Question

part a
friction: the coefficients of static and kinetic friction between a 3.0 - kg box and a horizontal desktop are 0.40 and 0.30, respectively. what is the force of friction on the box when a 15 - n horizontal push is applied to the box?
6.0 n
8.8 n
4.5 n
12 n
15 n

Explanation:

Step1: 计算最大静摩擦力

最大静摩擦力公式为\(f_{s,max}=\mu_{s}N\)，物体在水平桌面，\(N = mg\)（\(m = 3.0kg\)，\(g = 9.8m/s^{2}\)），则\(N=3.0\times9.8 = 29.4N\)，\(\mu_{s}=0.40\)，所以\(f_{s,max}=0.40\times29.4 = 11.76N\approx12N\)。

Step2: 比较推力与最大静摩擦力

已知推力\(F = 15N\)，因为\(15N>12N\)，此时物体相对桌面运动，摩擦力为滑动摩擦力，滑动摩擦力公式\(f_{k}=\mu_{k}N\)，\(\mu_{k}=0.30\)，\(N = 29.4N\)，则\(f_{k}=0.30\times29.4 = 8.82N\approx8.8N\)。

Answer:

8.8N