Fiitjee PDT Courseware FTRE -8th Moving to 9th-PHY-Colour

Friction FRICTION The force acting along the two surfaces in contact which opposes the motion of one body over the other is known as the force of friction or frictional force. The force of friction always opposes the relative motion between the two bodies in contact irrespective of the direction of motion. For example, when a body is pulled by force F towards the right, the force of friction acts along the left and when the applied force acts towards the left, the force of friction acts towards the right as shown in figure. Here R and W represent the normal reaction and the weight of the body respectively. Normal reaction R

Normal reaction R

F

F f

W (Weight)

f

W (Weight)

A frictional force always acts to oppose the motion of a body over a surface and is an example of a contact force. When solid bodies move through liquids and gases, they also experience this opposing force of friction. For example, a kite flying in the air, an aeroplane moving through air and a submarine moving through water. The force of friction experienced by a body moving over some liquid surface is very small compared to that experienced while moving over a solid surface. That is why a moving ship takes much longer time to come to rest than a car when their engines are switched off. Cause of Friction Friction arises on account of strong atomic or molecular forces of attraction between the two surfaces at the points of actual contact. On account of roughness of surfaces, the area of actual contact is much smaller than the 4 area of apparent contact. These areas are roughly in the ratio 1 : 10 . At the contact points, the molecules of the two surfaces are so close together that they exert very strong intermolecular force of attraction on one another. As actual area of contact is very small, therefore, the pressure (= force / area) at the points of contact becomes too large. So that many of the contact points may deform plastically and becomes ‘cold welded’. When one body tries to move over the other, it involves rupturing of thousands of such tiny cold welds. This gives rise to opposing force of friction.

Area of apparent contact

Area of actual contact

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Characteristic of Friction Force 1. Friction force is independent of the area of contact. 2. The maximum value of friction force depends on the normal contact force between the surfaces.

TYPES OF FRICTION FORCE Friction force can be divided in three groups 1. Static Friction 2. Kinetic Friction 3. Rolling Friction (i) Static friction: The friction that exists between the two surfaces in contact when there is no relative motion between them is called static friction. m F Static friction acts on a body to oppose the tendency of relative motion. For example if 5 N force is required to move a block, and horizontal force F is applied on the block and value of F is gradually increased from F= 0 N to 5 N the block will not move till F = 5 N. Upto 5 N the friction force is balancing the applied force. So the maximum value of friction force is 5 N, which is called the limiting value of friction. Before reaching the limiting value, static friction is variable because, say F = 2 N block will not slide, so the static friction is 2N and when F = 3N then static friction in 3 N. So we can say that static friction is self adjustable force, which is equal and opposite to the applied force upto the limiting value. (ii) Kinetic or sliding friction: Force of kinetic friction is the force that comes into picture, opposing the relative motion when one body is actually moving over the surface of another body. It is found that the force of kinetic friction is independent of the area of contact, and its magnitude is constant for the given surfaces. It does not depend on the velocity of the body. (ii) Rolling friction: When a body rolls on the surface of another the friction that exists between the surface is known as rolling friction. When a box is about to start moving, the friction is the static friction. it has started moving, the friction that exists is the sliding friction. When the drum is rolling on the surface, the friction is the rolling friction. Experimentally, it has been proved that, Static friction > kinetic or Dynamic friction > Rolling friction

EFFECTS OF FRICTION When one object moves over another object, the friction between them shows the following effects: (i) Friction opposes Motion: When a ball is thrown on the ground, it stops after sometime. So friction opposes motion. (ii) Friction produces Heat: When the palms of the hands are rubbed together for a few seconds, they become warm. Friction produces heat.

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(iii) Friction causes wear and Tear: If the soles of the old shoes and the new shoes are compared, we will find that the old soles are worn out. Soles wear out because of friction between the ground and the shoes.

DISADVANTAGES OF FRICTION Whenever there is a relative motion of two surfaces in contact, there is frictional resistance. When a body moves over another, it has to overcome friction. This needs energy. Hence, some energy is always wasted in overcoming friction. When two parts of a machine rub against each other, heat is produced, and the efficiency of the machine decreases. There will be greater wear and tear in different parts of a machine due to friction. Thus friction reduces the efficiency of machines.

