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#1
Monday, August 25, 2008
 Senior Member Join Date: Apr 2007 Location: karachi & hyderabad Posts: 528 Thanks: 155 Thanked 436 Times in 213 Posts

There are advantages and disadvantages of friction. Since friction is a resistance force that slows down or prevents motion, it is necessary in many applications to prevent slipping or sliding. But also, it can be a nuisance because it can hinder motion and cause the need for expending energy. A good compromise is necessary to get just enough friction.

Questions you may have include:
• How is friction necessary?
• How is friction a nuisance?
• What is a good compromise for using friction?

Important uses
In some situations, friction is very important and beneficial. There are many things that you could not do without the force of friction.

Walking
You could not walk without the friction between your shoes and the ground. As you try to step forward, you push your foot backward. Friction holds your shoe to the ground, allowing you to walk. Consider how difficult it is to walk on slippery ice, where there is little friction.

Writing
Writing with a pencil requires friction. You could not hold a pencil in your hand without friction. It would slip out when you tried to hold it to write. The graphite pencil led would not make a mark on the paper without friction.

A pencil eraser uses friction to rub off mistakes written in pencil lead. Rubbing the eraser on the lead wears out the eraser due to friction, while the particles worn off gather up the pencil lead from the paper.

Driving car
Your car would not start moving if it wasn't for the friction of the tires against the street. With no friction, the tires would just spin. Likewise, you could not stop without the friction of the brakes and the tires.

Problems from friction
Friction can cause problems or be a nuisance that you try to minimize.

Makes movement difficult
Any time you want to move an object, friction can make the job more difficult. Excess friction can make it difficult to slide a box across the floor, ride a bicycle or walk through deep snow.

An automobile would not move forward very well unless its friction was not reduced. Oil is needed to lubricate the engine and allow its parts to move easily. Oil and ball bearings are also used in the wheels, so they will turn with little friction.

Wastes energy
In any type of vehicle--such as a car, boat or airplane--excess friction means that extra fuel must be used to power the vehicle. In other words, fuel or energy is being wasted because of the friction.

Fluid friction or air resistance can greatly reduce the gas mileage in an automobile. Cars are streamlined to reduce friction. But driving at highway speeds with your windows open can create enough drag on the car to greatly reduce your gas mileage.

Heats parts
The Law of Conservation of Energy states that the amount of energy remains constant. Thus, the energy that is "lost" to friction in trying to move an object is really turned to heat energy. The friction of parts rubbing together creates heat.

You've seen how people will try to start a fire by vigorously rubbing two sticks together. Or perhaps you've seen an automobile spin its wheels so much that the tires start to smoke. These are examples of friction creating heat energy. Just rub your hands together to create the same effect.

Besides the problem of losing energy to heat, there is also the threat of a part overheating due to friction. This can cause damage to a machine.

Wears things out
Any device that has moving parts can wear out rapidly due to friction. Lubrication is used not only to allow parts to move easier but also to prevent them from wearing out. Some other examples of materials wearing out due to friction include the soles of your shoes and a pencil eraser.

Compromise
A compromise is needed between too much friction and not enough.

Fro example, if you wanted to slide a heavy box across the floor, you would want to reduce the friction between the box and the floor, so that it would be easy to move. Lubrication of some sort is often a way to reduce friction.

But you would also want to increase the friction of your shoes on the floor, so that you would be able to get good traction and be able to push effectively. Soles made of rubber material that include treads can reduce slipping when walking or running.

