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 asloj au f moc nx;9+) ,;8;q0qd+aaqtn sleigh as the sleigh iss g3j0s2lg(eh:v x9km pulled up a hill with an increasing speed. The crate is not fastened down to the sleigh. What force is :vm2ge9 s(g0 s3hkxl jresponsible 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 sleig6pu))y) )uqb gmeb/leh.
The normal force is 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 stbluqye)u/m6be) p)g )atic friction, which prevents the crate from sliding down the sleigh.
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