In this simulation, you see, on the left, a picture of a box at rest on a table. You can apply a vertical force (up or down) to the box. On the right, you can see the full free-body diagram of the box. The free-body diagram shows the gravitational force (green) exerted on the box by the Earth, the normal force (purple) exerted by the table, and the vertical force (dark blue) that you apply. Note that, on the free-body diagram, all three forces are along the vertical line passing through the center of the box, but the force of gravity and the normal force have been shifted a little so they can be seen more easily.
Simulation by Andrew Duffy. Amended on 8-16-2016 to include labels on the force vectors.
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