Final answer:
The net force on a box on a desk is based on the coefficients of static and kinetic friction and the applied force. If the applied force is less than the static frictional force, the box remains stationary. Once the object moves, kinetic friction applies and the net force is the difference between the applied force and the kinetic frictional force.
Step-by-step explanation:
The net force on an object, like a box on a desk, when a horizontal force is applied, is influenced by the coefficient of static friction (μs) prior to the object's movement and by the coefficient of kinetic friction (μk) once the object has started moving. The determination of the net force requires understanding the interaction between these coefficients, the applied force, and the box's weight.
For an object at rest, the static friction must be overcome by the applied force to initiate movement; this maximum static frictional force is given by fs(max) = μsN, where N is the normal force, which is equal to the weight of the object if the surface is horizontal. When the object is already in motion, the kinetic friction force that needs to be overcome to maintain a constant velocity is calculated by fk = μkN.
To determine if the box will move when a specific force is applied, you compare the applied force to the maximum static friction force. If the applied force is less than the maximum static friction, the net force on the box will be zero; the box will not move. However, if the applied force exceeds this value, the box will begin to move, and the kinetic friction force will come into play, which will be less than the maximum static friction due to the lower coefficient. The net force will then be the difference between the applied force and the kinetic friction force.