ADVANTAGES OF FRICTION From what has been discussed above, it may appear that if there were no friction, our life would have been much easier and we would not have to spend energy in overcoming the frictional resistance. This is not true. As a matter of fact it is only because of friction that we are able to move around. Try to run on a very smooth surface and see what happens. As we step forward, friction opposes the forward force, so that the foot does not slip forward force. When we raise our heel again to take another forward pace, the friction prevents the front part of the shoe from moving back.

WAYS TO REDUCE FRICTION (i) Streamlined Bodies: Air and water offer the maximum friction. Objects moving in air or water have streamlined bodies to reduce friction. Birds, cars, aeroplanes and rockets have special streamlined bodies to reduce air resistance Ships, fishes, boats and submarines are broad in the middle and narrow in front and at the back.

   

(ii) Polishing: Polishing rough surfaces also reduces friction. If the surfaces are in contact are rough, there is more friction. Hence to reduce friction, the surfaces in contact are kept smooth and polished. It is worth nothing that if the two surfaces in contact are of the same metal, the friction increases on polishing the two surfaces. This is why the bearing the axle is made of different metals. (iii) Wheels: If a suitcase is fitted with wheels, it is easier to move it because the friction between the wheels and the ground is less.

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(iv) Ball Bearings or Roller Bearing: They are made in various designs to reduce friction. They reduce friction because rolling friction is smaller than sliding friction. These bearings are also lubricated to further improve their function.

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Balls act as rollers between axle and inside surface of wheel

Wheel revolves

Axle Fixed

(v) Lubricants: Use of lubricants makes the surface smooth. In machines, friction is reduced by using lubricants such as oil, grease or graphite power. The lubricant fills the minute unevenness of the two surfaces and separates them by forming a very thin layer in between. This layer offers very little resistance and as a result, the friction gets reduced. (vi) Anti-friction Metals: When steel slides over an alloy of lead, the friction is less than when steel slides on steel. Bearings are sometimes packed with such an alloy.

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SOLVED PROBLEMS SUBJECTIVE 1.

Why it is difficult to move a bike with its brakes on?

Sol.

When the brakes of a bike are on, its wheels cannot rotate. They will simply skid. Thus rolling friction will be converted into skidding friction which is comparatively larger. Hence it becomes difficult to move the bike.

2.

Why a horse has to pull a cart harder during the first few steps of his motion?

Sol.

During the first few steps of his motion, the horse has to pull a cart harder, because the horse has to work against the limiting friction, whereas once the motion starts, the horse has to work against the dynamic friction which is less than the limiting friction.

3.

How does a lubricant help in reducing friction?

Sol.

When a lubricant is added to a machine it spreads between the two surfaces rubbing fills the irregularities present on the surfaces and forms a thin layer between the contact. As a result of it, the contact between the two hard surfaces is replaced by between the hard surface and lubricant layer. Due to it, the force of friction considerably.

4.

Sand is thrown on the tracks covered with snow. Why?

Sol.

Sand increases the friction.

5.

Why are wheels circular?

Sol.

They convert sliding friction into rolling friction.

each other, surfaces in the contact is reduced

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OBJECTIVE MULTIPLE CHOICE QUESTIONS (SINGLE OPTION CORRECT) 1.

Friction force acts (A) parallel to motion (B) perpendicular to motion (C) opposite to the direction of relative motion 0 (D) at an angle of 30 with the direction of motion

Sol.

C

2.

Friction between any two objects is due to (A) Attraction between them (C) Some adhesive forces between them

Sol.

D

3.

It is difficult to walk on ice because of (A) absence of friction (C) more inertia

Sol.

A

4.

Frictional force can’t be measured in (A) kg wt. (C) dyne

Sol.

D

5.

Graphite powder is used in machines to (A) enhance friction (C) reduce friction

Sol.

C

6.

Friction is a/an (A) Self-adjusting force (C) Important force in daily life D

Sol.

(B) Repulsion between them (D) irregularities on the surfaces.

