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Force of Friction

According to Newton's first law of motion every body continues to be in its state of rest or uniform motion along a straight line unless an external force acts on it. This means that a ball moving on a horizontal surface should continue to move indefinitely as no force is acting on it. But practically this does not happen. After sometime the speed of the ball decreases and eventually it comes to rest. Now the question is what stopped the ball? The ball stops because the surface opposes the motion of the ball i.e., the ball experiences a force, which opposes its motion. This force is called the force of friction or simply friction.

The force of friction or simply friction is that force which opposes the motion of an object over another object in contact with it.

The force of friction between two surfaces will retard the motion of the object and finally will bring it to rest. An external force, which is equal and opposite to the force of friction, must continuously act on the moving object in order to maintain the uniform motion.

 

What Causes Friction?

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The surfaces of bodies are never perfectly smooth. Even a very smooth surface seen under a microscope, is found to have depressions and projections as shown in the figure below.

depression and projections on a surface of body
Enlarged View of Apparently Smooth Surfaces in Contact

The interlocking of the irregularities of the surfaces in contact causes friction.

Friction - A Self-adjusting Force

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Consider a block of wood lying on a plane horizontal table. There is no force of friction as the block of wood is stationary. Push the block lightly with the hand. The block does not move because the force applied by the hand is exactly balanced by the force of friction acting horizontally in the opposite direction. Now push the block a little harder, still the block does not move as the force of friction increases so as to become equal and opposite to the applied force.

friction is a self adjusting force
A Gentle Force Being Applied to Push a Block

Thus the force of friction has a remarkable property of adjusting its magnitude so as to become exactly equal to the applied force tending to produce motion. However, after a certain limit the force of friction cannot increase further. If the applied force exceeds this particular limit then the block starts moving as the two forces, i.e., the force of friction and the applied force are not balanced.

The maximum value of the force of friction, which comes into play when a body just begins to move over the surface of another body is known as the limiting friction.

When there is no relative motion between the two surfaces in contact, then the friction is a static friction.

Friction is capable of adjusting itself not only with regard to its magnitude but also the direction of the applied force.

The force required to keep the body in steady motion after the motion has once started is called the sliding friction. The sliding friction is smaller than the limiting friction.

Rolling Friction

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When a body rolls over a surface, the frictional force developed between the surfaces is called rolling friction. For example, when a wheel rolls over a surface, the surface of the wheel and the surface on which it rolls are both slightly deformed. A resistance to smooth rolling is produced due to this deformation. This resistance or opposing force is called rolling friction. The deformation of a motor car tyre in contact with the road is an example. The tyre becomes slightly flattened at the point of contact with the road and also the surface in front of the tyre or wheel gets slightly raised forming a kind of moving ridge.

picture exhibiting rolling friction
A Much Smaller Force is Applied to Push the Block Placed on Rollers

wheel in contact with road exhibits rolling friction
Tyre of the Wheel in Contact With the Road

It is a matter of common experience that it is easier to roll a body than to slide it along the ground. That is because rolling friction is always less than sliding friction. It is probably due to this reason primitive man started using circular wheels for the carriages. In modern applications, the sliding friction between the shaft and the wheel is changed into rolling friction by using ball bearings.

Liquids and gases also exert friction. However, friction offered by them is less compared to that offered by solid surfaces.

The best example to demonstrate the friction offered by air is the meteor shower. Each time a comet passes the Sun it loses some of its gas and ultimately only the dust particles and bits of rocks are left in space. When these particles enter earth's atmosphere, they burn due to the heat produced by air resistance and produce a shower of meteors or shooting stars.

The friction due to water is much less than that between solid surfaces and hence it is difficult to stop ships or boats. To stop a ship the engine is fired in the opposite direction.

Now let us see how the force of friction can be increased and decreased.

Friction can be Decreased

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Friction can be Decreased by the following methods:

By Using Lubricants Like Oil, Grease or Graphite Powder

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Lubricants like oil or grease can reduce friction.

The lubricants form a thin film between the moving parts and fill the tiny pores and depressions on the surfaces.

This makes the surfaces smooth and reduces the friction.

oil grease graphite powder are types of lubricants

Oil                                                      Grease                                                   Graphite Powder

By Using Ball Bearings or Roller Bearings

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As rolling friction is less than sliding friction, ball bearings and roller bearings are also used for decreasing friction.

Small balls or rollers made of steel are placed between the moving parts.

Ball Bearings or Roller Bearings are rolling friction reducers
Ball Bearings or Roller Bearings

By Using Anti-friction Metals or Alloys

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When steel slides over an alloy of lead, the friction is less than when steel slides on steel.

Hence bearings are packed with an alloy of lead.

alloy of lead is an anti friction metal
Alloy of Lead

By Separating the Surfaces by An Air Cushion

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For example, a hovercraft can travel over rough land, swamp or sea. As the moving object does not come in contact with the other solid surface, the frictional forces are reduced.

frictional force is reduced in hovercraft
Hovercraft

streamlining the body

By Streamlining the Body

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The shape of many objects moving through air or water is designed in such a way that friction between the objects and the medium through which they move is minimised.

Aeroplanes, boats and paper rockets are examples of streamlined objects.

 

aeroplane boat paper rockets are examples for streamline bodies

Aeroplane                                        Boat                                                    Paper Rockets

 

By Polishing the Surfaces

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- By polishing the surfaces the friction between the moving surfaces can be reduced.

polishing surface reduces friction
Polishing of a Surface

Friction can be Increased by:

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  • By making both the surfaces very rough
  • By making irregular projections and depressions like those we see on the tyres

Friction has Both Advantages and Disadvantages

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Advantages of Friction

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  • Due to friction we are able to walk on the surface of the Earth
  • The brakes applied in automobiles work only due to friction
  • Nails, screws and the wooden boards are held together due to force of friction
  • The fibres of thread are held together due to force of friction

Disadvantages of Force of Friction

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  • In overcoming the friction, a lot of energy is wasted in the form of heat. Friction causes wear and tear of the moving parts
  • Due to friction, speed of automobiles cannot be increased beyond a certain limit


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