural relic is lost,it can never be recovered.   D. Varnish for violins will become more and more precious.