What type of force opposes the motion between two surfaces in contact?

The force that opposes the motion between two surfaces in contact is friction. Friction arises due to the interaction of the microscopic roughness of the surfaces and acts to resist the relative motion of the surfaces sliding against each other. It is essential in allowing objects to start moving as well as in controlling their speed when in motion.

When an object tries to slide over another, friction acts opposite to the direction of the applied force that aims to move the object. This resistance is crucial in everyday situations, such as when walking, driving, or stopping. Without friction, objects would slide indefinitely without slowing down, making it essential for both motion and stability.

In contrast, the normal force is the perpendicular force exerted by a surface to support the weight of an object resting on it, applied force refers to any external force applied to an object to cause motion, and gravity is the attractive force acting between two masses, primarily affecting the weight of the object rather than the movement at the level of contact between two surfaces.