Chapter 12 - Friction

FRICTION- It is a force which resists the relative motion of surfaces.

• All types of surface like solid, liquid and air offer the force of friction.

• This force of friction always opposes the applied force.

• Examples - A moving tyre on the ground stops after covering a certain distance because surface of ground offers opposite direction friction to the surface of wheel in motion.
  Gently pushing a book on a table. It stops after moving some distance because of friction between the surfaces of book and table.

 FACTORS AFFECTING FRICTION

• Friction is caused by the irregularities on the two surfaces in contact.
• Even those surfaces which appear very smooth but microscopic view show those irregularities.
• The irregularities of two surfaces get interlocked. We have to apply force to remove the interlocking.
• So the force of friction is greater if a rough surface is involved.
• Friction force will increase with increase in weight.
  Example: It is easier to dragging a mat when nobody is sitting on it than a person is sitting on it because of friction increases with increase in weight.
• Rougher surface offers more force of friction.

FRICTION: A NECESSARY EVIL

Friction is a necessary evil because it is both harmful and beneficial effects in everyday life.
Examples

• Soles of shoes have grooves on them. Grooved soles provide more friction to the ground which gives better grip when we walk.
• We can walk easily because ground offers friction. So, friction is necessary even in a simple task like walking.
• It is difficult task to move on a wet muddy track, or wet marble floor. This is because these surfaces offer very small friction to the surface of feet of a person.

Increase of friction

• Friction is desirable in many situations.
• Friction is necessary for us; therefore friction is increased to accomplish some of important task.
• Examples-
  (a) Treaded tyres of vehicle increase friction. Vehicle with treaded tyres provides mores friction which gives better grip with the road.
  (b) Kabaddi players rub their hands with soil for a better grip of their opponents.

(c)  Javelin thrower rubs some powder on his palm to get better grip on the javelin.
(d) Soles of shoes are treaded to increase friction between sole and the ground.

Reduction of friction

• In Some cases, friction is not desirable. We need to reduce it.
• The substance which helps to reduce friction is called
• Lubricants make a thin layer over surface and facilitate smooth movement.
• Example-In the engines of vehicles use of lubricants increases the efficiency as parts are not rubbed directly each other because of thin layer of lubricant.
• In many machines, graphite powder, oil, grease are used as lubricant for reducing friction.
• Few drops of oil are poured on the hinges of a door for reducing friction, the door moves smoothly.

TYPES OF FRICTION

1. Sliding Friction

This friction comes in to action when a surface slides over another surface and resist its motion is called sliding friction.
Example: sliding of book or a box over ground.

2. Rolling Friction

This friction comes into action when a surface rolls over another surface and resist its motion is called rolling friction.
Rolling friction is smaller than sliding friction. Due to this roll anything over the ground is easier than to slide. Example-  The use of ball bearings between hubs and the axles of ceiling fans and bicycles.

FLUID FRICTION

• In science, Gases and liquids are known as fluid.
• Similar to solid, fluids also provide friction. The friction of fluid increases with the speed of object moving through it and speed of fluid.
• The frictional force offered by fluids is called
• Objects which are made to move in fluid are made streamlined shape to reduce friction offered by fluid. The shape of aeroplane, ship, boat, birds and fish are made streamlined shape. A shape which is pointed on both ends and wide at the middle is called streamlined shape which facilitates easier movement through fluids.