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.

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