Friction

The friction always opposes the motion.

When the engine of a car is switched off, then there is no energy available for the engine to oppose the frictional forces. Thus, the engine slows down and eventually comes to rest.

(a) Smooth surfaces offer least friction.

(b) Rough surfaces offer high friction.

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

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

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

(a) Rolling friction is the smallest in magnitude.

(b) Static friction is the largest in magnitude.

Rolling friction comes into play when a book kept on cylindrical pencils is moved by pushing.

The well-polished floor offers very less resistance. Therefore, the shoes slip on it and its very difficult to walk properly.

It is very difficult to walk on a wet floor because the friction is less and the required grip cannot be maintained.

Frictional force is responsible the wearing of car tyres.

Frictional forces prevent us from slipping every time you take a step forward.

Frictional forces helps things to move and stop.

Friction enables us to fix nails in a wall and knots to be tied.

Frictional force is responsible the wearing of footsteps of over-bridges.

The tyre’s surface has treads on it. Treads improve the grip of the tyre on the road.

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

Kabaddi players rub their hands with dry soil before playing so as to increase friction. This enables them to grip the other player’s hand properly.

Ball bearing is used between the hubs and axles of bicycle wheels to reduce friction.

Ball bearing reduce friction between the support loads and axles of a vehicle.

Car and bicycle.

Pulley.

Car

Oiling the-axles of a bicycle reduces the friction and thus the bicycle moves easily.

The ball bearing help in reducing the friction between the wheels and it axle.

Castor oil and Grease are the two common lubricants.

Note: A lubricant is a substance that smoothens a surface. It decreases the friction of a surface.

Sprinkling fine powder on carrom board reduces friction between carom and striker.

Frictional force gets reduced when the two surfaces in contact are polished to make them smooth.

The surface of a slide polished to make it smooth in order to reduce friction.

Frictional force increases when the two surfaces in contact are made more rough.

Drag is the special name of frictional force exerted by fluids.

The special shape is more like a point shaped so that they cut the fluid friction.

Grooves in the soles of shoes help in increasing the friction when we walk on the roads. The grooves in the shoe sole help in maintaining a grip with the ground.

Treads are made on the surface of tyres help in increasing the friction. These treads help in maintaining a firm grip on the road which reduces the risk of slipping on the road thus avoiding accidents.

(a) Friction always opposes motion between the surfaces in contact with each other

(b) Sliding friction is less than the static friction.

(c) Friction produces heat

(d) Friction prevents our foot from slipping over the ground

(e) Sprinkling of powder on the carom board reduces friction.

(f) Ball bearings reduce friction because they roll rather than 'slide.

(g) The friction when something moves through a liquid or gas is called drag.

(h) Cars and speedboats are streamlined to reduce drag.

(i) Shapes that are designed to reduce air resistance are called streamlined shapes.

(j) Objects which can move quickly through the water have a streamlined shape.

(k) The shape of an aeroplane is similar to that of a bird in flight.

Whenever we push a very heavy box kept on the ground, it does not move at all because frictional force balances the force that we apply. So, there is no net movement of this box. It acts in a direction opposite to the force we apply.

A rough surface will cause more friction than a smooth surface. Because, rough offers more resistance to the other surface in contact as compared to the smooth surface.

The sliding friction is less than static friction because, in sliding friction, the interlinking of the two surfaces is for very short time.

Friction mainly arises due to the interlocking of irregularities between two surfaces, as shown below in the diagram:

In smooth surfaces, there are very fewer irregularities, thus friction is very very less. But, on rough surfaces, the irregularities are very large.

In sliding friction, the time of interlocking is very less, as one object slides over another object. Whereas, in static friction, one object is moved over another object thus the irregularities time is more. Thus, the two surfaces could not properly come in contact with each other in sliding due to which the magnitude of static friction is more than sliding friction.

Rolling friction is the friction or drag which opposes the motion when a body rolls.

