I found out something pretty interesting in Physics yesterday. I'm pretty certain I'd heard it before, but it's such an intriguing fact that I feel compelled to mention it anyway.
Go grip a piece of paper, a paper clip, a box, or any other object. Then place it on a flat surface. It stays there and doesn't move. This is the action of normal force: the force of the flat surface pushing up on the object as the object, under the action of gravity, pushes down on the surface. The normal force is perpendicular to the surface itself (hence its name). This normal force also explains why you're able to stand on the ground.
Take a book, and push it across a flat surface. When you do so, you apply a force to the book, thus changing its motion from rest to a certain speed. Push it a little faster then release your hand; it may slide slightly, slow down, and stop. This deceleration results from friction: the microscopic surface irregularities, nooks, crannies, bumps, holes, hills, chaffing and hitting against each other. In the same way, put on socks and run on a smooth, perhaps tiled floor; or even more fun, a wet floor. You keep moving, even after you stop running, though you decelerate. You aren't applying any more forward force, but you keep in motion until friction stops you. Simple demonstration of Newton's first law.
Now go take a piece of paper and drop it. As it descends to the floor under the action of gravity, it experiences drag: the air moving about its thin contour, changing its motion. Air, a fluid, causes its own brand of friction. All objects experience it when moving through the air, some moreso than others, depending on the shape of the object and density of the fluid.
Finally, take a pencil and write on a piece of paper. The friction between the paper and pencil causes a thin layer of carbon to break its bonds from the rest of the graphite. An arrangement of carbon atoms in layers composes graphite, with each layer loosely connected to the others. The bonds between each layer sever easily (which is why it's generally easy to write); the friction between it and the paper is enough to sever the bond.
What exactly is interesting about all this?
That they are all caused by the electric force: the forces of attraction and repulsion between molecules. This isn't a counterintuitive idea: your hand gets closer to a book, so repulsive forces between molecules at such small distances* will become noticably pronounced. Your ability to grip an object is likewise a result of this force. Just getting two objects close to each other strengthens the electromagnetic interactions; thus, the book stays just a microscopic distance away from your hand. Or, even, any other object it touches. Our ability to grip is intricately linked with the electrical forces between microscopic particles. These macroscopic phenomena and sensations, caused by electrical reactions. While most of these molecules are probably neutral in charge, they become so close that the repulsive forces between outter electrons sets in. The atoms never really "touch;" that's just a macroscopic perception we have. In reality, the atoms are close, but not touching.
Going deeper into the man, we see neural paths, driven by currents, spawning his thoughts, abstractions, churning his emotions. He muses about himself inwardly, and forms words to express himself. A stream of electrons divert their path; his stomach gurgles a bit, he knows he must eat.
He rises from his seat, his feet pressured slightly by the ground. His hands clasp together, he stretches a bit. He heads to the kitchen, and looks for the cutlery. He removes a knife from the drawer, and holds it in his mouth while he searches the pantry for bread. There it is, resting on the second shelf from the bottom. Pulling it out, he sets the knife down and glances around for some choice condiments. Butter perhaps? Nay. Peanut butter and jelly isn't filling enough. Meat seems like a fine choice. He opens the refrigerator door, pulls out some baked ham. Carefully, he lays the knife, just barely at the ham's edge; an uneven piece of ham slides off; he slices again, again, again, until he has enough meat. The slices of pork are dropped onto the bread. The man covers them with another slice of bread, and eats. While he eats, he ponders.
There go a couple neurons; pleasure from the taste of meat and satisfaction of fullness. Another stream of electrons zoom chaotically towards their destination; he feels an impulse to continue his work. Two streams of electrons crash into each other, and fuse into one; a mental lightbulb flickers, the vague recollections from his physics classes appear in his mind's forefront. The three streams of electrons meet at a point; inspired, his hand grasps a pen.
Coulomb's Law states that the magnitude of the electric force between two objects, one with charge q1 and the other with q2, is:
F = k |q1| |q2| / r2
Where k = 1 / (4 π ε0), ε0 is the permittivity of free space, approximately equal to 8.854 * 10-12 F/m; r is the distance between the two charges; and the force F is measured in Newtons. Note that whether the force is attractive or repulsive depends on the signs of the two charges; if both signs are the same, it's repulsive. If both signs are opposite, attractive.