TYPES OF FRICTION
Friction is classified into three types. They are
1) Static friction 2) Kinetic friction (or) Dynamic friction (or)sliding friction 3) Rolling friction
(A) STATIC FRICTION
"The resistance encountered by a body in static condition while tending to move under the action of an external force is called 'static friction' (f)". Static friction is equal and opposite to the applied force parallel to the contacting surfaces
Consider a body at rest on a rough horizontal surface. When an infinitesimally small force is applied on the body the body remains at rest. This is because of the frictional force of magnitude equal to the external force acting on the body in the opposite direction.
When the applied force is increased frictional force also increases equally until the body starts moving. When it is about to slide on the table, the static friction reaches a maximum value and it is equal and opposite to the applied force. Any further increase in the applied force makes the body slide on the table. The maximum value of static friction is called Limiting friction. (fL)
\(
f_{s_{\max } }
\) = fL
Note : Static friction opposes impending motion. The term impending motion means motion that would take place (but does not actually take place under the applied force, if friction were absent.
(B) KINETIC FRICTION OR DYNAMIC FRICTION
"The resistance encountered by a sliding body on a surface is known as kinetic friction or dynamic friction (fk) or sliding friction".
Consider a body sliding on a surface. It encounters a resistance to the motion due to which Buring the body eventually comes to rest.
The body moves with acceleration due to the resultant force on it. If the body slides with uniform way of velocity (a = 0) the resultant force on the body should be equal to zero. In other words the applied force required to keep the body moving with uniform velocity under friction, is numerically equal to the value of kinetic friction.
(C) ROLLING FRICTION
"The resistance encountered by a rolling body on a surface is known as Rolling friction".
A body like a ring or a spherical ball rolling without slipping over a horizontal plane will suffer no friction, in principle. At every instant, there is just one point of contact between the body and the plane and this instantaneous point of contact has no motion relative to the plane, if kinetic or static friction is zero and the body should continues to roll with constant velocity. We know, in practice, plied this will not happen and some resistance to motion cause does occur to keep the body rolling. When ever a emal body rolls over a surface at the point of contact,the body (as well as the surface) gets deformed. At the point of contact the flattened area element of the body tends to slide against the surface.
Because of the surface deformations, a rolling ball has to climb a hill as long as it is rolling. Thus sliding friction arises For the same weight, rolling friction is much smaller than static or sliding friction.
VARIATION OF FRICTION WITH APPLIED FORCE
Consider a block 'B' which is at rest on a horizontal table as shown in figure. A small pan is attached to the block by means of a thread passing over a frictionless pulley. When the weight in the pan is increased the applied force also increases, the static friction also increase in equal magnitude.
Let the static friction reach its maximum value for an applied force F1. This value of F1 is numerically equal to limiting friction fL. Therefore the net force on the body is zero and the body is in equilibrium. If the applied force is increased slightly more than fi the body starts sliding on the surface However, the magnitude of force required to keep the body sliding under friction with constant velocity is slightly less than the force required to start the motion. The friction in this condition is called dynamic friction. Hence dynamic friction is less than the limiting friction for the same body on the same surface. Even if the applied force is increased the dynamic friction remains constant.
Here we notice that until the static friction reaches its maximum value applied force is equal proportional to the frictional force. Thus the angle made by the straight line with X-axis is equal to 450.
Slope of the line "m"= tan\(\theta\)=tan450= 1.