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When we push an object with only a small amount of force, it does not move at all. It means that the surface on which the object is resting, exerts some force on the object and this force must be acting in a direction opposite to the force of our push. In other words, some force is acting on the stationary object which opposes its motion.
This opposing force is friction. A ball moving on the ground also slows down or stops due to the friction between the ball and the ground. From this discussion we conclude that: The force which always opposes the motion of our body over another body in contact with it is called the force of friction or just friction. Thus, the force of friction is always opposite to the direction of motion of the body
Cause of Friction
We have seen sand-papers. They have rough surfaces. When we try to move one sand-paper over the other, the movement is not smooth, that is, there is friction due to the interlocking of rough surfaces. In fact, every object has a rough surface, though the surface may appear to be smooth to the naked eye. When we see though a
microscope, it is found that the surfaces of all the bodies are rough — some are more rough and others are less. Some of the particles on the surface are in the form of tiny hills and the others form grooves. These tiny hills and grooves get entangled with one another (see figure). The interlocking of the two surfaces opposes the motion of one body over another and gives rise to frictional force. Figure shows the highly magnified diagram of two wooden blocks in contact with each other. We can see the tiny hills and grooves on the surfaces of both these wooden blocks which give rise to friction. So, in simple words we can say that: friction is due to the roughness of surfaces. Friction is of three types: Static friction, Dynamic friction and Rolling friction. Static friction is also known as limiting friction whereas dynamic friction is also known as sliding friction or kinetic friction. We will first
discuss the static friction and dynamic friction.
Static Friction and Dynamic Friction
The static friction (or limiting friction) is the friction between any two bodies when one of the bodies just tends to move or slip over the surface of another body. There is no actual momentum of the body in static friction (or limiting friction). If, however, one body moves slowly or slides over another
body, then the frictional force is called dynamic friction (or sliding friction). This will become more clear from the following example.
Consider a wooden block kept on a table top (see figure). We fix a light string to the wooden block with the help of a hook. Let us hold the string in our hand and pull the block with a small force (called applied force). The wooden block does not move because its motion is being opposed by the force of friction which acts in the opposite direction (see figure). Now according to Newton’s third law of motion, the force of friction should be equal and opposite to the force applied. So, the force applied by us in pulling the block is a measure of the force of friction.
As we increase the applied force (to pull the block), the force of friction also goes on increasing. But ultimately when the applied force becomes a little more than the maximum frictional force, the wooden block tend to move. This means that the frictional force has a value beyond which it cannot increase. The force which we are exerting in making the block just tend to move or slip is equal and opposite to the force of friction (present between the table top and the block of wood). The maximum frictional force present when a body just tend to slip is equal and opposite to the force of friction (present between the table top and the block of wood). The maximum frictional force present when a body just tends to slip over the surface of another body is called static friction or limiting friction. It should be noted that in this case the body is not actually moving or sliding, it only tends to move or slide.
Let us now increase the force applied in pulling the block a little more. We will see that the block begins to slide or move slowly on the surface of table. The frictional force present when one body moves slowly or slides over another body is known as dynamic friction or sliding friction. It has been found that the force required to keep the block sliding, once it has started sliding, is less than the static friction or limiting friction. In other words, when a body starts sliding, the friction is less. These results have been shown very clearly in the form of a graph between applied force and frictional force in figure.
We can see from figure that as the applied force is increased, the force of friction increases from A to B. The force of friction is maximum at point B when the body (or block) just tends to move. So, point B shows the static friction or limiting friction. And when the body (or block) starts moving slowly or sliding, then the friction decreases from B to C and the remains constant (as shown by the line CD in figure). From the above discussion we conclude that the static friction or limiting friction (when the body just tends to slide) is more than the dynamic friction or sliding friction (when the body is sliding slowly). Please note that when a body has already started moving or sliding, the interlocking of the two surfaces is less and hence the dynamic friction is less. We will now give the laws of friction.
Laws of Friction: The four important laws of friction are given below:
1. Friction opposes the motion.
2. Friction depends on the nature of the two surfaces in contact. In other words, friction depends on the kind of materials; smoothness; and degree of lubrication.
3. Friction is independent of the area of contact between the two surfaces.
4. Frictional force is directly proportional to the weight of the body which tends to move. This is why it is difficult to move heavier objects (but easier to move light objects).
