FRICTION

Our common experience is that a ball rolled on the ground gradually slows down and finally stops even when no one stopped it. Then, why does a rolling ball stop through a certain distance ? This is because the surface offers a force that opposes the motion called as frictional force.

FRICTIONAL FORCE

It is defined as the force acting along the two surfaces in contact which opposes the motion of one body over the other is called force of friction.

It always opposes force irrespective of the direction of motion.

CHARACTERISTICS OF FRICTIONAL FORCE Self adjusting force Is a contact force Always opposes the direction of motion Always acts in tangential direction

CAUSE OF FRICTION Mainly occurs due to two factors:

1) Due to interlocking of surface

No solid surface is perfectly smooth. There are present hills and valleys on the surfaces ( say grooves). When a body (say a wooden block) is pulled over another surface, these hills and valleys interlock with each other and oppose the relative motion between the two bodies. This gives rise to the frictional force. Thus, friction is due to the roughness of the two surfaces in contact with each other. More the rough surface, more the irregularities and more the friction

2) Due to force of Adhesion between the two surfaces

Two rough surfaces when placed together meet only at certain points. The atoms present at such points of contact attract each other due to electrostatic attractions. These attractions are called FORCES OF ADHESION. When one body is made to move over the other, the force of adhesion opposes the motion. This force which opposes the motion also gives rise to frictional force. So the force of adhesion between the two surfaces gives rise to friction.

FACTORS ON WHICH THE FORCE OF FRICTION DEPEND

The force of friction is directly proportional to the weight of the body which moves. The force of friction depends on the nature of surface in contact. For example, the

force of friction between a pair of highly polished surface is very small compared to the force of friction between the two rough surfaces.

It does not depend upon the area of the surfaces in contact.

FRICTION IS A SELF ADJUSTING FORCE

Consider a wooden block on a horizontal table. The forces W and R balance each other. So, there is no motion in the vertical direction. Now, suppose a small horizontal force F is applied on the block, and the block does not move. At this stage, the force F should be exactly balanced by the force of friction (f)

That is F = f body stays at rest

Important noteSo if the applied force (F) is increased and the block does not move, then the force of friction (f) will also increase at the same rate.. Hence the force of friction changes itself accordingly to the applied force, so is called self adjusting force.

TEST YOURSELFQ Why frictional force is referred to as self adjusting force?Q At times even when the block of wood is pushed on the horizontal surface it does not move. Why?Q What is the cause of friction between the two surface in contact?

TYPES OF FRICTION

There are three types of frictions: STATIC, SLIDING, ROLLING

Static friction

Friction is a self adjusting force, hence, when the applied force is gradually increased, the force of friction also increases at the same rate and the body remains stationary.This force is called Static friction (f s ) If the applied force is increased further, a stage reaches when the body begins to just move. At this stage the force of static friction is maximum. And after this sliding friction acts on the body.

The maximum force of friction when the body is just starting to move is called limiting value of friction or limiting friction. ORThe maximum value of the static friction is limiting friction.

Sliding friction or kinetic friction or dynamic friction

The force of friction between the two sliding surfaces in contact when on of them just slides over the other is called sliding or kinetic friction

The force required to move the body is less than the force required to start the motion.

Hence, SLIDING FRICTION < STATIC FRICTION

WHY SLIDING FRICTION IS LESS THAN STATIC FRICTION?

Motion between two surfaces starts only when the irregularities have been unlocked. Once the motion starts the contact points do not get enough time to lock with each other, they only act as obstruction. HENCE THE SLIDING FRICTION BECOMES LESS THAN STATIC FRICTION

Rolling Friction

The force of friction between the two surfaces in contact when one of them is rolling on the other is called rolling friction.

ROLLING FRICTION < SLIDING FRICTION

HENCE, STATIC > SLIDING > ROLLING

Q Why ball bearings are used in all machines where rotatory motion is required? Q Give one example where sliding friction can be converted into rolling friction

LAWS OF LIMITING FRICTION

Always opposes the motion of a body over another. Always acts in the direction opposite to the direction of motion. Increases with the increase in the weight of the body and decreases with the decrease in the

weight of the body. Is independent of the area of contact between the two surfaces. Does depends on the nature of the two surfaces in contact.

TEST YOURSELFQ A ball rolled on the ground stops after some time. Why? Name the force responsible for it.Q Can we walk on a frictionless surface? Give reasonQ A ball is rolled on the frictionless surface and no other force Acts on it. Will its speed decrease, increase or remains unchanged?

