Interactive · 1 question
If you hit a rock with a hammer, or pull on a rope, or walk across the ground, you're exerting and experiencing forces. Specifically, these are called contact forces.
But which of the four fundamental forces are contact forces?
Atomic Structure and Contact Forces
Everyday materials are composed of atoms, with low-mass and negatively-charge electrons arranged around high-mass and positively-charged atomic nuclei. The electrons form a cloud around the nuclei.
If we force two atoms next to one another, they don't directly contact one another. Instead, the negatively charged electron clouds usually repel one another electromagnetically. This produces a contact force; pushing the two atoms away from one another.
Friction
Friction exists because no surface is entirely smooth. If we had two entirely uniform electron clouds, infinite planes going off in all directions and always separated by the same distance, then there would be no friction between them - any motion parallel to the planes would give the system the same energy, and there would be no net force in that direction between the clouds.
But it is impossible to make a perfectly uniform distribution of electrons - they tend to clump up a bit near the nuclei; the nuclei aren't all in an ideal uniform arrangement themselves. So given two objects with a contact force between them, moving one sideways relative to one another will run electron clouds into one another. The potential energy of the system changes; perhaps momentarily as the surfaces slip to a new equilibrium. But that converts energy from kinetic energy of motion; to potential energy of how the electrons are arranged; and then to random motions of the electrons and nuclei. And so we have friction and drag.
Pulling
If overlapping electron clouds makes atoms repel one another, then how can we have contact forces that let us pull on things?
The answer is that overlapping electron clouds does not always make atoms repel one another. It is frequently a lower-energy state for the outer electrons of an atom to be shared with one or more other atoms. These are called chemical bonds. The arrangements atoms make are the subject of chemistry, but if you push on one atom that is bonded to other atoms, the whole arrangement moves.
Consider a simple example: pulling on a block with a rope. The friction between fingers and the rope is due to irregularities in the electrons in the fingers pushing against irregularities in the surface of the rope. The bonds between the atoms in the rope transfer the force to the block.
And that's how electromagnetism produces contact forces - pushing, pulling, and friction.