We will now discuss the rolling friction.
Before we discuss the rolling friction, we should know the meaning of the ‘roller’. A roller is a cylindrical object which can ‘roll’ over a surface. The rolling pin (belan) which we use for making ‘chapatis’ is the most common example of a roller. The round pencils are also rollers. A wheel is also a kind of roller. As we will learn after a while, the rollers and wheels give rise to ‘rolling friction’.
We have already studied that we have to apply a considerable force to pull a block of wood kept on the table due to large force of friction between them. Let us now place four or five round pencils below the wooden block and then pull the block from its string. We will find that it is much easier to pull the block now. This means that when the block of wood is moved by keeping over round pencils (which act as rollers), then the friction is much less. In this case, when we pull the block of wood from its string, then the round pencils kept below start rolling (like wheels). And this rolling action of the pencils reduces the friction. The friction which acts in this case is called rolling friction. Thus, when a body (like a roller or wheel) rolls over the surface of another body, the friction is called rolling friction. When we pull the block of wood on the table top (without keeping pencils), then we have sliding friction. But when we pull the same block of wood by keeping round pencils below it, then we have rolling friction. In this way, the rollers (here pencils) convert sliding friction into rolling friction which is much less.
The rolling friction is due to two reasons:
i) the rolling body deforms the surface a little bit on which it rolls, and
ii) the rolling body itself gets deformed at its point of contact with the surface. Let us make it more clear by taking an example.
Suppose a car is moving on the road. The wheels of the car act as rollers which roll on the road. Now, the tyres of the car cause a slight deformation on the road (the deformation produced on the road by a car tyre is so small that we cannot see it). At the same time, the tyres themselves get deformed or flattened at their point of contact with the road (see figure) (the deformation of tyres is quite large which we can see easily). Now, as the wheels of the car roll along continuously, the various parts of the tyres and the road are successively deformed and cause rolling friction. And some force has to be applied by the car engine to overcome this rolling friction. It is obvious that if the wheel is hard and the road is also hard, then the deformation will be less and hence the rolling friction will also be less. So, we should have sufficient air pressure in the car tyre to reduce the rolling friction. But we cannot put too much air pressure in car tyres because then the car will not give a smooth ride on the road. The wheels of a train are made of metal and the railway line is also made of metal, so the deformation in them is very small and hence the rolling friction between them is also very small. But the rolling friction can never be zero.
Note that rolling friction is much less than sliding friction. Since the rolling friction is much less than the sliding friction, therefore, it is easier to roll a heavy drum than to drag it. The purposes of attaching rollers (or wheels) to the bodies is to convert the sliding friction into rolling friction so that these bodies can be moved easily. For example, it is quite difficult to move a heavy body by dragging it due to the high value of sliding friction. But if the same body is provided with rollers (or wheels), then the sliding friction is converted into rolling friction (which is much less), and it becomes easy to move the body with rollers or wheels. Most of the suitcases these days are fitted with small wheels (called rollers) due to which it becomes very convenient to pull them from one place to another.
Friction Exerted by Liquids and Gases
All solid surfaces exert friction on solid bodies moving over them. Even liquids and gases exert friction on the solid bodies moving over them (or moving through them). The liquids and gases, however, exert much less
friction as compared to solid surfaces. The most common liquid around us is water and the most common gas around us is air. So, we can also say that even water and air exert friction on solid bodies moving over them or moving through them. But the friction exerted by water and air is much less than that exerted by solid surfaces. We will now discuss the friction exerted by water and air in a little more detail.
When a person swim in water, the water opposes his motion. This is because water exerts a force of friction on the swimmer (in a direction opposite to his motion). Similarly, the b oa t s a nd s hip s which move in wa t er experience the friction of water. But, the friction exerted by water is much less than that exerted by solid surfaces. Though the small friction offered by water is good for the movement of boats and ships in water but it creates problems when the boats and ships which are moving in water are to be slowed down or stopped. Since the friction exerted by water is very small, it becomes difficult to apply brakes to a moving ship or stop a moving ship, its engines are fired in the opposite direction as brakes which slows down or stops the ship moving on water. Similarly, to stop a moving boat in water,, the oars are operated in the reverse direction (which act like brakes to the moving boat).
