Question

How many objects must be there to interact for a force to come into play?

Answer 1

If force has to come into play, at least there must be two objects.Force comes into play when external or internal forces are applied on the objects or bodies.

Complete answer:
At least there must be two objects for a force to come into play, because force is applied in between the bodies or objects. Force comes into play, when a body interacts or collides with each other. If there is no interaction or force applied in between bodies or objects then there is no force applied.

Force is calculated by the multiplication of mass and acceleration in between the bodies or objects. Force is a push or pull that acts on the object or bodies.There are some specific forces that apply on bodies or objects wherever the force is applied, those are frictional force, tensional force, normal force, air resistance force and applied force.

Frictional force: Friction means the force between two bodies that are sliding. Mainly friction depends on the material. Mainly the force affected by the surface texture and force impelling.
Tensional force: The force that is transmitted in the form of spring or rope pulled tight then the force acts from opposite ends.
Normal force: Normal force is a constant force. When two surfaces are in content, then they contact each other without force applied.
Air resistance force: The forces that are in opposite directions relative to the motion of an object as it passes through air.
Applied force: The force applied to an object by a person is called applied force.

Note: To apply force at least there must be two objects. Any objects either they are placed on a flat surface or sloped surface. Force is applied only between the bodies.Force is an external agent capable of changing the state of rest or motion of a particular body. It has a magnitude and a direction.