Taking into account body interactions with a surface. In this case, one of the bodies is the surface and it has components of force that it exerts on the body its interacting with.
The direction of the friction force is always such as to oppose relative motion of the two surfaces.
The magnitude of kinetic friction increases when the normal force increases (the y component of the pull or pushing force contributes to the normal force ? )
Its proportional to a constant depending on the surface
fk=μkn
μ is a quotient of forces, therefore it has no units
When there is no relative motion between the surfaces there can still be the effects of static friction.
The static force is kind of elastic, or responsive to whatever force is taking the body on the surface out of equillibrium.
A force greater than fs takes the body out of equilibrium
fs≤μsn
If there is no applied force, there is no static friction
The direction of fluid (gas or liquid) resistance on a body is always opposite the direction of the body’s velocity relative to the fluid.
Fluid resistance at high speeds f=Dv2
Is the fluid not acting from multiple angles though? Its not just on one axis.
Is there not some part of the fluid force propelling the body forward? Buoyancy
Resistance force increases until its equal to the downward force (weight in some example). Then there is no further increase in speed.