如图所示，质量为m的木块放到倾角为θ的斜面上，恰好可以沿斜面匀速下滑，问：
（1）木块匀速下滑过程中受到摩擦力的大小为多少？
（2）若对木块施加一个平行于斜面向上的力F，使小块沿斜面向上匀速运动，则力F的大小为多少？
（1）木块匀速下滑过程中，受力如图（1）所示，根据平衡条件可得：
mgsinθ=f
（2）木块匀速上滑过程中，受力如图（2）所示，根据平衡条件可得：
F=mgsinθ+f
由以上两个式子可得：
F=2mgsinθ
答：（1）物体与斜面的动摩擦力为mgsinθ；
（2）保持倾角不变，为使物体可以匀速上滑，推力为2mgsinθ．
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| If we were asked exactly what we were doing a year ago, we should probably have to say that we could not remember. But if we had 1 a book and had written in it an account (叙述) of what we did each day, we should be able to give an answer to the question. It's the 2 in history. Many things have been 3 because we do not have any written account of them. sometimes men did 4 the most important happenings in their country, but often the material was destroyed by fire or in a war. Sometimes there was never any written record at all 5 the people of that time and place did not know 6 to write. For example, we know a good deal about the people who lived in China 4,000 years ago,because they could write and leave written records for those who lived 7 them. But we know almost nothing about the people who lived even 200 years ago in central Africa, because they had not learned to write. Sometimes, of course, 8 the people cannot write, they may know something of the past. They have heard about it from older people,and often songs and dances and stories have been made about the most important happenings, and these have been sung and acted and told for many 9 . For most people are proud to tell what their fathers did in the past. Such 10 stories are often very helpful where there are no written records. |
| ( )1．A．bought ( )2．A．question ( )3．A．forgotten ( )4．A．experience ( )5．A．although ( )6．A．how ( )7．A．on ( )8．A．while ( )9．A．people ( )10．A．long | B．written B．same B．checked B．know B．when B．what B．by B．since B．generations B．different | C．kept C．story C．damaged C．study C．unless C．when C．after C．even if C．reasons C．personal | D．opened D．mystery D．imagined D．record D．because D．why D．with D．as long as D．purposes D．spoken |