Modes of Friction
by Ron Kurtus (11 August 2016)
For a given pair of objects or materials, there are different modes of friction, depending on the relative motion of the object.
When a force pushes on an object, but there is no resulting motion, the object is in the static mode of friction. When there is relative motion between the objects or materials they are in the kinetic or dynamic mode of friction.
Joining the static and kinetic modes is the transitional phase or mode of friction. This is where the objects or materials change from static to moving with respect to each other or from kinetic to the static mode.
Questions you may have include:
- What is the static mode of fiction?
- What is the transitional phase?
- What is kinetic friction?
This lesson will answer those questions. Useful tool: Units Conversion
Static mode of friction
The static mode of friction is when the resistance from friction is greater than the force trying to move the object or material, such that there is no resulting motion. During this mode of friction an object will not slide, roll, or move.
During the static mode, the force F applied on an object or material is less than the resistive force of friction Fr (written as F < Fr), such that there is no motion and the objects remain static with respect to each other.
Note: Some sources call static friction a type of friction, along with sliding, rolling, and fluid friction. However, static friction can occur in all three of those types.
(See Types of Friction for information on the three types.)
When you apply a force on a dry object to slide it across another dry object, static friction prevents the object from moving.
Likewise, if the force or torque on a wheel must be large enough to overcome the static friction and allow the wheel to roll. Also, static friction can prevent of fluid from moving along a solid or fluid surface.
Static fluid friction is very small and thus difficult to measure. However, a small enough force will not move a lubricated object or an internal fluid.
When the force applied on an object is sufficient to overcome the static friction, the object starts to move. This does not happen instantaneously. There is a transition from static friction to kinetic friction, where the object starts to move and the coefficient of friction changes.
Note that this also happens in going from moving to static.
There have been a few studies and measurements of what happens in the transition phase of sliding friction but none that I know of when rolling or fluid static friction transitions to kinetic. This probably because the frictional forces are so small in those cases.
Kinetic mode of friction
The kinetic mode of friction is when the applied force is greater than the resistive force (F > Fr). This causes the object or material to move across the other object or material.
(Kinetic friction is sometimes called dynamic or moving friction.)
This means a solid object will slide along another solid object, a wheel will roll along an object or material, a fluid will move along a solid object, a solid object will move over a fluid, and a layer of a fluid will move with respect to some other fluid layer.
What is interesting is that the kinetic coefficient of friction—and thus the kinetic frictional force—is less than the static coefficient.
When you apply a force on a dry object to slide it across another dry object, kinetic friction works to slow the object.
When a wheel is rolling, there is rolling friction or resistance to the motion. Rolling kinetic friction is considerably less than sliding friction. However, if the wheel is somewhat soft—such as with a tire—or the ground is soft—such as driving through mud—rolling friction can be large.
Kinetic fluid friction is very small and thus difficult to measure. However, a small enough force will not move a lubricated object or an internal fluid.
Friction can occur in the static mode, where is no motion, and in the kinetic mode. where there is relative motion between the objects or materials.
Between the static and kinetic modes, there is a transitional phase of friction, where the objects or materials change from static to moving with respect to each other.
Try to define things clearly in your mind
Resources and references
Friction Resources - Extensive list
Types of Friction - CK12.org
Friction Concepts - HyperPhysics
Friction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub. (1995) $89.95
Physics of Sliding Friction (NATO Science Series E:) by B.N. Persson, E. Tosatti; Springer Pub. (1996) $358.00
Questions and comments
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