Contact forces
Whenever two bodies interact by direct contact (touching) of their surfaces, we describe the interaction in terms of contact forces
Friction force
The component vector parallel to the surface is the friction force.
Kinetic friction force
The kind of friction that acts when a body slides over a surface is called a kinetic friction force.
Static friction
When a body is not moving relative to a surface, the friction force is called static friction.
What is the direction of the fluid resistance force acting on a body
The direction of the fluid resistance force acting on a body is always opposite the direction of the body’s velocity relative to the fluid.
Fluid resistance force for a small object at low speed
The magnitude f of the fluid resistance force is approximately proportional to the body’s speed v
f=kv
where k is a proportionality constant that depends on the shape and size of the body and the properties of the fluid.