[tex]\begin{gathered} m=5\operatorname{kg},g=9.81m/s^2 \\ \mu_k=0.21 \\ Ff=\text{?} \\ Ff=\text{ N}\cdot\mu_k \\ \text{From the fr}ee\text{ body diagram} \\ \uparrow+\Sigma Fy=0 \\ N-mg=0 \\ N=mg \\ \text{Hence} \\ Ff=\text{ N}\cdot\mu_k \\ Ff=\text{ }mg\cdot\mu_k \\ Ff=(5\operatorname{kg})(9.81m/s^2)(0.21) \\ Ff=10.3N \\ \text{The friction force is 10.3N} \end{gathered}[/tex]
