Final answer:
The correct answer is a) Friction, with static friction affecting an object at rest and kinetic friction affecting an object in motion.
Step-by-step explanation:
Definition and Types of Friction
Friction is a contact force that occurs between systems in contact with each other and opposes motion or attempted motion between them. The two types of friction are static and kinetic. Static friction acts on an object that is not in motion and opposes the start of motion. It is proportional to the normal force and has a maximum value determined by the coefficient of static friction and the normal force, described by the equation ℓₛ ≤ Mₛ N. On the other hand, kinetic friction acts on an object that is already moving, opposing its continued motion, and is calculated as fₖ = Mₖ N, where Mₖ is the kinetic coefficient of friction.
Answering the Question
The correct answer to the student's question about static and kinetic types of force on objects that are in contact with each other is a) Friction. Kinetic friction acts on an object in motion, while static friction acts on an object at rest.