Physics form three topic two

 Concept of Friction The Concept of FrictionExplain the concept for frictionFriction

is the force which opposes (resists) motion of the body for example: If
a block of wood is placed on a table it remains at rest Unless it is
acted upon by an external force.The
applied force must exceed the maximum friction force between the block
of wood and the table if the block is to be moved along the table.

Friction force between two surface in contact exists only when there is relative motion between the two contacting surface.The
friction between two surfaces exists because of the nature of the
surface of the bodies in contact. (Strictly speaking all surfaces are
rough because they are made up of peaks and valleys)DemonstrationThe
peak and valleys on the surfaces of bodies may be due to a random
arrangement of coarse particles, for example the surface of a gridding
stones.Smooth surfaces exert almost no friction. What happens when two objects are pushed?When
one body is pushed against another, the peaks of one surface have to
rise up over the peaks of the other surface. Hence opposition to motion
or friction occurs the force acting to restore the bodies to their
original position.The Advantages and Disadvantages of Friction in Daily LifeRealize the advantages and disadvantages of friction in daily lifeAdvantages of FrictionThefriction force has several advantages which includes the following:

1. It helps in walking process (because friction force stop us from slipping over).
2. It
   helps cars to move onroads easily due to friction between car tyres and
   road (makesa car stay on road) NOTE: Most car tyres have deep dreads to
   increase friction between the tyres and the road surface.
3. it
   helps in car braking system. Brakes rely on friction between the brake
   drum or pads and the wheels. In a bicycle, there are brake pads which
   clamp onto the wheel to slow it down.

Disadvantages of frictionFrictional force has several dis-advantages which includes the following:

1. Cause machinery to heat up and can cause wear and tear.
2. Cause machinery to be less efficient.
3. Cause machinery to produce noise.

Methods of Reducing FrictionDescribe methods of reducing frictionThedifferent methods ofreducing friction between surfaces includes the following:

1. Polishing:Polishing the surface, reduces the irregularities and hence, reduces friction.
2. Lubrication:It provides a layer of the lubricant on which the bodies can easily slide.
3. Use
   of ball-bearing or roller bearing:Roller bearing help to convert
   sliding friction into a milder form-rolling friction. Rolling friction
   is lesser than sliding friction.
4. Streamlining:Fast cars, boats,
   planes etc., have a streamlined body. This is to allow air (or water in
   the case of boats) to easily flow by, without offering much resistance.
   Flying birds have streamlined bodies.
5. Use of correct combination
   of surfaces in contact:Use of alloys on moving and sliding parts
   reduces friction because alloys have a low coefficient of friction.

Roller

• It is a simple cylinder on which a body to be pulled RESTS.
• It is used to eliminates sliding friction.
• Conveyor belts use metal rollers.

Wheels

• These
  arerollers that are fixed to a moving body, held in place by cylinders
  or axles which are threaded in their centers. Example; a trailer towed
  by vehicle.
• Wheels are used to eliminate sliding friction.
• They
  are used to reduce friction (the friction between surfaces can be
  reduced by smoothing and polishing the surfaces in contact).
• In
  order to make surfaces slippery, a lubricant such as oil or graphite is
  used. Oil and Grease are commonly used invehicles and machines to reduce
  friction between moving parts. In engines, differential air is also an
  effective lubricant in machine.

Types of FrictionTypes of FrictionIdentify types of frictionThere are three main types of friction in daily life which include the following:

1. Static
   friction: an opposing forcebetween two solid objectsat rest.In simple
   words, when there is no relative motion between two solid objects in
   contact with each other, we describe the frictional force between them
   asstatic.
2. Limiting friction: numerically equal to the minimum external force required to make a body just move over another.
3. Dynamic friction: numerically equal to the force of opposition when a body is moving over the rough surface.

Limiting FrictionDetermine limiting frictionLimiting
friction is equal to the minimum external force required to make a body
just move over one another. Is the maximum possible value of static
friction. It is the frictional force that must be overcome before an
object starts moving. The coefficient of friction will be the same for
all masses. The limiting frictional forces is independent of applied
force but depends on nature of surface.Example 1A
block of mass 20 kg is pulled along a horizontal surface. If the
coefficient of friction is 0.4, what force is acting on the block?SolutionForce = coefficient of friction ×mass × acceleration due to gravityF = 0.420×10 = 80N.Laws of FrictionLaws of FrictionState laws of frictionThe five laws of friction

1. When an object is moving, the friction is proportional and perpendicular to the normal force (N).
2. Friction is independent of the area of contact.
3. The coefficient of static friction is slightly greater than the coefficient of kinetic friction.
4. Within rather large limits, kinetic friction is independent of velocity.
5. Friction depends upon the nature of the surfaces in contact.

The Coefficient of FrictionDetermine the coefficient of frictionCoefficient of friction is the ratio of the frictional force that acts between two objectsin contact to the normal reaction, R.

Types of Coefficient of frictionThere are three main types of coefficient of frictions which includes the following:Coefficient of static friction is the ratio of the static friction to the normal reaction.Us =Frs/RCoefficient of dynamic friction USIs the ratio of the dynamic friction to the normal reaction that acts onthe body.Ud = Frd/RLaws of Friction in Solving ProblemsApply laws of friction in solving problemsDemonstration to determine coefficient of dynamic friction.Method of calculationThe coefficient of dynamic friction U isU = Frictional force, Fr/Normal Reaction, RU= Fr/R_______________________________ ( I)By Resolving forces we getFr = W SinQ __________________________(II)R= W CosQ ________________________(III)Put Eqn (iii) ad (ii) into eqn (i)U =W SinQ/Wc osQU =W SinQ/WCosQBut SinQ/CosQ=TanQU=TanQThusTanQ = (AB/CB) FROM Angle ABCButU = TanQu = (ab/cb)Example 2Find
the static friction between a block of wood of mass 10kg and the table
on which it rests. A minimum force of 50N is required to make the block
just move on the table top.

SolutionLimiting Friction, Fr = 50NNormal Reaction, r =(10 X 10 ) = 100NCoefficient of static friction; UsUs = (Fr/ R)Us = (50/100)Us = 0.5Therefore,coefficient of static friction, Us = 0.5Example 3A
mass is placed on an Inclined plane such that it moves at a constant
speed when tapped tightly. If the angle the plane makes with the
horizontal is 30º. Find the coefficient of dynamic friction.

The Coefficient of friction U =Fr/RFr = WSinQ/R = WCosQAt EquilibriumU =WSinQ/wCosQU = (W/W) (SinQ/CosQ)But (SinQ/CosQ) = TanQU = Tan 30ºU = 0.56Coefficient of friction, U = 0.56