DISADVANTAGE OF FRICTION (friction is waste)

Leads to loss of energy( when machine parts rub , it reduces their efficiency) Leads to wear and tear of moving machine parts. Friction reduces the speed of the moving vechicle

FRICTION A NECESSITY( ADVANTAGES OF FRICTION) Nails and screws are able to hold the wood due to friction Friction between ground and sole of the shoes makes us walk on the ground. Rough tyres increases the Friction between the road and the tyre, helps the vehicle not to skid

on the road. Friction between the paper and pen/pencil allows us to write on paper. Otherwise it’s difficult

to write on the glazed paper. Application of brakes also works due to friction. The lightning of the match stick is due to the force of friction, which raises the temperature

of match head to such an extent, that the chemicals catch fire. Spikes are provided in the shoes of athletes to increase the friction and prevent slipping

FRICTION A NECESSARY EVIL

Without friction we cannot set the bodies in motion or stop the moving bodies. This friction is absolutely necessary for carrying out day to day activities like walking, driving, writing etc,On the other hand, friction wastes energy and brings about wear and tear and slows the motion.

TEST YOURSELF

Q Why is the tyres of the automobiles made corrugated and rough?Q Why is it difficult to pull a boat on the beach than on the water?Q Give two examples showing that friction is necessary in everyday lifeQ Give two examples showing the harmful effects of friction.Q Why driving on the icy road dangerous?Q Why friction is considered a necessary evil?Q Why do we slip on oily or wet floor?The oil forms the thin layer in between our feet and the floor. This decreases the friction between our feet and ground and we no longer are able to grip the ground firmly and we slip.Q why the soles of your shoes are grooved?Q Kabbadi players rub their hands with soil while playing. Why?

WAYS TO REDUCE FRICTION By polishing the surfaces: the grease or oil applied between two surfaces like machine parts,

reduces the interlocking and makes the surface to slip over each other. Hence reduces friction.

Graphite powder is used as a lubricant to reduce friction between machine parts.(a material that reduces the force of friction between two surfaces in contact is called lubricant)Compressed air also acts as a lubricant that provides an elastic cushion between moving parts and reduces friction.

By sprinkling soft slippery fine powder like talcum on the surface: The talcum powder fills the hills and valleys making the surface smooth and hence reduces friction. That is why talcum powder is applied on the carom board.Talcum powder also acts as a solid lubricant

By using ball bearings or roller bearings When a body rolls over a surface, the force of friction is much less than that on the flat surface . Hence by using ball bearings the axle rotates on the balls without sliding. By streamlining the body of an object: Properly shaped bodies called streamlined experience less friction from air or water. By converting sliding friction into rolling friction Heavy suitcase are provided with small wheels , as they provide rolling friction which is less than the sliding friction and it becomes easily to move them from one place to another.

Friction Due to Liquids and Gases

Like solid, gases and liquids also experiences friction but less than solids. Friction offered by fluids (gases and liquids) is called DRAG.

Force of friction in fluids depends upon the

o Nature of the fluid o Shape of the moving body (like a crumpled paper and plain paper) o Speed of the moving body with respect to the fluid

The meteors enter the earth’s atmosphere at very high speeds. At such speeds the friction due to air is also very large. Due to this, temperature of the meteors entering the earth atmosphere rises to a very high level. As a result meteors burn out soon after entering the earth atmosphere.

To protect space ships from burning during entry to the earth atmosphere, they provide with heat shield.

A plain paper sheet falls slowly than a crumpled sheet since in plain sheet surface area is large , hence it experience a larger force of friction and it speed slows down, whereas in crumpled form the surface area is small, hence experiences less friction and reaches the ground quickly.

To minimize the friction due to water (any liquid) and air, the moving body should have streamlined shape.

The special shape of the body which offers least resistance to the friction(that is around which liquid or gas can flow easily) is called streamlined shape.

Streamlined bodies are pointed towards front so that air flows with same speed and in the same direction at every particular point, called streamlined flow.

EXAMPLES The bodies of ships and boats are streamlined so that they experience minimum amount of

friction while moving through water. The body of fishes is streamlined so that they experience less amount of friction in water.

Nature has shaped birds in such a way that they offers least possible friction and they can fly at very high speed.

The bodies of airplanes are streamlined so that the air offers least possible friction.

TEST YOURSELFQ State one way of increasing the friction between two surfaces.Q Name four ways by which friction can be reduced between two surfaces in contact.Q state two advantages of frictionQ Why is friction called a necessary evil?Q a) what is a lubricant? b) How does a lubricant reduces friction? c) What kind of lubricant is used in i) sewing machine ii) axle of the machine.Q Why do meteors burn on entering into the atmosphere?Q Why is boats and aero plane given special shape?Q why do carom coins move faster on the carom board when dusted with talcum powder?