### Kinetic vs. Static Friction

A box of weight mg = 50 N rests on a flat surface. A force of 25 N
directed right is applied. The static and kinetic friction coefficients
between the box and the surface are μ_{s}=0.6 and
μ_{k}=0.4, respectively. What is the friction force acting on the
box?

- 20 N to the left
- 25 N to the left
- 30 N to the left

The applied force is insufficient to overcome static friction, so the box
remains at rest. To give a net force of zero, the friction force exactly
balances the applied force, so the friction force is 25 N to the left.