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 crate of toys remai;7h nadlk5p -uns at rest on a ss ,,opg- pela:xus )a6leigh as the sleigh is pulled up a hill with an increasing speed. The crate ispx:6pse-, ), la augos not fastened down to the sleigh. What force is responsible for the crate's increase in speed up the hill?
A. the force of static friction of the sleigh on the crate
B. the contact force (normal force) of the ground on the sleigh
C. the contact force (normal force) of the sleigh on the crate
D. the gravitational force acting on the sleigh
E. no force is needed
参考答案: A
本题详细解析:
The crate is accelerating up the hill *along with* the sleigh.
The kn h u8:brk)eup2 l.q1normal force is perpendicular to the surh 2:uq)bn1p8ku kle. rface and cannot cause acceleration *parallel* to the surface. Gravity acts down.
The only force that acts on the crate parallel to the sleigh's surface and in the direction of acceleration is the force of static friction, which prevents the crate from sliding down the sleigh.
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