What is frictional force and its causes to increase or decrease?
frictional force is a macroscopic effect of atomic size electromagnetic forses.
It usually causes the decreasion of the velocity and because of being a nonconservative force, has a path dependent integral. (I mean the work) and decreases the total energy of the body
There are two kinds of friction: dry and fluid. Dry friction increases with the roughness of the two surfaces in contact and the normal force between the surfaces.
Fluid friction depends on the viscosity and density of the fluid (air, water, oil,...), the cross section of the body perpendicular to the direction of movement, the shape of the body, the angle of incidence (angle between the velocity vector and the symmetry axis of the body) and to the velocity (for high speeds it is proportional to the square of the velocity).
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