Seema will have to apply larger force than Iqbal.

Because Seema’s box is heavier and Seema will have to apply greater force to overcome the larger friction in case of the heavier box.

While walking on the road, a man slips if he steps on a banana peel because the banana peel acts a lubricant between the road and the man’s shoe which reduces the friction. Thus, man slips on it.

A car spins on icy road because the treads of the car can no longer maintain the proper friction to keep it moving. Due to ice, the friction reduces and the car spins.

(a) A pencil is made up of graphite. The paper surface offers more friction to the pencil when we write on the paper, in this way when the pencil is rubbed on the paper, some bits of graphite come on the paper and we are able to write. But glass offers no friction to the pencil, therefore we can’t write on a glass with pencil.

(b) Climbing a greasy pole is very difficult because the greasy pole offers very less friction to the feet of the person climbing on it. Du to which, the person slips down again and again.

When a matchstick is rubbed on a rough surface, the surface produces friction. And we know friction produces heat, therefore the matchstick lightens up.

The smooth surface offers very less friction, due to which very less heat is produced by it and it is not sufficient to lighten up the matchstick.

The heels of the shoes get wear and tear first than the rest of the shoe sole. This is because, the heel of the shoe is interacted with the ground more than the other part and due to friction, the treads of the shoe sole wear out.

The brake pads of bicycles have to be replaced quite often because they gather more dirt, dust and the filthy matter on itself. So to allow proper functioning, they have to be replaced.

This is because the paper surface offers more friction to the pencil rubber when we rub on the paper, some bits of rubber come on the paper.

By rubbing our hands against each other we create friction between our hands and this friction produces heat. Therefore, out hands become warm.

When step forward and walk, we are applying force (denoted by “F”) on the ground. Now, in return ground also applies the reaction force to us. But we can fall if friction (denoted by “f”) is not present as the large ground can apply a large force . This is explained in the diagram:

It is easier to hold a kulhar (an earthen pot) because it more rough surface as it is made from mud. Mud offers more friction than glass and we are able to from a grip on the earthen plot.

When we apply brake sot the bicycle, a force of friction generates between the rim of the bicycle and the brakes. The friction is produced in opposite direction of motion. Thus, the bicycle is stopped.

The shoes rub on the ground while walking. The ground offers friction due to which the sole of shoes wear and tear out.

The tyres of the car wear out due to friction produced between he tyres and the car when a car is moving.

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.

The sports people use shoes with spikes, spikes increase the friction with the ground and does not let the person slip or fall when running fast.

By applying any lubricant , say grease, one can reduce friction between those machine parts which rub against each other to avoid wear and tear.

Lubrication means applying some additive to the parts of a machine to reduce the friction. Its important because it reduces the friction between the parts of the machine. But excess lubrication can generate more heat. Example- Castor Oil, Grease.

Wheels are important because they are used for transportation. Heavy machines can be moved from one place to another with the help of wheels because wheels can be rolled and the amount of force is also comparatively less.

Lubricants (oil or grease) are applied to rubbing surfaces of machines because when the parts rub against each other, heat is produced. To avoid the wearing and tearing of the machine parts, lubrication is done. Because it reduces the friction and thus less heat is produced.

If we oil between the two surfaces, say part1 and part 2 of a machine, then the oil layer will allow less contact of the two surfaces with each other as you can see in the figure below:

In this way, the friction between the two surfaces is decreased.

The cars, aeroplanes and rockets are streamlined so as to reduce the fluid friction. The air drag obstructs the vehicle to move forward and the fuel is consumed mainly for opposing this opposite frictional force. Therefore, the objects are given special shape called streamline shape, which itself cuts the air friction.s

A speedboat has a streamlined shape so as to reduce the fluid friction as it moves in the waters.

Fluids are those whose which do not have any shape and flow like liquids. Example- Air and Blood.

The force that opposes the motion of an object is called friction. Example, when we walk, friction acts in opposite direction we apply force to walk.

