Frictional force opposes relative motion.If two bodies are in the state of rest and a force is applied to move one body over the other body and the applied force is not sufficient to move the body,the bodies will be in the state of rest.Still there will be a frictional force acting against the motion.This frictional force is called static frictional force.Static frictional force is self adjusting force. Until applied force reaches maximum frictional force, with the increase in applied force,frictional force also increases and the body will be in the state of rest.Once the applied force reaches the limiting friction,static frictional remains constant and body starts moving.The frictional force acting between the bodies that are in the state of motion is called dynamic or kinetic friction.
Once if the body starts moving, there is no enough time for interlocking of irregular surfaces in contact and hence kinetic frictional force is less than kinetic frictional force.Even with the increase of applied force,kinetic friction remains constant.
A graph is drawn taking applied force on x axis and frictional force on y axis.Until the body is at rest, the graph varies such that with the increase in applied force also increases.The graph reaches its maximum at limiting friction.From here on wards the body starts moving and the graph comes down a little bit as kinetic frictional force is lesser than static friction.The graph further remains parallel to the applied force that is x axis.
If any one body is rolling over the other body, there is less area of contact between the bodies and hence interlocking is less.Hence rolling frictional force is even less than kinetic frictional force.
The frictional force is directly proportional to normal reaction and the proportionality constant is called coefficient of friction.As there are three types of frictions, there are three types of coefficient of friction respectively.They are called coefficient of static friction,coefficient of kinetic friction and coefficient of rolling friction. Coefficient of friction is a measure of nature of frictional surface. Hence coefficient of static friction is higher than coefficient of kinetic friction and coefficient of rolling friction is even less than that of coefficient of kinetic friction.
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