Short notes-2 of chapter-3(Force) of class-6

 *Kinds of Frictional Forces:

- Static friction

    ●The maximum opposing force between the object and the surface in contact with it,so long the object remains stationary even on applying the external force.

      ●static friction is self adjusting in nature.

      ● static friction is maximum.

- Sliding friction

   ● The frictional force  exerted by the surface on the object while it is sliding.

    ●It is equal to the force required to keep the object in motion over the surface.

    ●It is less than the static friction but more than the rolling friction.

-Rolling friction

   ●When an object rolls over a surface,the force which opposes the rolling motion of the object.

   ● It is minimum among three.

    ●The vehicles like cars , trucks , scooters, bicycles, and luggage trolles etc., are provided with wheels because rolling friction is minimum compared to sliding and static friction.

*Disadvantages of friction:

-It opposes the motion of a body so as to decrease efficiency.

- It causes wear & tear in the moving parts.

-It produces heat.

Because of frictional force it is not possible to have a 100% efficient machine.

*Methods or ways of reducing friction:

-By making the surfaces smooth by polishing.

-By use of lubricants such as oil,grease & spreading fine powders.These fills the fine pores or depressions in the surfaces and thus reduces friction.

-By the use of ball bearings in place of wheels and axles because rolling friction is much less than then the sliding friction.

- By streamlining a moving body.special shape given to a body in which the front portion of the body is made sharp or pointed as compared to its rear portion, to reduce friction.eg arrow,aeroplane,fish,birds,boat etc are streamlined bodies.

*Advantages of friction:

-Frictionless world would not at all be convenient because to move a body there must be a some minimum frictional force between the surface & the moving body.In absence of that minimum frictional force body may slip or skid.

-Burning of a match stick due to heat produced.

- Braking system of vehicles to slow down its speed and finally stops it.

-Writing is possible due to friction .

*Methods or ways of reducing friction:

-By making  the surfaces rough.eg Boots of a footballer,atheletes uses spike shoes for running etc.,

-By using dry surfaces .eg during tug of war in your school students uses dry soil to make their hand dry for a better grip.

-By increasing the weight.Its well known fact that to move a heavy body, more force is required to over come its frictional force.