半圆柱体P放在粗糙的水平面上，有一挡板MN与半圆柱不接触，在P和MN之间放有一个光滑均匀的小圆柱体Q，如图是这个装置的截面图，整个装置处于静止状态．若用外力缓慢地移动挡板MN，并保持挡板MN始终与两圆心的连线平行，在MN到达水平位置前，发现P始终保持静止，在此过程中，下列说法中正确的是（ ）
A．Q所受的合力逐渐增大
B．地面对P的弹力逐渐增大
C．P、Q间的弹力先减小后增大
D．MN对Q的弹力逐渐增大
设两圆心的连线与水平方向夹角为α．
A、缓慢地移动挡板MN的过程中，Q所受合力始终为零，保持不变．故A错误．
B、C、D以Q为研究对象，分析受力，如图1，根据平衡条件得，MN对Q的弹力N1=GQcosα，P对Q的弹力N2=GQsinα，GQ不变，α减小，则N1增大，N2减小．再以P为研究对象，分析受力如图，由平衡条件得，地面对P的弹力N3=GP+N2sinα，GP不变，N2减小，α减小，则地面对P的弹力N3减小．故BC错误，D正确．
故选D
Directions: There are 10 blanks in the following passage. For each numbered blank, there are 4 choice marked A, B, C and D.
| During recent years we have heard much about "race": how this race does certain things and that race believes certain things and so on. Yet, the (51) phenomenon of race consists of few surface indications. We judge race usually from the coloring of the skin: a white race, a brown race, a yellow race and a black race. But (52) you were to remove the skin you could not tell anything about the race to which the individual belonged. There is nothing in physical structure, the brain or the internal organs to (53) a difference. There are four types of blood. All types are found in every race, and no type is distinct to any race. Human brains are the (54) No scientists could examine a brain and tell you the race to which the individual belonged. Brains will (55) in size, but this occurs within every race. (56) does size have anything to do with intelligence. The largest brain ever examined belonged to a person of weak (57) . On the other hand, some of our most distinguished people have had (58) brains. Mental tests which are reasonably (59) show no differences in intelligence between races. High and low test results both can be recorded by different members of any race. (60) equal educational advantages, there will be no difference in average standings, either on account of race or geographical location. |
A．same
B．identical
C．similar
D．alike