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When an object with some mass is placed on a surface (non smooth), and when it is pushed or pulled, it usually experiences a resistive force which acts in a direction opposite to the push or pull (or strictly speaking, it makes and angle of 180 degrees with the horizontal component of the applied force).This resistive force is called friction.
For example, when we try to push a table across a room, we experience a resisting force which makes our work much more difficult, and this is the frictional force.
When an object is stationary on a horizontal surface, due to friction, it offers a resistance whenever some horizontal force is appplied to it. This friction is upto a certain limit, proportional to the horizontal force applied, and is known as static friction, and during this time the body remains stationary (does not move). The maximum possible static friction between a particular body and a particular surface is given by Fmax stat. = kstat.N where Fmax stat. is the maximum possible static friction for that body and that particular surface, kstat. is a constant of proportionality called co-efficient of static friction and N is the normal reaction force experienced by the body.
NOTE: Notice that neither the surface area of contact nor the horizontal force on the body affects the maximum static friction.
When the horizontal force applied exceeds a certain limit, the body sort of "breaks away" and begins to move. The frictional force drops sharply, and from this point onwards, it remains approximately constant.
This friction is now known as kinetic friction and for a particular body on a particular surface is given by Fkin. = kkin.N where Fkin. is the kinetic friction, kkin. is a constant of proportionality
called co-efficient of kinetic friction and N is the normal reaction force on the body.
NOTE: Notice that here too, neither the surface area of contact nor the horizontal force on the body affects the maximum static friction.
The graph below illustrates the relation between the applied horizontal force and the frictional force on a particular body
on a particular surface.
Here, the x-axis represents the applied horizontal force and the y-axis the frictional force.
The gray line represents the static frictional force, and throughout this range, the body does not move.
As the "Breakaway point" is reached the frictional force drops rapidly, and the body begins moving.
The red line represents the kinetic frictional force, and is approximately constant. It is also much less than
the maximum static frictional force.
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