We have just studied that the friction due to water is small. But the friction due to air is still smaller. The very small friction of air is used in developing hovercrafts which move a little above the surface of water in the sea. These hovercrafts face much less friction (because they move in air) than ships (which move on water), due to which hovercrafts are able to move fast. Before we go further, we should know the meaning of the term ‘streamlined shape’. The special shape of a body (or object) around which a fluid (air on water) can flow past easily, is called streamlined shape. A body having streamlined shape faces the minimum resistance while moving in air or water. The shapes of boats and ships are made ‘streamlined’ so that they experience the minimum friction while moving in water. Even the shapes of fishes and other marine animals are such that they face the minimum friction while moving in water. Nature has made their bodies streamlined.
We will now discuss the friction of air (which is a gas); Air exerts the force or friction on all the bodies which move through it and opposes their motion. But the friction of air is so small that we are not able to experience its effect easily. We can observe the effect of friction of air only when an object falling through air is either very light having a large surface area, or when the object falling through air has a very, high speed. This will become more clear from the following examples.
If we drop a coin and a feather from the same height simultaneously, we find that the coin reaches the ground first but the feather takes a little more time to reach the ground. The feather reaches the ground a little later because the friction of air opposes its motion and slows it down .Please note that though air also exerts friction on the falling coin but since the coin is compact and heavy, the effect of friction of air on it cannot be observed by us. On the other hand, since the feather is light and has a large surface area, the effect of friction of air is more obvious on the falling feather which can be observed easily.
The most interesting example of the friction of air is the case of a meteor (or shooting start). Meteors are the stone like objects which enter into the earth’s atmosphere from the outer space with a very, very high speed. When the meteors fall through the earth’s atmosphere, their motion is opposed by the friction of air. Due to the very high speed of meteors through air, the heat produced by the friction of air is so large, that the meteors start burning. The burning meteors are seen by us as shooting stars coming down the sky during night. Most of the meteors falling from the sky are small and burn up completely before reaching the surface of earth. Only the very large meteors burn partially and reach the earth’s surface. The meteors which land on earth’s surface are called meteorites.
The automobiles (like cars), aeroplanes and rockets are specially designed to streamlined shapes to minimise the effect of friction of air when they move with high speeds through air. Even nature has shaped the bodies of all the birds in such a manner that they experience the minimum friction of air when they fly in air.
Friction is a Necessary Evil: Friction plays an important role in our daily life. In some cases, friction is useful to us and we wish to keep it (or increase it). But in other cases, friction is harmful and we want to reduce it. We will first describe those cases where friction is useful to us. After that we will discuss those cases where friction is harmful to us. And finally we will describe the various methods of reducing friction.
Where Friction is Useful
slippery ground is difficult because the frictional force is not great enough toWe are able to walk because friction prevents us from slipping. When we push the ground with our foot, the friction provides a forward reaction to our push and sends us forward. If there were no friction between the ground and the soles of our shoes, if would not be possible to walk. Walking on prevent slipping. It is also difficult to walk on a road covered with green algae in the monsoon because algae make the road surface very smooth and hence the friction is not great enough to prevent us from slipping. When we step on to a banana skin thrown on the road, our foot slips because the friction gets reduced because of the smoothness of the banana skin.
Without friction, belts could not drive machines and the brakes could not be applied to cars or other vehicles. Friction between the brake-shoe and the brake- drum slows down the wheels and friction between the tyres and the road brings the car to a stop. If there were no friction between the tyres of a car and the road, the wheels of a car would spin at one place and the car would not move. Without friction, nails and screws could not be used to hold things together and knots could not be tied.
Friction also enables us to write on paper. We are able to write because there is friction between the tip of pen (or pencil) and paper. Writing with a a pen (pencil or chalk) would have been impossible without friction. The lighting of a match stick is another useful application of friction. When we rub a match-stick against the side of the match box, then the friction between the head of match-stick and the side of match box produces heat. This heat heats up the chemicals present on the tip of the match-stick due to which the match-stick lights up. Thus, the lighting of a match-stick would not have been possible without friction.
In some situations we have to even increase the friction (to make it more useful). The friction is increased by increasing the roughness of surfaces. For example, the surface of the head of a match-stick and the sides of the match-box are deliberately made rough to increase the friction. Due to increased friction, greater frictional heat is produced on rubbing the head of match-stick against the side of the match box because of which the match-stick lights up easily.
