Friction

When two objects in contact try to slide past each other, friction keeps them from moving or slows them down."

A climber slowly moves up the side of a rock. This is possible due to a force called 'friction' which is exerted on the climber at each place where he touches the rock. Friction balances gravity's downward pull on the climber and keeps him from sliding down the rock.

1.0Introduction

Friction is found everywhere in the world. You can feel the effects of friction when you ride a bicycle, while walking, and even when you sit on a sofa. You must have seen a driver of a car slowing down the vehicle at a traffic signal. You too, slow down when you stop pedaling a bicycle or by applying brakes. Imagine pushing a ball away from you on a floor. As the ball rolls, it slows down and then stops. Every object moving over

Friction can act when an object is moving or when it is at rest, it is present in almost all reallife situations. It always acts in the direction opposite to the direction of motion of the object.

the surface of another object slows down when no external force is applied on it and finally it stops. In each of the above cases, one force is responsible for slowing down of the objects, that force is called friction.

2.0Friction

Friction is a force that opposes the movement between two surfaces in contact.

The magnitude of the frictional force depends on the types of surfaces in contact. The frictional force is usually larger on the rough surfaces and smaller on the smooth surfaces.

Friction is always parallel to the surfaces in contact

We know that, when an object is in contact with a surface, there are forces acting on the object. One such force we have discussed in the previous chapter was normal force, that is perpendicular to the surface in contact. When the object moves or attempts to move along the surface, there is also a force that is parallel to the surface. This parallel force is called the frictional force, or simply friction.

In some cases, friction is desirable while in other cases, friction reduces the effectiveness of machines. Without friction, you would not be able to walk ; the wheels of a vehicle would have no grip on a road surface and the vehicle would not be able to move forward or backward.

On the other hand, friction is undesirable in most of the cases. Friction causes mechanical parts to seize and wear out, and mechanical energy to be converted into heat.

Friction is present whenever, particles come in contact, it is present in every medium namely, solids, liquids and gases.

3.0Spring Balance

Spring balance is a device used for measuring the force acting on an object or the weight of an object. It consists of a coiled spring which gets stretched when a force is applied to it. The stretch in the spring is measured by a pointer moving on a graduated scale (see figure). The reading on this scale gives the magnitude of the force. Spring balance usually measures force in terms of grams or kilograms. The force in kilograms can be converted in newtons by multiplying it by a factor 9.8. For example, if spring balance measures 1.5 kgs , then it is $1.5\times 9.8=14.7\mathrm{N}$.

Q. A man falls down when he steps on a banana peel. Why?

Explanation: When a man walks on a road, the friction between soles of his shoes and the road surface is quite large. Thus, he has an adequate grip on the road surface and he can walk easily. When he steps on a banana peel, a smooth layer of banana peel between his shoes and the road reduces the friction significantly. This causes the boy to slide easily (see figure).

4.0The Cause Of friction

Friction is caused by the irregularities on the two surfaces in contact. Even those surfaces which appear very smooth have a large number of minute irregularities on them. If you look through a microscope at two smooth surfaces touching each other, you would see tiny hills and valleys on both sides (see figure). These tiny hills and valleys on both the surfaces lock into one another. As surfaces slide or try to slide across each other, the hills and valleys grind against each other and cause friction. In order to move an object, we have to apply a force to overcome interlocking (or friction).

The tiny hills on the surfaces in contact may change shape or wear away when the two surfaces slide across each other. For example, if you rub sandpaper on a piece of wood, friction affects the wood's surface and makes it either smoother (hills wear away) or rougher (they change shape).

5.0Types Of Friction

There are many kinds of friction that exist in different media.

• Static friction : It exists when two surfaces try to move across each other but not enough force is applied to cause motion.
• Sliding friction : It exists when two surfaces slide across each other.
• Rolling friction : It exists when one object rolls over another object.
• Air friction (air resistance) : It exists when air moves around an object.
• Viscous friction : It exists when objects move through water or other liquids. (see figure)
  
  Types of friction

Friction depends on the force between surfaces

Force of friction increases if the two surfaces are pressed harder. For example, you can drag a mat easily when nobody is sitting on it. But, when a person is sitting on the mat, it is very difficult to drag the mat. The greater the force pressing the two surfaces together, the greater will be the force of friction between them.