Summary
Friction is necessary in many applications to prevent slipping or sliding. But also, it can be a nuisance because it can hinder motion. A good compromise is necessary to get just enough friction or a proper combination of frictions.

regards

faryal shah
__________________
Go confidently in the direction of your dreams. Live the life you have imagined.
 The Following User Says Thank You to Faryal Shah For This Useful Post: eram ismail (Friday, January 16, 2009)
#2
Monday, August 25, 2008
 Senior Member Join Date: Apr 2007 Location: karachi & hyderabad Posts: 528 Thanks: 155 Thanked 436 Times in 213 Posts

Causes of Friction

The causes of the resistive force of friction are molecular adhesion, surface roughness, and the plowing effect. Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Trying to slide objects against each other requires breaking these adhesive bonds. For years, scientists thought that friction was caused by surface roughness, but recent studies have shown that it is actually a result of adhesive forces between the materials. But surface roughness is a factor when the materials are rough enough to cause serious abrasion. This is called the sandpaper effect. When one or more of the materials is relatively soft, much of the resistance to movement is caused by deformations of the objects or by a plowing effect.

Questions you may have include:
• How does adhesion cause friction?
• How does surface roughness cause friction?
• How do deformations cause friction?

When two objects are brought into contact, many atoms or molecules from one object are in such close proximity to those in the other object that molecular or electromagnetic forces attract the molecules of the two materials together. This force is called adhesion. Trying to slide one object across the other requires breaking these adhesive bonds. Adhesion is the essence of friction.

You've seen a water drop adhere to a window pane. The force of friction prevents this liquid from sliding down the solid material. But most cases of friction you see concern a solid object sliding or moving against another solid.

Sliding objects against each other requires breaking these millions of contact points where the adhesion force takes effect, only to result in millions of new contact points of adhesion.

Sticky materials
Some solid materials may have a composition that greatly increases their adhesion and makes them even "sticky" to the touch. This stickiness greatly increases the fiction. Rubber and adhesive tape are examples of sticky materials that have this type of friction.

Fluids
Fluids often exhibit molecular adhesion, increasing the friction. This adhesion force is often seen in the capillary effect. This is where water will be pulled up a glass tube by the forces of molecular adhesion. That same force can slow down fluid motion.

One example is how a coin will easily slide down a ramp. But if you wet the coin, it will stay in place. That is because of the molecular friction of the fluid on the hard surfaces.

The motion of two fluids or two sections of a fluid against each other is also slowed down by the molecular attraction factor. This type of fluid friction is usually not considered as friction and is studied under the complex field of fluid dynamics.

Surface roughness
All solid materials have some degree of surface roughness. If you looked at what seems to be a smooth surface under a high-powered microscope, you would see bumps, hills and valleys that could interfere with sliding motion.

At one time it was thought that the surface roughness of materials was the cause for friction. In reality, it only has a small effect on friction for most materials.

If the surfaces of two hard solids are extremely rough, the high points or asperities can interfere with sliding and cause friction because of the abrasion or wear that can take place when you slide one object against the other. This is the "sandpaper effect" where particles of the materials are dislodged from their surfaces. In such a case, the friction is caused by surface roughness, although the adhesion effect still plays a part in the abrasion.

Deformations
Soft materials will deform when under pressure. This also increased the resistance to motion. For example, when you stand on a rug, you sink in slightly, which causes resistance when you try to drag your feet along the rug's surface. Another example is how rubber tires flatten out at the area on contact with the road.

When materials deform, you must "plow" through to move, thus creating a resistive force.

When the deformation becomes large, such that one object sinks into the other, streamlining can affect the friction, similar to what happens in fluid friction.

Summary
The causes of the resistive force of friction are molecular adhesion, surface roughness, and the plowing effect. Adhesion is the molecular force resulting when two materials are brought into close contact with each other. Surface roughness is a factor in friction when the materials are rough enough to cause serious abrasion. When one or more of the materials is relatively soft, much of the resistance to movement is caused by deformations or a plowing effect.

regards

faryal shah
__________________
Go confidently in the direction of your dreams. Live the life you have imagined.
 The Following 2 Users Say Thank You to Faryal Shah For This Useful Post: ASP imran khan (Monday, August 16, 2010), Qaiserks (Wednesday, April 01, 2009)