Action-at-a-Distance Forces. Frictional Force. Gravitational Force. Tension Force ... The force of gravity on earth is always equal to the weight of the object as ... The friction force is the force exerted by a surface as an object moves across it or ... The force of air resistance is often observed to oppose the motion of an object
Because with friction you need to be rubbing multiple object together, in tension two objects must be pulling against each other, and in air resistance the air must be touching the object either pushing or pulling it. While in gravity the mass of an object is pulling another object toward it, the objects don't have to be touching each other making it an at-a-distance force.
