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?
A) 15 N
B) 8.8 N
C) 12 N
D) 4.5 N
E) 6.0 N

Question

Quizplus · Accepted Answer

The force of friction when the box is moving is given by the kinetic friction formula: $f_k = \mu_k N$, where $\mu_k$ is the coefficient of kinetic friction and $N$ is the normal force. The normal force for a horizontal surface is equal to the weight of the object, which is $mg$, where $m = 3.0 \, \text{kg}$ and $g = 9.8 \, \text{m/s}^2$. Therefore, $N = 3.0 \times 9.8 = 29.4 \, \text{N}$. The kinetic friction force is $f_k = 0.30 \times 29.4 = 8.82 \, \text{N}$, which rounds to $8.8 \, \text{N}$, indicating that the box is moving and the kinetic friction force applies.

The Coefficients of Static and Kinetic Friction Between a 3