# Why is there a force between two objects?

1. Jul 15, 2009

### 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.

Last edited: Jul 15, 2009
2. Jul 15, 2009

### 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.