Two crates, a and b, sit at rest side by side on a frictionless horizontal surface. the crates have masses ma and mb. a horizontal force f⃗ is applied to crate a and the two crates move off to the right.

The diagram for crate A has all the forces acting on crate A. This would be a horizontal applied force to the right, a horizontal normal force from crate B to the left, a vertical gravitational force downward, and a vertical normal force from the supporting surface upward. 

The diagram for crate B has all the forces acting on crate B. This would be a horizontal normal force from crate A to the right, a vertical gravitational force downward, and a vertical normal force from the supporting surface upward. 

There are two things to remember if you are going to use these diagrams to solve a problem. The horizontal normal forces from each crate on the other are equal in magnitude (Newton's Third Law). And since the crates move away together, they must have the same acceleration.

Independent variable is something that is changed in a experiment.