Friction

An object starts moving when the applied external force $F$ exceeds the maximum value of static friction (limiting friction $f_{max}$).

1. Normal Force ($N$): For a block on a horizontal surface, $N=mg$. $N=5\text{ kg}\times 10{\text{ m/s}}^2=50\text{ N}$.
2. Limiting Friction ($f_{max}$): $f_{max}={\mu }_{s}N=0.4\times 50\text{ N}=20\text{ N}$. To start motion, the applied force must be at least equal to $20\text{ N}$. If $F<20\text{ N}$, static friction will exactly cancel the applied force, and the block remains at rest.