On a molecular level, friction arises due to the interactions between the atoms or molecules of two surfaces in contact. When an object is experiencing a constant external force and is in motion, friction opposes the relative motion between the object and the surface it is in contact with.
There are two types of friction: static friction and kinetic friction. Static friction occurs when the object is at rest or not moving relative to the surface, while kinetic friction occurs when the object is in motion.
When an external force is applied to an object, the atoms or molecules at the interface between the object and the surface start to interact. These interactions involve electrostatic forces between charged particles, as well as intermolecular forces such as van der Waals forces. The strength of these forces depends on the nature of the surfaces and the molecules involved.
In the case of static friction, as the external force increases, the intermolecular forces at the interface also increase. This causes an opposing force to build up, preventing the object from moving. The magnitude of static friction increases until it reaches a maximum value, called the limiting friction. Once the applied force exceeds the limiting friction, the object starts to move, and kinetic friction comes into play.
Kinetic friction occurs when the object is in motion. As the object slides or moves across the surface, the intermolecular forces are constantly breaking and reforming between the atoms or molecules of the object and the surface. These interactions generate heat and dissipate energy, which is why friction is associated with energy loss. The magnitude of kinetic friction is generally lower than static friction, but it remains relatively constant as long as the conditions, such as the nature of the surfaces and the normal force, remain unchanged.
Overall, friction on a molecular level involves complex interactions between atoms or molecules at the interface of the object and the surface. These interactions lead to opposing forces that resist the motion of the object and result in energy dissipation in the form of heat.