Why static friction is a reactionary force and kinetic friction is not

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.