Which of the following is an example of a contact force?

• A. gravitational force
• B. magnetic force
• C. air resistance force
• D. electric force

Answer: (C)

The essential idea is distinguishing forces that require contact from those that act through a field or at a distance. Air resistance is the force the air medium exerts on a moving object as it travels through the air, arising from collisions and viscous interactions with air molecules at the surface. Because this force comes from the surrounding air in direct interaction with the object, it is a contact force. In contrast, gravitational, magnetic, and electric forces can act without the objects touching—gravity through the field of mass, and magnetic and electric forces via fields that propagate through space. So air resistance is the example of a contact force.