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 μs=0.6 and μk=0.4, respectively. What is the friction force acting on the box?

  1. 20 N to the left
  2. 25 N to the left
  3. 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.