Let's start with the definition of Static Friction since friction is an often misunderstood topic.
As with any definition of friction we must remember that friction is surface (interfacial) dependent, that is, friction is dependent on the types of surfaces under consideration for the surfaces in the system. Static Friction
is that force that opposes movement, i.e., there is no movement and the static friction force
holds an object motionless
unless a greater force is applied to overcome it which then results in Dynamic Friction (friction due to motion). Every system has a Static COF.Dynamic Friction
= Kinetic Friction = Sliding Friction which is a force opposing, but not preventing, the actual movement of objects between two surfaces. Every system has a Dynamic COF.
Coefficients of Friction (u
or Mu in the literature) are dimensionless values and are determined experimentally.
Dry surfaces (no lubricant present) means we will have high Coefficients of Friction (COF).
For example, a rectangular wood crate weighing 20 kg. on a wood floor will have to have a force greater than 70.8 N to move it horizontally since the opposing Friction force = u
*Normalforce = (0.3)*236 N = 70.8 N; where the Normal force is that force acting vertically down and Mu is the COF. Apply 71.8 N of force and the crate will begin to move.
Wet lubrication is the application of a substance known to reduce friction between two surfaces. Lubricated Friction results in motion of the crate with greater ease since the lubricant reduces the Dynamic COF. Apply some floor wax to the floor and the COF may be reduced to say 0.1, so now we can move the crate horizontally (from another room and a rope) ) with only 23.6 Newtons of force, Grandma may not appreciate the experiment and she may have you wax the whole floor.
In Step-Shift automatic transmissions and in limited-slip differentials with clutches, we want a wet lubrication
Dynamic Coefficient of friction Mu(V) to overcome stick-slip phenomenon and it is Velocity and Force dependent.
"Stick" means an opposing static force whereas "Slip" means an opposing dynamic force. Friction Modifiers in ATF and in limited-slip differentials with clutches provides a "compromise" solution between those two frictional forces. How they do this involves precise shearing forces at the molecular level of the FM chemistry and is beyond the scope of this short note.
Here is another view: The FM chemical compound modifies (changes) the Coefficient of Friction (COF) between the surfaces "on-the-fly"
depending on the relative speeds
of the rotating components and the applied pressure forces
. That is, it decreases OR increases friction depending
on the relative speeds of the rotating components and the applied pressure forces in order to prevent Stick-Slip which causes shudder.
Below is an actual Mu(V) plot for some experimental ATF Friction Modification (FM) compounds.