(B) absence of inertia (D) more friction

(B) Newton (D) kg m/s

(B) enhance profit (D) reduce efficiency

(B) necessary evil (D) all the above

7.

When a ball moves on a rough surface, its speed (A) Increases (B) decreases (C) Remains the same (D) none of these

Sol.

B

8.

Some statements about friction are given (I) frictional force can accelerate a body. (II) Frictional force is proportional to the area of contact between the two surfaces. (III) Kinetic friction is always greater than static friction. (A) only I is true. (B) only (III) is true (C) I & II are true. (D) II & (III) are true. A

Sol. 6

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9.

An external horizontal force P acts on a block placed on a rough horizontal surface. The force of friction between them is f. Which of the following graphs represents the relation between P and f correctly? (A) (B) f

f P

(C)

P

(D) f

f

P

Sol.

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PDT Coursewar e (FTRE) 8th moving to 9th PH

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P

B.

True / False 10.

Limiting friction is independent of the area of contact.

Sol.

True

11.

Limiting friction depends on the nature of surfaces in contact.

Sol.

True

12.

Friction force always apposes the motion.

Sol.

False

13.

The magnitude of friction force acting on a body may be zero.

Sol.

True

14.

When a person walks on a rough road, the frictional force exerted by the surface on the person is opposite to the direction of motion.

Sol.

False

Fill in the blanks: 15.

The maximum value of forces which makes the surfaces just to slide is called the ____.

Sol.

Limiting friction

16.

Frictional forces also arise when solid bodies move through ________________.

Sol.

Liquids and gases

17.

Ball bearing is commonly used to _____________friction.

Sol.

Reduce

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PDT Coursewar e (FTRE) 8th moving to 9th PH

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ASSIGNMENT SUBJECTIVE 1.

Define force of friction.

2.

What do you mean by limiting friction?

3.

Why is it easier to maintain the motion than to start it ?

4.

Is force of friction a contact force?

5.

If a ball is moving on a frictionless horizontal surface and no forces are applied on it, will its speed decrease, increase or remain constant?

6.

What is rolling friction?

7.

What are the various ways of reducing friction?

8.

Give any two examples where friction supports motion.

9.

How does frictional force vary with change in roughness of the surfaces?

10.

Is friction a self –adjusting force? Give two disadvantage of friction.

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OBJECTIVE MULTIPLE CHOICE QUESTIONS (SINGLE OPTION CORRECT) 1.

2.

3.

SI unit of force of friction is (A) N 2 (C) kg m/s

(B) kg wt (D) none of these

The use of lubricants makes the surface (A) smooth (C) very rough

(B) rough (D) none of these

Ball bearings are used to (A) increase friction (C) optimize friction

(B) decrease friction (D) none of these

4.

The maximum force of friction when the body is just beginning to move is known as the (A) limiting friction (B) rolling friction (C) static friction (D) sliding friction

5.

The friction that exists between a surface sliding on another surface is called the (A) dynamic friction (B) rolling friction (C) static friction (D) limiting friction

6.

The force of friction between two bodies is (A) Parallel to the contact surface (C) Inclined at 30° to the contact surface

(B) Perpendicular to the contact surface (D) Inclined at 60° to the contact surface

7.

It is easier to maintain the motion than to starts it because (A) kinetic friction > limiting friction (B) kinetic friction < limiting friction (C) kinetic friction = limiting friction (D) none of these

8.

In an experiment, four students plotted graphs between force of limiting friction F and normal reaction R, Which one is correct

(A)

F

(B)

F

R

R

F

F

(C)

(D) R

R

9.

In the shown arrangement,

1 kg

2 kg

F=(12 )N

If coefficient of friction between blocks and surface is 0.1 the tension in the string would be (A) 4N (B) 3N (C) 2N (D) 1 N

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PDT Coursewar e (FTRE) 8th moving to 9th PH

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10.

If the normal force is doubled, the coefficient of friction is (A) not changed (B) halved (C) doubled (D) tripled

11.

A balloon of gross weight W Newton is falling vertically downward with a constant acceleration a(