如图所示，小球从高为h的斜面上的A 点，由静止开始滑下，经B点在水平面上滑到C点后停止，现在要使物体由C点沿原路径回到A点时速度为零，那么必须给小球以多大的初速度？（设小球经过B点处无能量损失）
解：以小球为研究对象，在斜面上和水平面上它都受到重力、支持力、摩擦力三个力作用，在整个运动过程中支持力始终垂直于运动方向不做功，重力做正功，摩擦力做负功，由动能定理可知在下滑到C点静止的过程中有：W重-W阻=0，
所以W阻= W重=mgh，
当它沿原路径返回时，摩擦力所做负功与滑下过程完全相同，而此时重力也做负功，由动能定理得：- W重- W阻=0 - 
，所以
=2W重=2mgh，
解得
即要使小球从C点沿原路径返回到A点时速度为零，必须给小球以
的初速度．
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| In traditional education, the teacher __1__ feel that the students are not very grown-up. Indeed, teachers are older than students __2__ teachers feel that students are __3__ and don't know very much about the world. The teachers feel that they must tell the students __4__ to do most of the __5__ , and that they must also__6__ the students study specific things. In__7__ education, the teacher's means is __8__ . These teachers feel that the students are beginners first, and school second. They __9__ the students to be responsible for (负责) the things they do, just as grown-ups __10__ . A student's ideas and feeling are just as important as the teacher's. The teacher allows the students to decide what they don't want to do, and doesn't make them study what they don't want to. The teacher lets them decide what to study and how much to study. It is very important for the teacher to show how he or she feels for the students. | |||
| ( ) 1. A. will ( ) 2. A. and ( ) 3. A. older ( ) 4. A. that ( ) 5. A. day ( ) 6. A. force ( ) 7. A. open ( ) 8. A. the same ( ) 9. A. like ( ) 10. A. are | B. should B. however B. young B. what B. week B. ask B. higher B. different B. hope B. do | C. may C. stll C. youngest C. how C. vear C. order C. strict C. important C. expect C. will | D. need D. yet D. old D. which D. time D. make D. public D. useful D. help D. would |