Friction
A force applied by a surface to any object sliding along that surface or subject to forces that would cause it to slide in the absence of friction. The force of friction will always resist the existing or intended sliding motion.
Motivation for Concept
It takes effort to get an object sliding along a surface, and sustained effort to keep the object moving once started. The effort required will depend on the surface characteristics and the object's characteristics. Sliding a wooden block along a tiled floor is much easier than sliding it along a rubber mat. The effort required will also depend upon the contact force between the object and the surface. Brushing sandpaper lightly across wood is easy, but when the sandpaper is pressed hard against the wood, movement requires substantial effort.
Static Versus Kinetic Friction
Friction has two basic manifestations that are qualitatively different.
friction which attempts to resist efforts to move an object that is currently at rest with respect to a surface. If possible, static friction provides just enough force to keep the object stationary, and no more. When the net force attempting to create sliding motion exceeds a certain limiting value proportional to the normal force exerted by the surface on the object, static friction will be unable to prevent motion. ">Static Friction: The specific manifestation of friction which attempts to resist efforts to move an object that is currently at rest with respect to a surface. If possible, static friction provides just enough force to keep the object stationary, and no more. When the net force attempting to create sliding motion exceeds a certain limiting value proportional to the normal force exerted by the surface on the object, static friction will be unable to prevent motion.
friction that is directly opposed to an object's sliding motion along a surface. The force of kinetic friction has a size independent of the speed of the object, and proportional to the normal force exerted on the object by the surface.">Kinetic Friction: The specific manifestation of friction that is directly opposed to an object's sliding motion along a surface. The force of kinetic friction has a size independent of the speed of the object, and proportional to the normal force exerted on the object by the surface.
Definition of Static Friction
If an object is at rest with respect to a surface, friction will attempt to resist efforts to start the object sliding along the surface. Friction has the goal of keeping the object static with respect to the surface. This static friction is a response force – it provides just enough force to keep the object stationary, and no more. Static friction is characterized by a limiting value. When the net force attempting to create sliding motion exceeds a certain value, static friction will be unable to prevent motion.
Defintion of Kinetic Friction
Whenever sliding motion is occuring, friction will apply a force that is directly opposed to the sliding motion. This force will have essentially constant size independent of the speed of the object for a given object sliding on a given surface. The size of the friction force will depend, however, on the contact force existing between the object and the surface and also on the material characteristics of the surface and the object.
Quantitative Model of Static Friction
The Limiting Size of Static Friction
The basic characteristics of static friction are well approximated by the limit expression:
\begin
[ F_
\le \mu_
N]\end
where μs is the coefficient of static friction. The coefficient of static friction is a dimensionless number, usually less than 1.0 (but not required to be less than 1.0). Rough or sticky surfaces will yield larger coefficients of friction than smooth surfaces. N is the normal force exerted on the object by the surface which is creating the friction, which is a measure of the strength of the contact between the object and the surface.
Determining the Force of Static Friction
To determine the force of static friction on an object, calculate the net force in the absence of any friction and compare it to the limiting value of the friction force. If the maximum static friction force is larger than the net force in the absence of friction, then friction will provide the force necessary to make the total net force equal zero assuming that the net force has no component perpendicular to the surface. If, however, the maximum static friction force is less than the net force in the absence of friction, static friction will not apply (it will not provide a force). Instead, kinetic friction will apply.
It is very important to remember that for an object at rest on a surface and subject to no forces that would act to cause sliding, the static friction force will be zero! (The object will not move without friction, so friction "has no job to do".)
Quantitative Model of Kinetic Friction
Magnitude
For an object that is already sliding along a surface or is accelerating from rest on a surface, the size of the friction force will be given by:
\begin
[ F_
= \mu_
N]\end
Note that the size of the kinetic friction is fixed by the normal force and the coefficient. There is no limit expression as there was for static friction. Thus, it is not necessary to consider the complete net force to find the friction force for the kinetic case.
where μk is the coefficient of kinetic friction. The coefficient of kinetic friction is a dimensionless number, usually less than 1.0 (but not required to be less than 1.0). Rough or sticky surfaces will yield larger coefficients of friction than smooth surfaces. N is the normal force exerted on the object by the surface which is creating the friction, which is a measure of the strength of the contact between the object and the surface.
The coefficient of kinetic friction for a given object on a given surface will usually be different than the corresponding coefficient of static friction. It is usually the case that μk < μs.
The fact that μk is generally less than μs has important consequences for cars. Antilock brakes are specifically designed to prevent skids, which change the tire-road friction from static to kinetic. Changing braking friction to kinetic by skidding reduces the force of friction and so the effectiveness of the braking.
Direction
There are two possibilities to consider when determining the direction of kinetic friction:
- For a sliding object, the direction of the kinetic friction must be opposite to the direction of the velocity.
- For an object just beginning to slide (the object still has zero velocity) then the friction must oppose the acceleration.