Diagrams of atoms show a tiny nucleus surrounded by electrons, so atoms are often described as mostly empty space. That does not mean solid objects can simply pass through one another. Electron clouds in neighbouring atoms interact through electromagnetic forces, and quantum rules strongly resist forcing electrons into the same states. These effects create the rigidity of ordinary matter. When you push on a wall, the atoms in your hand and the wall repel and rearrange slightly rather than passing through one another. The phrase 'mostly empty space' describes scale inside atoms, but it does not mean that the space behaves like an open doorway.

The useful part of this physics is that it helps people predict and design real things. Engineers often begin with the same simplified model used in school, test where it works, then add more detail when measurements show that the real situation is more complicated.

The motion can be understood by adding the forces as vectors. If they balance, the resultant force is zero and the object stays at rest or continues at constant velocity; if they do not, it accelerates. Friction and drag transfer energy to the surroundings, usually as heating, so maintaining a steady speed can still require continuous work even when the speed itself is not changing.