If a wagon is moving on a sidewalk, why doesn’t that wagon fall through the sidewalk?
The wagon and sidewalk cannot occupy the same space at the same time. Although the wagon’s weight pulls it downward, the sidewalk pushes the wagon upward with a force that prevents the wagon from moving into the sidewalk.
The type of force the sidewalk exerts on the wagon is known by several different names: support force, contact force, or normal force. It derives from the repulsive forces that atoms experience when they are too close together. When the wagon and sidewalk are pushed together by the wagon’s weight, the atoms of the wagon and sidewalk become too closely spaced and they push apart.
When solid objects are pressed into one another, they always respond with support forces that act to separate them. The harder they are pressed together, the more their atoms overlap and the harder they push apart. There are limits, however, beyond which the objects begin to break apart. Pressing them together causes them to dent or deform, a large-scale behavior related to the small-scale overlapping of their atoms. They can only dent or deform so much before they break.