Why is there a force between two objects?

[mark1950]

I don't get it. Two objects (A and B) on a smooth horizontal plane in contact with each other. A force is applied on object A and the objects move to the right together. However, if we were to draw a free body diagram of each object, there would be a force acting like a frictional force on object A but not B.

Imagine a free body diagram of each object in that system. P is the force between each object A and B. P acts against object A but in the diagram of object B, P is in the same direction as object B. I understand that this force is not frictional force as the objects are on a smooth plane. What force is it? Is it the same as tension of a string? Thanks.

 

[Fenn]

I suppose you could think of it like tension in a string, as these forces act to maintain the relative position of the objects. The forces acting between each object keep the objects from collapsing into each other.

Do you understand the concept of the normal force when an object is pressed against a surface? It is basically the same concept. When an object rests on a surface, for example, and gravity pulls that object down, the surface applies a normal force on that object to keep it from crushing down into the surface. When one object is pressed against another, there exists a force in the opposite direction to keep the objects from crushing into each other.

As for the free body diagram for the objects A and B which you describe, you can draw each object individually. For object A, there would be the externally applied force in one direction, and the force from object B on A in the other. Be sure not to confuse this as a frictional force. You've identified the objects sitting on a smooth plane, so there would be no need to consider gravity and a normal force to determine a frictional force.

For a free body diagram of object B, there would be the force from object B acting on A, which is equal in magnitude as the force from B acting on A in the previous free body diagram. This force will act in the same direction as the applied force on object A. If these forces acting between the objects didn't exist, the objects would be pushed together and overlap. Think of a two big jelly blobs that happily squish together.

 

[charng]

The force between two objects is known as the contact force or the normal force. This force exists due to the interaction between the two objects at their point of contact. In this scenario, as object A is pushed to the right, it exerts a force on object B in the opposite direction, causing both objects to move together. This contact force is necessary for the objects to maintain their physical contact and move together.

The force acting on object A in the free body diagram is not a frictional force, as you correctly stated. This is because the objects are on a smooth horizontal plane, so there is no resistance to their motion. Instead, the force acting on object A is the normal force exerted by object B. This force is also known as the reaction force, as it is a reaction to the force applied on object B by object A.

The normal force is not the same as tension in a string. Tension is a pulling force that occurs when an object is suspended or pulled by a string. The normal force, on the other hand, is a pushing force that occurs when two objects are in contact with each other.

I hope this explanation helps clarify the concept of the force between two objects. It is important to understand that this force is a result of the interaction between the objects and is necessary for them to maintain their contact and move together.