FRICTION Class VIII Chapter 12

INTRODUCTION –

When we go somewhere by a bicycle we often apply brakes to slow or stop the bicycle. When we are running or walking and unknowingly under our feet some banana peel comes, we often fall and get injured.

Look at the tyres of bicycle, bus; truck etc. like vehicles there is special design to help in its stopping on applying brakes. The rough surfaces help in stopping of vehicle unlike the smooth slippery surfaces. It is due to presence of friction in between the road or surface and the tyres of the vehicle the helps in stopping the vehicle on applying the brakes.

The force that comes in between two surfaces in contact due to their relative motion is called friction or frictional force.

SALIENT FEATURES OF FRICTION-

Following are the salient features of frictional force-

1. The force of friction opposes the applied force. e.g.- If you push a wooden box on a surface, the frictional force will come into action. It opposes the push force and reduces the net force.

2. The force of friction results in between two surfaces in contact. The nature of surfaces (rough or smooth) or irregularities, inter-molecular force of attraction (Adhesion or Cohesion) and deformation of surface due to weight are factors which come in action during friction.

3. Friction can be reduced by using lubricants, polishing, using ball bearings and making the streamlined body.

4. Friction can be increased by making the surfaces in contact rough and increasing the weight of moving object.

5. Friction is necessary evil. It has many advantages as well as many disadvantages. It plays crucial role in our day to day life.

FACTORS AFFECTING FRICTION-The force of friction results in between two surfaces in contact. The nature of surfaces (rough or smooth), inter-molecular force of attraction (Adhesion or Cohesion) and deformation of surface due to weight are factors which come in action during friction.

The nature of surfaces in contact determines the amount of friction. Smooth surfaces cause less friction while rough surfaces cause comparatively more friction. Plane Glass will produce very less friction while plane cemented rough floor will produce more friction. It is due to irregularities in between two surfaces in contact.

e.g.- Drop of rainwater in glass of window remain hang on.

Fig. Water droplets adhered on Glass

There comes an inter-molecular force of attraction between two surfaces in contact. The inter-molecular force between similar molecules of same material is called cohesion. The inter-molecular force between dissimilar molecules of different materials is called adhesion.

The deformation of surface due to weight is also a factor. When weight of moving object is increased and surface on which it is moving becomes changed, the frictional force increases.

e.g.- It is difficult to walk on sand than on plane surface.

FRICTION AS A NECESSARY EVIL- There are certain advantages and disadvantages of friction. So, it is called as a necessary evil.

Advantages-

1. Friction develops between our feet or shoes and the ground. So, it helps in walking and running.

2. Friction helps in writing with pen, pencil on paper and on blackboard with chalk.

3. Friction helps to light the matchstick when rubbed on the side of the matchbox.

4. Friction that develops between tyres and the road helps the vehicles to run on the road.

5. Friction between breaks and tyres help to stop the vehicle on applying the force.

Disadvantages-

1. Friction slows down or stops the motion of the objects.

2. Friction generates heat. So, the running vehicles and machines become hot. It is wastage of energy.

3. Friction causes wear and tear of the soles of shoes and slippers.

INCREASING OF FRICTION- Friction can be increased by –

1. Making the surface more rough-

e. g.- The new tyres with more roughness give better grip than the old weared out tyres.

e. g.- The carpets help in comfortable walking on granite floor.

e. g.- ‘Kabaddi’ players rub their hands with soil for better grip of their opponents.

e. g. –Pens are provided with rubber grip for better holding and comfortable writing.

e. g. – Weight lifters and Gymnasts apply powder on their hand for better grip.

2. Increasing the weight of moving object-

e. g.- The pulling of a mat is comparatively easy than the pulling of mat with more load kept on that.

REDUCING OF FRICTION-

1. By Polishing- due to polishing the roughness of the surface get reduced. So, the irregularities due to contact of two surfaces become less and friction reduces.