Q. It is quite easy to move a cloth alone on a surface. But when you place a book on it, you have to pull hard to move the same cloth. Why?

Explanation: The two surfaces in contact are still the cloth and the table. But, the book causes the cloth to press harder into the table's surface. The tiny hills and valleys in the cloth and in the table are pressed together with a much greater force, thus, the interlocking between the two surfaces increases. This increases the friction between the two surfaces in contact. Thus, it becomes harder to pull the same cloth.

Friction increases with weight

You know that the friction force between two hard surfaces is approximately proportional to the force pressing the surfaces against each other. If an object is placed on a floor, the force between the object and the floor is the weight of the object. For a heavy object, the weight is quite large, therefore, the force between the object and the floor is also large. Thus, the friction force between them is large.

Q. Suppose you have to push a heavy box across the floor. You can push the box with its face having larger area touching the floor as shown in figure(a). Also, you can push it with its face having smaller area touching the floor as shown in figure(b). In which case, the force of friction acting on the box is more?

• Explanation: You know that, the force of friction is proportional to the weight of the object with hard surface is placed on another hard surface. In both the cases (a) and (b), box and the floor remains the same, this means, the weight of the box is also the same. Since, the weight is same in both the cases, the force of friction acting on the box in each case is also the same. Here, we get a good result that 'for hard contact surfaces, the force of friction does not depend on the 'area of contact' between the two surfaces.

• The rule that 'the force of friction does not depend on the area of contact' is true for dry and hard surfaces. It is not true if the surfaces are wet, or if they are soft. Rubber is soft as compared to the surface of a road. The friction between rubber and surface of road also depends on how much rubber is contacting with the surface of road. Thus, wide tyres made of rubber have more friction than narrow tyres.

6.0Static Friction

Suppose you push on a heavy box and the box doesn't move. This means the forces acting on the box are equal in magnitude but opposite in direction i.e., the net force on the box is zero [see figure (a)]. The force you exert on the box is balanced by a force acting on the box in the opposite direction. This force is called static friction.

Static friction is the force between two surfaces in contact that tends to prevent a stationary object to start moving. In other words, 'static friction is the force exerted on an object at rest that prevents the object from sliding'.

Static friction is greater than sliding friction. This is because it takes more force to break the interlocking between two surfaces than it does to keep them sliding once they are already moving.

The static friction is exerted on the bottom of the box where it touches the floor. Suppose you push harder and the box still doesn't move [see figure (b)]. This means that the force of static friction has increased to balance the force you apply. However, there is an upper limit to the value of the static friction force between two surfaces. If your applied force becomes greater than the maximum value of static frictional force, then the box begins to move.

The maximum static friction is called the starting friction or limiting friction. It is the amount of force that must be overcome to start a stationary object to move.

Static friction prevents the box from sliding: Friction depends on weight of object and nature of surfaces in contact and does not depends on area of contact.

7.0Sliding Friction or Dynamic Friction

When the force pushing on the box is larger than the maximum static friction force, the box begins to slide. When the box is sliding, a different frictional force acts on the box. This force is sliding friction.

The force that opposes the movement of two surfaces that are in contact and are sliding over each other is called sliding friction.

In other words, 'sliding friction is the force exerted on an object in motion that opposes the motion of the object as it slides on another object'.

The magnitude of the applied force is usually greater than that of sliding friction (see figure). Unlike static friction, the magnitude of sliding friction does not change if you push on the box harder. As long as the object is sliding, the force of sliding friction is constant. If the force you apply is greater than sliding friction, the box speeds up as it slides. If the force you apply is equal to sliding friction, the box slides with a constant speed.

The direction of sliding friction is opposite to the applied force. Also, the direction of sliding friction is always opposite to the motion of the sliding object.

