Hai Sindhuja

Frictional Forces:

It is caused by direct contact between surfaces. However, while the normal force is always perpendicular to the surface, the frictional force is always parallel to the surface. To fully describe the cause of friction requires knowledge beyond the scope of classical mechanics. For our purposes, it is enough to know that friction is caused by electrical interactions between the two surfaces on a microscopic level. These interactions always serve to resist motion, and differ in nature according to whether or not the surfaces are moving relative to each other. 

Static Frictional Forces:

Consider the example of two blocks, one resting on top of the other. If friction is present, a certain minimum horizontal force is required to move the top block. If a horizontal force less than this minimum force is applied to the top block, a force must act to counter the applied force and keep the block at rest. This force is called the static frictional force, and it varies according to the amount of force applied to the block. If no force is applied, clearly there is no static frictional force. As more force is applied, the static frictional force increases until it reaches a certain maximum value; once the horizontal force exceeds the maximum frictional force the block begins to move. The frictional force, defined as Fsmax, is conveniently proportional to the normal force between the two surfaces:

F_k = \mu_s FN

Kinetic Frictional Forces:

Once a force is applied to an object that exceeds Fsmax, the object begins to move, and static frictional forces no longer apply. The moving object does still experience a frictional force, but of a different nature. We call this force the kinetic frictional force. The kinetic frictional force always counteracts the motion of the object, and is independent of speed. No matter the speed of the object (as long as v≤ 0) it experiences the same frictional force. Also, for the same reasons as explained with static friction, the kinetic frictional force is proportional to the normal force:

F_k = \mu_k FN

Affects of friction:

Newtons first law of motion:

it slows down the moving object

Newtons second law of motion:

Friction decreases the acceleration compared to a frictionless situation.

Newtons third law of motion:

Friction is encompassed by Newton's laws, as opposed to operating out side of it. If a block slides down a wedge, there is friction acting against the block up the slope. From the wedge's perspective, the block produces a frictional force against it, trying to drag it downwards along the slope.