题目

英文物理题（牛二律）
An 92 N box of oranges is being pushed across a horizontal floor.As it moves,it is slowing at a constant rate of 0.80 m/s each second.The push force has a horizontal component of 40 N and a vertical component of 45 N downward.Calculate the coefficient of kinetic friction between the box and floor.

提问时间：2021-03-14

答案

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分析
由slowing at a constant rate of 0.80 m/s each second可知其加速度为-0.80m/s^2.
因为盒子的重量为92N,其质量就是92N/g=460/49kg.由F=ma可知,盒子所受合力为-0.8*460/49 N=-368/49N.
合力是水平方向产生的（竖直方向受力始终平衡）,合力即摩擦力和推力的矢量和：F=T+f,可得f=F-T=-368/49N-40N=-2328/49N
摩擦力和摩擦系数的关系：f=μN,即μ=f/(G+Nv)=|2328/49/(92+45)|=2328/6713=0.347
a=Δv/Δt=-0.8m/s /1s=-0.8m/s^2;
m=G/g=92N/(9.8m/s^2)=460/49kg;
F=ma=-0.8m/s^2*460/49kg=-368/49N;
f=F-T=-368/49N-40N=-2328/49N;
μ=f/(G+Nv)=|2328/49N/[(92+45)N]|=2328/6713=0.347.