Sliding friction is smaller than static friction

The force required to overcome friction at the instant an object starts moving from rest is a measure of static friction (starting friction). On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction. Static friction is not created by motion, but it results from the interlocking of hills and valleys. When the box starts sliding, the contact points on its surface, do not get enough time to lock into the contact points on the floor. So, the sliding friction is slightly smaller than the static friction and you find it somewhat easier to move the box already in motion than to get it started.

8.0Friction : A Necessary Evil

Friction is often undesirable, it causes wear of machine parts, engines, soles of shoes, etc. Thus, friction is an evil. But, it can also be useful. For example, walking on the road is not possible without friction. Walking or running on the road involves static friction. Friction has advantages as well as disadvantages and thus, friction is a necessary evil. There are many applications where friction is both useful and necessary.

Friction is useful/necessary in many cases

• Friction is required to hold vessels like glass tumbler, a Bottle. Without friction it would not be possible to hold such vessels (see figure).
• You could not fix a nail in the wall without friction. Friction keeps nails in place. When a nail is hammered into wood, the wood pushes against the nail on all sides (see figure). The force of the wood against the nail surface creates a lot of friction. Each hit of the hammer pushes the nail deeper into the wood. The deeper the nail goes, the more is the push force of the wood on the nail surface, thus greater is the force of friction to keep the nail in place.
• You could not write with pen or pencil if there were no friction. When your teacher is writing with chalk on the blackboard, its rough surface rubs off some chalk particles which stick to the black board. It could not happen if there were no friction between the chalk and the board. Here, wear caused by friction is useful for writing with chalk !!
  
  Without friction, a man cannot hold the 'bottle' in his hand.
  
  Friction keeps the nail in place in a piece of wood.
• Friction is useful for brakes and tyres. The brakes on a bicycle create friction between two rubber brake pads and the rim of the wheel. Friction between the brake pads and the rim slows down the bicycle. Friction is also necessary to make a bicycle go on the road. Without friction the bicycle's tyres would not grip the road. All the automobiles/vehicles move on the road due to the friction between their tyres and the road.
• You could not tie a knot without friction. A knot tied on a rope made of smooth fibre like nylon can be opened easily as friction between the surfaces of nylon fibres is quite small. A knot tied on rope made of jute is difficult to open as the friction is quite large between the surfaces of jute fibres involved in the knot.

Q. Why is it hard to pull out a nail fixed in a piece of wood or a wall?

Explanation: A nail fixed in a piece of wood experiences a large push force exerted by wood on its surface from all sides. This force of the wood against the nail surface creates a lot of friction. When we try to pull out the nail from the piece of wood, this large friction opposes the motion of nail (see figure). Thus, it is hard to pull out nail fixed in a piece of wood or wall.

9.0Rolling Friction

Suppose a bicycle is moving on a road. The wheel of the bicycle rotates about the axle and the axle itself is moving in a straight line. As a result, the motion of the wheel is a combination of both spin (rotational) motion and linear motion (see figure).

• Static friction > Sliding friction > Rolling friction

Q. What type of motion will you predict for automobiles if there is no friction?

Explanation: If there is no friction between the tyres of the automobiles and the road, they cannot be started or stopped or turned to change the direction of motion. If a push force is given to them, they will just slide over the surface without rolling of their tyres. They will continue to move with a constant speed in a straight line i.e., they will never stop.

10.0Friction And Energy

Friction changes energy of motion into heat energy. You may have noticed that rubbing your hands together quickly can make them warmer on a cold day. You are feeling the effect of friction by changing energy of motion into heat.

• Heat in machines: Friction is always present in any machine with moving parts. If the machine is small, or the forces are low, the amount of heat produced by friction may also be small. A sewing machine gets warmer as it runs, but usually not so hot that parts melt or break. You might have observed that the jar of a mixer becomes hot when it runs for a few minutes. Larger machines have more problems with heat.
• Igniting a matchstick involves friction. When you strike (rub) a matchstick against the rough surface of matchbox, it catches fire (see figure). Here, friction between the matchstick and the rough surface of matchbox produces enough heat to ignite the matchstick that causes fire.
  
