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Following are the three types of friction:

• Static Friction: The minimum amount of force that is required to overcome the force of friction is called static friction. In other words, the minimum force that is required to make an object moving is called static friction.
• Sliding Friction: Once an object starts moving on a surface, some force is required to keep the object moving. The minimum force that is required to keep a moving object in motion at a constant speed is called sliding friction. Sliding friction is always less than static friction. Due to this, it is easier to push a moving box compared to pushing a static box.
• Rolling Friction: When an object rolls over a surface, the friction created is called rolling friction. Rolling friction is less than sliding friction.

Friction opposes the relative motion between two surfaces in contact. It acts on both surfaces.

Friction depends on the nature of the surface in contact.

For a given pair of surfaces, friction depends upon the state of smoothness of those surfaces.

Friction depends on how hard the two surfaces pressed together which is a normal force acting on the body.

Friction is independent of the area of contact.

Force of friction is desirable in many cases for example:–

I. Stopping of vehicles – If there were no friction then we cannot stop our vehicles and the vehicles keeps on going and never stop but there is friction which helps us to stop our vehicles because it opposes motion always.

II. Writing – When we write on paper or blackboard then the pencil lead gets dragged on the paper due to friction and gets deposited on the sight where we move it, if there were no friction then there would be no writing on paper with pencil.

III. Walking on ground – When we walk on the ground it is due to friction which gives us forward reaction force due to which we are able to walk and if there were no friction then we were not able to walk.

IV. Picking of objects – When we pick objects this is only due to force of friction because gravity is always acting on objects and if we pick an object it’s net force should be zero in order it to not to move.

Disadvantages of Friction: 

1. Friction always opposes he motion. 

2. Friction causes wearing and tearing.

Advantages of Friction: 

1. Friction help us to walk. 

2. A horse can’t pull a cart without friction.

If we does not change the tyre then the frictional force decreases due to threading worn out tyre there by the vehicle may skid.

1. If there were no friction, many problems could have been arisen, as one will not able to walk if they were no friction at all. 

2. One could not write with pen or pencil.

3. One could not write on blackboard with a chalk. 

4. If any object just started moving, it would never stop. 

5. Had there been no friction between the tyres of the automobiles and the road, they could not be started or stopped or turned to change the direction of motion. 

6. One could not fix the nail in the wall or tie a knot. 

7. Without friction no buildings could be constructed.

(b) the force of friction between the bodies is zero

(d) the bodies may be rough but they don't slip on each other.

Explanation:  

 Contact force F is the resultant of normal force Fn and friction force f. F and Fn can only be equal if f-friction force is zero.Option (b) is correct.                   

f can be zero if the contact surfaces are smooth or they are not trying to slip. If the surfaces are rough but not trying to slip then f is zero and contact force is equal to normal force, hence option (d) is correct but option (a) is not correct as the surfaces may be smooth but it is not the only condition.                  

Option (c) is not correct because
magnitude of Fn and f are equal then magnitude of contact force F=√2.Fn  if

(b) Coefficient of static friction is always greater than the coefficient of kinetic friction

(c) Limiting friction is always greater than the kinetic friction. 
(d) Limiting friction is never less than static friction

(b) Coefficient of static friction is always greater than the coefficient of kinetic friction.

(c) Limiting friction is always greater than the kinetic friction.

(d) Limiting friction is never less than static friction.

Explanation:  

Static friction is not constant. It varies from zero to maximum value of Limiting friction depending upon the force applied on the body to move it while kinetic friction is a bit less than the Limiting friction. So static friction may or may not be greater than the kinetic friction. So option (a) is not correct.              

Coefficient of static friction is equal to Limiting friction force divided by Normal force while coefficient of Kinetic friction is equal to force applied to just move the body with a uniform velocity divided by the Normal force. Since Limiting friction force is always greater than the force applied to just move the body with uniform velocity, hence Coefficient of static friction is always greater than the coefficient of kinetic friction. So options (b), (c)  and  (d) are correct.

(c) The graph is a straight line of slope 45° for small F and a straight line parallel to the F-axis for large F.
(d) There is a small kink on the graph.

Explanation: 

Until the applied horizontal force F is increased to a Limiting force the block does not move because this horizontal force is balanced by force of friction f. Up to this point the graph is a straight line of slope 45°. Beyond this point the block moves with a uniform velocity for even less than the Limiting force, at this point the graph has a small kink. Now even if the F is increased f remains constant, so now the graph gets parallel to F-axis. Hence options (c) and (d) are correct.

