Concept:A contact force requires physical touching between objects, while a non-contact force acts even when objects are separated in space.
Explanation:Force of friction is a contact force because it arises when two surfaces touch and oppose relative motion.
Normal reaction is a contact force because a surface pushes perpendicularly against an object only when they are in contact.
Viscous force is a contact force because it acts between adjacent layers of a fluid in relative motion.
Gravitational force is a non-contact force because it acts at a distance between two masses without any physical contact.
The magnitude of gravitational force is expressed as
F=r2Gm1m2, where
G is the gravitational constant,
m1 and
m2 are the masses, and
r is the distance between them.
This equation confirms that gravity operates over a distance, requiring no direct contact between objects.
Hence, among the given options, gravitational force is the one that is NOT a contact force.
Answer:Option C: Gravitational force.