Which type of friction opposes motion once it has already started?

The type of friction that opposes motion once it has already started is kinetic friction. This friction occurs between two surfaces that are in relative motion. When an object begins to move, kinetic friction acts to resist that motion, effectively slowing it down. This frictional force is generally lower than static friction, which acts to prevent motion from starting in the first place.

In practical terms, once an object begins moving (like a box sliding across the floor), it experiences kinetic friction, which determines how easily it continues to slide. The level of kinetic friction depends on factors such as the surfaces in contact and any external forces acting on the object. For example, a box on a smooth surface will experience less kinetic friction and will thus be easier to slide than if it were on a rough surface.

The other types of friction, while relevant in various contexts, are not the focus here:

• Static friction prevents motion from starting and is typically higher than kinetic friction.

• Rolling friction acts on rolling objects and is generally less than kinetic friction, aiding in smoother wheeled motion.

• Fluid friction pertains to objects moving through a fluid, such as air or water, and differs from the contact friction experienced with solid surfaces.