(c) The graph is a straight line of slope 45° for small F and a straight line parallel to the F-axis for large F.
(d) There is a small kink on the graph.

Explanation: 

Until the applied horizontal force F is increased to a Limiting force the block does not move because this horizontal force is balanced by force of friction f. Up to this point the graph is a straight line of slope 45°. Beyond this point the block moves with a uniform velocity for even less than the Limiting force, at this point the graph has a small kink. Now even if the F is increased f remains constant, so now the graph gets parallel to F-axis. Hence options (c) and (d) are correct.

(a) is towards east if the vehicle is accelerating

(b) is zero if the vehicle is moving with a uniform velocity

Explanation: 

When the vehicle is accelerating towards east the frictional force must be towards east according to Newton's Second Law of Motion. So option (a) is correct. If the the vehicle is moving with uniform velocity no force is applied in the direction of motion according to Newton's First Law. So option (b) is correct. 

Due to above two conditions frictional force is not always towards east, it may not even exist. So option (c) is not correct. Frictional force will not be towards west if the vehicle is going towards east, so option (d) is not correct.

(a) is towards east if the vehicle is accelerating

(b) is zero if the vehicle is moving with a uniform velocity

Explanation: 

When the vehicle is accelerating towards east the frictional force must be towards east according to Newton's Second Law of Motion. So option (a) is correct. If the the vehicle is moving with uniform velocity no force is applied in the direction of motion according to Newton's First Law. So option (b) is correct. 

Due to above two conditions frictional force is not always towards east, it may not even exist. So option (c) is not correct. Frictional force will not be towards west if the vehicle is going towards east, so option (d) is not correct.

The Right option is : (d) gravel.

Larger force will be required to move the heavier block.

D) i and ii only True

On the surface covered with sand, it will cover the least distance because sand offers maximum friction against its motion.

Because initially they had to apply force to set the card in motion but once the car started rolling,they had to apply force only to balance rolling friction of the car, the value of which is very less.

Gymnasts apply a coarse substance to their hands to increase the frictional forces on their hands so that they can hold firmly.

Sportsman use shoes with spikes to increase the force of friction between the shoes and the ground. The increased force of friction give them a better grip while running.

 A tug of war can not happen on a friction-less surface because the rope will not remain tight as soon as it is pulled. Feet of both party will slip and they will dash against each other.

When two surfaces are polished the undulations on the surfaces reduce and it offers less resistance to sliding on each other. The actual points of contact where molecular bonds are formed is less. So the friction coefficient between them decreases.  

But when these surfaces are made highly smooth the actual contact points between them increases many folds. At these points molecular bonds are formed. To slide them over each other a large amount of force is needed to break these molecular bonds. So the coefficient of friction becomes very large. 

On a level road the full weight of the car presses the road and the normal force is maximum and so is the force of friction. While on an incline only a component of weight (mg.cosθ) is available to press the road normally. So normal force is always less than the weight and the force of friction also less than the level road. So the grip of the tires is less.

Since no friction force can develop to move me, I would throw my bag horizontally with a force in the opposite direction to which I want to go. According to Newton's third law of motion the bag too will push me with the same force in the opposite direction. This force will push me and since ice is so slippery that it can offer no friction, that is why me and my bag will come to the edge of the pond sliding though on the opposite directions.

A) less friction

1. We will increase friction by using brake pads in the brake system of bicycles and automobiles. 

2. Kabaddi players rub their hands with the soil for an easy grip of their opponents. 

3. Gymnasts apply some coarse substances on their hands to increase friction for better grip.

B) Covered with hard cement

Correct option is A) force

D) Above all

1. When we rub our hands together vigorously for few minutes we feel heat due to friction. 

2. When we strike a matchstick, against rough surface, it catches fire due to friction.

A) friction force increases

B) carrom board

D) Above all

Correct option is B) 2 > 1

Tyres are treaded to Increase friction.

A spring balance is used to measure the weights of object with the help of tension force.

a) Friction may cause fire

Dynamometer is used to measure the forces acting on any object.

The friction forces always act in the opposite direction of motion.

Spring balance is a device used for measuring the force acting on an object.