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 remains at rest on a sleigh as the sleigh is pullx sl. zdup 8mx;11t/myz( gar4ed ) r7 60fdddjksup a hill with an increasing speed. The crate is not fastened down to the sleigh. What force is responsible for the crate's increase in speed up the s0k7d d6jf rd)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.
kd *+e syby2*cfa0o;bThe normal force i;bk*y cseao*df y2+0bs perpendicular to the surface 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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