Friction
| Explain the concept for friction |
| Friction 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 r elative 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) |
| The 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 Life |
| Realize the advantages and disadvantages of friction in daily life |
| Thefriction force has several advantages which includes the following: |
| It helps in walking process (because friction force stop us from slipping over). |
| 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 tyr es |
| it helps in car braking system. Brakes rely on friction between the brake drum or pads |
| and the wheels. In a bicycle, there ar e brake pads which clamp onto the wheel to slow it down. |
| Disadvantages of friction |
| Frictional force has several dis-advantages which includes the following: |
| Cause machinery to heat up and can cause wear and tear. |
| Cause machinery to be less efficient. |
| Cause machinery to produce noise. |
| Methods of Reducing Friction |
| Describe methods of reducing friction |
| Thedifferent methods ofreducing friction between surf aces includes the following: |
| Polishing:Polishing the surface, reduces the irregularities and hence, reduces friction. |
| Lubrication:It provides a layer of the lubricant on which the bodies can easily slide. |
| 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. |
| 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 |
| 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. |
| 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. |
| 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 eff ective lubricant in machine. |
| Identify types of friction |
| There are three main types of friction in daily life which include the following: |
| 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. |
| Limiting friction: numerically equal to the minimum external force required to make a |
| body just move over another. |
| Dynamic friction: numerically equal to the force of opposition when a body is moving |
| Determine limiting friction |
| Limiting 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 |
| A 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? |
| Force = coefficient of friction ×mass × acceleration due to gravity |
| The five laws of friction |
| When an object is moving, the friction is proportional and perpendicular to the normal |
| Friction is independent of the area of contact. |
| The coefficient of static friction is slightly gr eater than the coefficient of kinetic friction. |
| Within rather large limits, kinetic friction is independent of velocity. |
| Friction depends upon the nature of the surfaces in contact. |
| The Coefficient of Friction |
| Determine the coefficient of friction |
| Coefficient of friction is the ratio of the frictional force that acts between two objectsin contact to |
| Types of Coefficient of friction |
| There 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 r eaction. |
| Coefficient of dynamic friction U |
| Is the ratio of the dynamic friction to the normal reaction that acts onthe body. |
| Laws of Friction in Solving Problems |
| Apply laws of friction in solving problems |
| Demonstration to determine coefficient of dynamic friction. |
| The coefficient of dynamic friction U is |
| U = Frictional force, Fr/Normal Reaction, R |
| U= Fr/R_______________________________ ( I) |
| By Resolving forces we get |
| Fr = W SinQ __________________________(II) |
| R= W CosQ ________________________(III) |
| Put Eqn (iii) ad (ii) into eqn (i) |
| TanQ = (AB/CB) FROM Angle ABC |
| Find the static f riction 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. |
| Limiting Friction, Fr = 50N |
| Normal Reaction, r =(10 X 10 ) = 100N |
| Coefficient of static friction; Us |
| Therefore,coefficient of static friction, Us = 0.5 |
| A 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/R |
| Coefficient of friction, U = 0.56 |
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