Explain the differences between static friction and dynamic (kinetic) friction like I'm 9.
EDIT: I simply don't understand how static friction works, because I'm thinking "How can non-moving objects have friction" that may make no sense, but that is what I'm getting from books. But from my available resources, I think that static friction is when the roughness nearly perfectly interlocks and that both surfaces are stronger connected. I might need a more detailed description of this concept.
No surface is perfectly flat, there's always some roughness. If you look at a flat hard surface with a microscope, at some high magnification level it's probably going to look like sandpaper with lots of tiny bumps (asperities).
So an intuitive, math-free model for two hard surfaces in contact would be: imagine two sheets of sandpaper on top of each other, with the sand covered faces in contact.
Static friction: when there is no movement the asperities on both surfaces can interlock.
Dynamic friction: when one surface is sliding on top of the other, asperities will bump into each other but they have less time to find a solid interlocking position.
This explains why dynamic friction coefficient is lower than static friction coefficient.
In both cases, the harder the surfaces are pressed together, the more friction there will be. So the maximum force due to friction will be proportional to the force pressing the two surfaces together (like the weight of the object if it is on the floor).
Then, as a model, we multiply this by a "coefficient of friction" which depends on surface roughness, material, static or dynamic, etc.
Static friction is that force of friction, that exists as the object is still at rest or by other means work = 0. Formula:
$$F_{static} \rightarrow u_{static}\cdot N$$
$$F_{static max} = u_{static}\cdot N$$
point just literally before the object does work, where it finally moves.
$N$ is the normal force which is perpendicular to the surface over which an object is put. $N = mg$ when at ground since it opposes force of gravity. $N = mg\cos(\theta)$ when on incline since it opposes the $y$-component of the force of gravity.
$u$ is the coefficient of friction depends on multiple factors such as the texture etc, found experimentally.
Kinetic friction is the friction during motion, after (max point)
$$F_{kinetic} = u_{kinetic}\cdot N$$
