OpenStax College Physics for AP® Courses, Chapter 4, Problem 32 (Test Prep for AP® Courses)

This is College Physics Answers with Shaun Dychko. The atoms within this object that is resting on the floor are surrounded by electrons. The electrons surrounding an atom in this box are going to repel from the electrons surrounding an atom in this surface here. While there are many, many electrons that are repelling each other and doing so in a variety of different directions, the net result is that while there might be an electron here, repelling one here in a vector that's to the right on an angle, there will be another equal least equal magnitude interaction of an electron here with an angle like this compensating for the horizontal components of these vectors resulting in a net vector upwards. That is just one example to show that there's a net force upwards perpendicular to the surface. If you consider all the other millions of possible interactions that are happening between the millions of electrons in the surface and in the object, you'll find that the net result is to have a force perpendicular to the surface.