  Stricking a match stick out produces fire by friction
  
  Rocks or stone worn out by the friction due to flowing water
  
  Surface of tyres wear due to friction
• Friction causes wear: Another way friction changes energy is by wearing away moving parts. You have probably noticed that objects that slide against each other often get rounded or smoothed. Each time two moving surfaces touch each other, tiny bits of material are broken off by friction. Breaking off bits of material consumes energy. Sharp corners and edges are rounded off and flat surfaces may be scratched or even polished smooth and shiny.
• Water flowing over stones in a stream or river causes the stones to be rounded and smooth due to friction (see figure). Friction wears out the surface of tyres (see figure), ball bearings, screws, etc.

11.0Fluid Friction

• Fluids are the substances which can flow on application of force or pressure on them. Gases and liquids are fluids. The force of friction exerted by the fluids on the objects moving through them is called fluid friction.
• The frictional force exerted by fluids is also called drag. Fluid friction affects a boat moving through water and an aeroplane flying through air.
• Air is a mixture of gases and it is also a fluid. Though air is very light, yet it exerts frictional force on objects moving through it. This friction is called air resistance.
• Air resistance is a form of friction that acts to slow down any object moving in the air.
• Similarly, water and other liquids exert force of friction when objects move through them. This friction is called viscous friction.
• The resistance offered by the liquids to the motion of the object moving through them is called viscous friction.

Factors that affect fluid friction

• The fluid friction (air resistance or viscous friction) on an object moving in a fluid depends on Speed of the object in the fluid. The faster an object moves in a fluid, the greater is the fluid friction acting on it.
• Shape of the object moving in the fluid. For example, a piece of paper crumpled into a ball falls faster than a flat piece of paper falls. Here, the force of friction on the flat piece of paper is more than the piece of paper crumpled into a ball.
• Nature of the fluid. This means a given object experiences different amounts of friction in different fluids. For example, an object moving with a certain
• The effect of fluid friction is negligible at low speeds. But at very high speeds its effect is quite significant, it can heat up the object moving in the fluid.

When a person falls in air using parachute, the fluid friction (air resistance) that acts on a parachute slows the fall (see figure). In this case, friction allows a parachutist to land safely without any injury.

This parachutist will land safely on the ground as friction offered by air slows down it's speed.

Q. Why the force of friction on a carrom 'striker' is very small when it is sliding on ice?

Explanation: When the 'striker' slides on ice, a thin layer of water between the striker and the ice allows the striker to slide easily. Water and other liquids, such as oil, can greatly reduce the friction between surfaces.

12.0Reducing Friction

Frictional forces are unavoidable. Any motion where surfaces move across each other or through air or water always creates some friction. Unless a force is continually applied, friction eventually slows down all motion to a stop. For example, bicycles have very low friction, but even the best bicycle slows down if you stop pedaling on a level road. Friction cannot be completely eliminated but it can be reduced. Many clever inventions have been devised to reduce friction. You use them every day.

Lubricants reduce friction in machines

• Keeping a fluid such as oil between sliding surfaces keeps the surfaces from actually touching each other. Instead of wearing away each other's hills and valleys, surfaces separated by oil stir up the oil instead. The force of friction is greatly reduced, and surfaces do not wear out as fast.
• A fluid used to reduce friction is called a lubricant. You add oil to a car engine so the pistons can slide back and forth with less friction. Even water can be used as a lubricant under conditions where there is not too much heat.
• A bicycle and a motor mechanic uses grease between the moving parts of these machines. In some machines, it may not be advisable to use oil as lubricant thus, an air cushion between the moving parts is used to reduce friction.
• Powdered graphite is also used as a lubricant in machines where high temperatures are involved. At high temperatures liquid lubricants like oil may burn out or decompose, that may increase the friction instead of reducing it.

Q. How does oil or grease reduce the friction between moving parts of a machine?

Explanation: When oil or grease is applied between the moving parts of a machine, a thin layer is formed there and moving surfaces do not directly rub against each other (see figure). Interlocking of irregularities is avoided to a great extent. This reduces the friction and the movement of the machine parts become smooth.