We also increase friction in the case of tyres of bicycles, cars, buses and other vehicles. The friction in tyres is increased by making grooves in them. The tyres having grooves on their outer surface are called ‘corrugated’. Thus, the tyre surfaces are made corrugated and rough so that the friction between the tyres and the road increases. Due to greater friction, the tyres get a better grip on the road which prevents skidding of the vehicles. Smooth tyres (worn out tyres) and wet roads have very small friction which can make the motion of a vehicle uncontrollable and lead to accidents.
Another point to be noted is that the friction in liquids is much less than the friction between two solid surface. For example, the force of friction between a ship and the sea water is very small. It is because of very small friction exerted by water on the ship that it requires much more time and force to stop a moving ship in water. On the other hand, due to large friction between car tyres and the road, it takes much less time and force to stop a moving car on the road.
Spikes are provided in the shoes of players and athletes to increase friction and prevent slipping. The machine belts are also made of special materials to increase friction so that these belts can drive the machine wheels properly without slipping off the wheels.
Where Friction is Harmful
There are a large number of situations in our everyday life where friction is harmful or causes inconvenience to us. Friction is particularly harmful to those machines which have moving parts in them. Friction is harmful in machines because of the following reasons:
i) Friction reduces the efficiency of machines: Some of the force applied to run a machine are wasted in overcoming the friction between its moving part This reduces the efficiency of the machine. The friction increases the energy consumption in the operation of a machine.
ii) Friction produces heat which could damage the machine: When the moving parts of a machine rub together, a lot of heat is produced due to friction between them. This heat can damage the machine gradually. In some cases, the excessive heat generated due to friction is often removed continuously by circulating cold water around the moving parts of the machine.
iii) Friction wears out the rubbing machine parts gradually: There are many moving parts in machines (like gears, etc.) which rub against each other constantly. Due to friction the rubbing parts of a machine wear out gradually. These worn put parts have to be replaced by new ones periodically.
In our daily life, friction wears away the soles of our shoes. This is because when we walk on the road, there is friction between the soles of our shoes and the surface of road. Due to this the soles of our shoes wear out gradually.
Methods of Reducing Friction
Friction is due to the roughness of surfaces. The smooth surfaces have much less friction. Thus, any process which makes the two surfaces smooth, will reduce the friction. Please note that we can only reduce the friction between two surfaces, it can never the made zero. We also know that the rolling friction is much less than the sliding friction, so wherever possible, friction can also be reduced by converting sliding friction into rolling friction (by using ball bearings, etc.). The friction due to water and air can be reduced by the suitable designing of the shape of the bodies which move fast in water or through air. The important methods of reducing friction are given below.
1. By Polishing: If we polish the rough surfaces, they become smooth and friction is reduced.
2. By Applying Lubricants (Oil or Grease) to Surfaces: When we apply some lubricant, oil or grease, to surfaces, some of the lubricant sticks to the sliding surface In this way the sliding surfaces are separated by thin layers of oil. Now the friction will be between the layers of the oil and this is much less. In some cases graphite, due to its softness, is used as a dry lubricant in machines.
Powder is also a dry lubricant. Friction can also be reduced by applying powder to the rough surfaces. For example, when a small quantity of powder is applied to the wooden carrom board, the depressions on the surface of carrom board and the coins of carrom get filled with powder and they become smooth. Due to this smoothness, the friction between carrom board and coins gets reduced considerably. Since the powder reduces friction, so a coin travels much farther on a powdered carrom board than on an unpowdered one.
3.of bicycles, the axles of cars, the shafts of motors and many other machines are provided3. By Using Ball-Bearings: Ball-bearing is a hollow, circular device containing small metal balls which is fitted around the moving part of a machine (like an axle). The ball-bearing reduces friction by converting sliding friction into rolling friction. For example, when the axle of a machine fitted with ball-bearing rotates, then the metal balls also roll and hence the friction is reduced (figure). The free wheels with ball bearings to reduce friction.
4. By Using Rollers and Wheels: Many heavy objects (like big suitcases) are provided with small wheels (called rollers) to reduce friction so that they may be carried easily by pulling. The automobiles (like cars, buses and trucks) are provided with wheels to reduce friction so that they can be moved easily.
5. B y Streamlining : T he b odies of ca r s , aeroplanes and rockets are streamlined to reduce air friction. And the bodies of boats and ships are streamlined to reduce the friction of water.
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