Final answer:
When no horizontal force is applied to a resting box, the friction force is zero since friction only reacts to oppose movement and there is none in this scenario.
Step-by-step explanation:
When a box is resting on a surface and no horizontal force is applied, the friction force acting upon it is zero. This is because friction is a reactive force that only opposes motion. Since the box is at rest and there is no applied force, there is no motion to oppose, so the static friction force is not active.
However, if a force were to be applied, the static friction force would initially match the applied force up to its maximum value, which can be calculated using the coefficient of static friction and the normal force. In this case, the maximum static friction force (fs,max) before the box begins to move can be determined using the formula fs,max = μs × N, where μs is the coefficient of static friction and N is the normal force (which is equal to the weight of the box, 40.0 N here), resulting in fs,max = 0.40 × 40.0 N = 16.0 N.