Ball bearings reduce friction

A ball bearing is a device containing many balls that reduce friction while supporting a load [see figure (a) and (b)]. Ball bearings change sliding friction into rolling friction, thereby reducing friction by up to 100 times. For example, a metal shaft rotating in a hole rubs and generates a lot of heat due to large friction [see figure (c)]. Ball bearings that go between the shaft and the inside surface of the hole allow the shaft to spin more easily [see figure (d)]. The shaft rolls on the bearings instead of rubbing against the walls of the hole. Well oiled (or well greased) bearings rotate easily and greatly reduce friction.

Air cushion reduces friction

Another method of reducing friction is to separate two surfaces with a cushion of air. For example, a hovercraft floats on a cushion of air created by large fans. A hovercraft (air-cushion vehicle, ACV) is a craft capable of travelling over any surface while supported by a cushion of high-pressure air which is ejected against the surface below and contained within a 'skirt'. Hovercrafts (see figure) are unique among all other forms of ground transportation in their ability to travel equally well over land, ice, and water. Small hovercrafts are used for sports or passenger services, while giant hovercrafts have civilian and military applications.

A hovercraft moving just above water. Air cushion present between hovercraft and water reduces friction. This gives a smoother and faster drive as compared to a boat.

A magnetically levitated train (maglev train) uses electromagnets to float above the track once the train starts moving (see figure). There is no contact between train and track i.e., only air is present between them. So there is far less friction than for a normal train on tracks. The ride is smoother, allowing for much faster speeds.

A magnetic train (or maglev train) moves just above the track with very small friction allowing for much faster speeds and a smoother ride.

Streamlining reduces fluid friction

When objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. To minimise fluid friction, objects are given special curved shapes. When an object with flat surfaces moves through a fluid, it experiences much more fluid friction as compared to that experienced by an object with curved surfaces. An object with special curved shape such that the fluid friction on it is minimum is called a streamlined object.

Q. Where do you think the scientists get hints for the special curved shapes which offer least friction in fluid?

Explanation: The scientists get hints for these special shapes from nature, of course. Birds and fishes have to move about in fluids all the time. Their bodies must have evolved to shapes which would make them not to lose much energy in overcoming friction (see figure). If you look carefully at the shape of an aeroplane, you find that its shape is similar to that of a bird.

13.0Increasing Friction

Friction is desirable in many real life situations. Thus, efforts are being made to increase friction for such situations.

Grooving the soles of shoes increases friction

Shoes are designed to increase the friction between your foot and the ground. Thus, the soles of your shoes are grooved (see figure). It is done to provide the shoes

Many athletes, including football and soccer players, wear shoes with cleats (see figure). Cleats are teeth like structures, on the bottom of the shoe that dig into the ground. Players wearing cleats can apply much greater force against the ground to help them move and to keep them safe from slipping.

Treads on tyres increase friction

Friction is also important to anyone driving any vehicle like car, truck or a motorbike. Tyres of such vehicles are specially designed to maintain friction on pavement in dry as well as wet conditions. Treaded tyres of cars, trucks, etc. provide better grip with the ground. Tyre treads have grooves that allow space for water to be channeled away where the tyre touches the road if the road surface is wet (see figure). This allows good contact between the rubber and the road surface.

14.0Mind Map

15.0Some Basic Terms

• Magnitude - Amount of quantity or maximum object's size.
• Effectiveness - Ability to produce desired output.
• Seize - To take hold of something suddenly.
• Adequate - Just good enough.
• Hills and valleys - In physics it refers to irregularities on any surface.
• Sandpaper - Strong paper with sand on one side that is used for rubbing surfaces to make them smooth.
• Drag - To pull something.
• Interlocking - Having parts that overlap or fit together.
• Resistance - Opposition.
• Eliminated - To remove that is not needed.
• Coarse - Rough, not smooth.
• Tapered - Reduce in thickness toward one end.
• Foe - An enemy.
• Inertia - An inability to move or change.