Which force is an example of a contact force?

The friction force is a prime example of a contact force because it occurs when two surfaces are in direct contact with one another. It arises from the interactions between the microscopic irregularities on the surfaces, and it acts to oppose the relative motion between the surfaces. When you push an object across a surface, it is the friction force that resists that motion, demonstrating its role as a contact force.

In contrast, other forces mentioned do not require physical contact to act. For instance, magnetic force operates at a distance and involves magnetic fields, while gravitational force acts between masses through a field, regardless of whether they are touching. Nuclear force, which operates between particles in an atomic nucleus, also does not require contact but instead acts at a subatomic level. Hence, the friction force distinctly stands out as a contact force due to its necessity for direct interaction between materials.