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Why do rough surfaces increase friction?Give 3 ways wherein increased and decreased...

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dcutesarah | Student, Grade 9 | eNotes Newbie

Posted November 4, 2009 at 7:03 PM via web

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Why do rough surfaces increase friction?

Give 3 ways wherein increased and decreased friction are used.

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giorgiana1976 | College Teacher | Valedictorian

Posted November 4, 2009 at 8:04 PM (Answer #1)

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First, it is obvious that the roughness of the two surfaces plays an important role - as the rough surfaces are greater, the greater will strike , opposing the motion. However as we think of the roughness of two surfaces in contact and how they are deformming and start hitting each other, all sorts of problems occur:

  • Although the area has a large roughness, if the other is very smooth and sufficiently rigid it will "float" on top "mountains" rough surface;
  • Given two surfaces with known roughness  is important how much  the materials are prone to bruising - a more rigid material tends to form fewer "mountains"=elevations, and it will make them easier than a more pliable material makes ;
    Even if two materials with identical surface roughness and elastic qualities, it matters how easy they break, because according to this, "mountain" formed at contact surface,break easier or more difficult.

Give the above factors and still others, the most convenient solution is to simultaneously consider the qualities of both areas, determining the coefficient of friction for each combination. The coefficient of friction is denoted by μ and is a dimensionless measure.

Ff=μ *Fn, where Ff=friction force, Fn=the force normal to the surface

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mamta88 | Student, Undergraduate | eNoter

Posted November 4, 2009 at 11:19 PM (Answer #2)

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In rough surfaces there are holes and cracks which contain air in them.When a body slides on rough surface,air in the holes is pushed out to create vaccum and holds the body and don't let it to slide..Hence ,rough surfaces increase friction.

3 ways where increase friction is used are:

1.In vehicles,tyres are retreated to increase the friction.

2.Mountaineers use hooked shoes to increase the frriction.

3.Feet of the horses are fitted with horse shoes to increase the friction.

3 ways wherein decrease friction is used are:

1.Skating ranks are made plain and smooth to decrease the friction.

2.iceskates are made smooth and plain for easy skating.

3.Ball bearings are oiled to decrease the friction in machines.

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krishna-agrawala | College Teacher | Valedictorian

Posted November 5, 2009 at 1:35 PM (Answer #3)

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Friction refers to the forces that resist the relative motion of two surfaces sliding against each other. There are three major causes of friction - molecular adhesion, ploughing effect, and surface roughness.

Molecular adhesion refers to the forces that attract molecules to each other. This is the most important cause of friction. Ploughing refer to the deformation of the surface of a softer material due to a harder material sinking in to it. Thus some force is needed for the changing deformation as the two surface i contact slide against each other. The surface roughens resists relative movement of surface due to higher points of one surface obstructing the movement of higher points in the other surface. To overcome this obstruction either the higher points must be broken down. This process is called abrasion. Effect of surface roughness is also overcome by the higher points of one surface moving up and down the higher points of other surface. Both these processes require application of some force, which contributes to the total frictional force.

Increased surface roughness means that the average height and size of these high points on the surface is increased. This requires higher abrasive force to break them or to move up and down over them. This is why rougher surfaces have higher friction.

Three most common method of reducing friction are:

  1. Making surfaces smooth
  2. Applying lubrication between surfaces
  3. Introduce a rolling member between two surfaces eliminate direct contact between two surfaces. The most common form if this is seen in use of rolling bearings.

Three most common method of reducing friction are:

  1. Increase roughness of surface. For example some small knobs to be adjusted by movement of fingers have knurled surfaces to prevent friction.
  2. Use of softer material such as a piece of cloth or a rubber sheet to avoid slipping. Foe example when cap of a bottle is stuck, we may use wrap piece of rough cloth on the cap to increase its roughness. This method works by increasing the ploughing effect,
  3. Increase the normal pressure pressing the two surfaces together.

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