Final answer:
In the context of a sliding block and surface, both the block alone and the surface alone are considered nonisolated systems due to external forces acting on them, such as friction and reaction forces. When combined as one system, they approach being an isolated system but, realistically, are still subjected to external forces like gravity.
Step-by-step explanation:
When considering the system where only the block is included, it would be a nonisolated system. This is because there are external forces acting on the block, such as friction from the surface, gravity, and potentially air resistance. According to our options, an isolated system is one where the net external force is zero, which is not the case for the block alone.
If the system is the surface alone, this is also a nonisolated system. Although the surface seems immobile, it experiences a reaction force due to the block sliding over it, which arises from Newton's third law of motion—every action has an equal and opposite reaction. Therefore, the surface is not free from external interactions either.
When considering the system as both the block and the surface combined, this could be closer to an isolated system in some contexts—if we assume no other external forces act upon it. However, in reality, it's nearly impossible to have a perfectly isolated system as there are always external influences like gravity, especially in typical settings such as a classroom or laboratory.
The direction of the friction force is always opposite to the direction of the block's motion. This is due to molecular interactions that resist the movement of the sliding block, thus generating what is known as kinetic friction.