Static Friction: $f_{s}$

Static friction is a responsive force; i.e. if there is no force pulling an object along a surface, there would be no static friction.
The magnitude of static friction depends on the magnitude of the force attempting to move the object.
- If the force pulling the object is small, static friction would be small.
- As the pulling force increases, our static friction too will increase to match the pulling force.
After a certain point, the static friction would not be able to match our pulling force, that’s when our object finally slides.

Kinetic Friction: $f_{k}$

Kinetic friction does not have a variable magnitude. It has a fixed magnitude and always exerts that same amount of friction regardless of the pulling force.