本题目来源于试卷: 2007美国US F=MA物理竞赛，类别为 美国F=MA物理竞赛
[单选题]
A large wedge rests on a horizontal frictionless surface, as ux d ov8rw75y;e bq6m6sh +0met 67 ig0yb7nc3vscfeji4 own. A block starts from rest an je +g37 cvfcytn4s im7e0 b06id slides down the inclined surface of the wedge, which is rough. During the motion of the block, the center of mass of the block and wedge 
A. does not move
B. moves horizontally with constant speed
C. moves horizontally with increasing speed
D. moves vertically with increasing speed
E. moves both horizontally and vertically
参考答案: D
本题详细解析:
Consider the (block + wedge) as a single sy7 9u.,)kf i-iejnymt:cl spq-stem.
There are no external horizontal forces (the surface is frictionless).pc7qitm-eus,n:9f- .kjiy l)
Therefore, the horizontal component of the center of mass ($x_{cm}$) has zero acceleration.
Since it starts from rest, $x_{cm}$ does not move.
However, there are external vertical forces (gravity on both objects and the normal force from the ground).
As the block slides down, its vertical position decreases. The wedge does not move vertically.
Therefore, the center of mass of the system must move vertically downwards.
Since it starts from rest and gravity is acting, it moves with increasing speed.
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