2. By using lubricants – A lubricant is a substance which reduces friction. e. g.- Oil, Grease, Talcum powder and Graphite. Lubricant is applied in machines to reduce the friction.

e.g.- The movement of doors and windows become smooth when oil is applied on the hinges.

e.g.- The moving parts of a machine or vehicle are made smooth by using grease.

e.g.-The coins of carom-board move faster easily on striking due to spreading of talcum powder.

3. By using ball-bearings – The ball bearing change the sliding friction into rolling friction. Rolling friction is less than sliding friction so it is easy to move the vehicles, fans etc where it is found.

4. By making streamlined body design- The streamlined bodies are narrow from front and back. It reduces the friction on moving body in air and water.

TYPES OF FRICTION-

A.STATIC FRICTION- The friction which develops and balances a force applied on an object and do not allow its motion upto a threshold level is called static friction.

The static friction is exactly equal to the applied maximum force until the object does not start moving. This friction is called limiting friction.

B.SLIDING FRICTION- The friction that develops when an object slides over other is called as sliding friction.

C.ROLLING FRICTION- The friction that develops when an object rolls over the other object is called rolling friction.

D.FLUID FRICTION (DRAG)- (Daniel Bernoulli) The liquids and gases are commonly called as fluids. The resistance or friction that develops when a solid object travelling in liquid or gaseous medium is called fluid friction. It is also called as drag.

Fig. Four types of forces on flying Aeroplane

Four types of forces on a flying aeroplane are-

1.Fluid friction /Drag

2. Thrust

3. Weight due to gravity

4. Lift or Upthrust

The viscosity (thickness) of fluid, surface texture i.e. roughness or smoothness and shape of moving object are factors that affect the fluid friction.

EXERCISE QUESTIONS–

Q.1 Fill in the blanks:

(a) Friction opposes the ………between the surfaces in contact with each other.

(b) Friction depends on the …..of surfaces.

(d) Sprinkling of powder on the carom board ……………………….. friction.

(e) Sliding friction is ……………….than the static friction.

Ans–

(a) movement

(b) roughness

(d) decrease

(e) less

Q.2 Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below.

Choose the correct arrangement.

(a) rolling, static, sliding

(b) rolling, sliding, static

(d) static, sliding, rolling

Ans– (c) static, sliding, rolling

Q.3 Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be:

(a) wet marble floor, dry marble floor, newspaper, towel

(b) newspaper, towel, dry marble floor, wet marble floor

(d) wet marble floor, dry marble floor, towel, newspaper

Ans-(a) wet marble floor, dry marble floor, newspaper, towel

Q.4 Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it.

Ans–

Frictional force will be obliquely upward and sliding of book on tilted surface is obliquely downward.

Q.5 You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why?

Ans– Soapy water on the marble floor will reduce the friction on walking. So, it becomes very difficult to walk over it.

Q.6 Explain why sportsmen use shoes with spikes?

Ans– The shoes with spikes increase the friction with the ground. So, the chance of sleeping on the ground is reduced.

Q.7 Iqbal has to push a lighter box and Seema has to push a similar box on the floor. Who will have to apply a larger force and why ?

Ans– Seema will have to apply a larger force because she has to push a larger box.

Q.8 Explain why the sliding friction is less than the static friction.

Ans-The contact points of irregularities are more in static friction. But, in sliding friction the contact points of irregularities are less. Therefore, sliding friction is less then static friction.

Q.9 Give examples to show that friction is both a friend and a foe.

Ans– Friction as a friend– Friction helps in easy walking as it develops between the soles of foot wares and the ground. The roughness of the tyres help it it’s stopping on applying the break. Friction causes burning of matchstick on rubbing the head of matchstick on the side of matchbox.

Friction as a foe– Friction produces heat. So, the machines and engines become hot. The soles of the foot wares wear out in a few months due to friction.

Q.10 Explain why objects moving in fluids must have special shapes.

Ans– The special shapes of objects moving in the fluids like water and air help to reduce fluid friction (drag) during movement. Such shapes are called streamlined body designs which are narrow from front and back but thick in the middle.

FRICTION Class VIII Chapter 12

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