The factor affecting friction are:

1. The weight of the object.

2. The type of contact between the two surfaces.

The cause of friction is surface roughness and adhesion.

When we apply force, F on a book, the book slide (Fig A). Then acts a force of friction, f (Fig B)

The friction is produced due to the contact of the two surfaces of the table and the book. It acts in opposite direction of the force to stop it. And, we see after some distance, the book again comes to the rest (Fig C).

The friction depends on the roughness as well as smoothness of the surface. If the surface is rough, then the surfaces in contact have more interlockings and the thus the friction is more. Thus, the object is not able to move easily due to more friction.

If the surface is smooth, then the surfaces in contact have very less interlockings and the thus the friction is less. Thus, the object is able to move easily due to less friction.

Suppose, a person is pushing a heavy box, then he is applying say force F1.

Now if the same person pushes two boxes of same mass as the previous case. Then, in this case he will have to apply more force to overcome the larger friction as in the earlier case. Thus is because friction depends on the mass of the object. Heavier the mass, more the force is exerted by the object on the surface and thus more friction is produced.

When the body is in stationary state or is at rest, the frictional forces acting on the body is known as static frictional forces.

But when the object is pushed and it moves, the friction that comes into action is sliding friction.

The static friciton is greater than sliding friction because more force is required to remove the objectf rom rest state into dynamic state. Once the obejct comes into motion, it is easier to keep the object moving by applying comparetively lesser force.

A very heavy machine can be moved from one place to another with the help of wheels. Because then rolling friction comes into action and its value is the least among static and sliding friction.

The following diagram shows, how a heavy cylinder can be rolled easily.

The drag is the other name for fluid friction. It resists the motion of any object that travels in fluid.

Examples: 1. A boat travelling in water.

2. An aeroplane moving in the sky.

The objects are built in special streamline shape to reduce the drag while moving in the air. Example, birds and Aeroplane have streamlined shapes.

A streamline shape is the one in which the body is pointed at the ends and broad-shaped in the middle.

Example : Speedboat has streamlined shape.

The objects moving in fluids have streamlined shape so as to reduce the air drag or fluid friction that opposes their motion in any fluid.

As the roughness of the surface increases, the friction force increases.

The static friction > Sliding Friction > Rolling friction.

The force of friction acts opposite to the direction of motion.

A spring balance can measure both the force acting on it as well as the weight of the object.

Friction does not depend on the area of contact.

The static friction is always greater than sliding friction.

Friction does wear and tear of the objects.

The static friction > Sliding Friction > Rolling friction.

The relationship between the three types of friction is:

The static friction > Sliding Friction > Rolling friction

Thus the rolling friction will be less than 20N

The static friction > Sliding Friction > Rolling friction.

As the roughness of the surface increases, the friction icreases.

As the distance travelled is the least in 4.8 m , thus friction would have been the largest.

The spring balance measures weight of an object.

The book starts sliding down slowly sliding slowly, because the sliding friction is less than force of gravity.

A body shape which offers very little resistance to the flow of a fluid around it, is called streamlined shape.

Using talcum powder to reduce the friction on carom board.

Bus is not streamlined shape. It is a cuboid shape.

The frictional force exerted by fluids is called drag force.

Mustard oil reduces the friction on his hands. So, Glass tumbler will be most difficult to handle.

Because rolling friction allows to move heavier objects from one place to other easily.

Floor B offers greater friction because it stops the moving pencil cell stop at a lesser distance of 20 cm on the floor. More the friction, more the surface is rough.

The direction of force of friction acting on this car due to surface of road is along the south direction i.e. opposite to the direction of motion.

It will make more difficult to walk on the floor. As soapy solution decreases the friction on the floor.

(a) Sliding friction

(b) Static friction

(c) Rolling friction

When no person is sitting on the mat, then the mat and floor are not pressed together.

Thus, there is no friction.

But, when a person is sitting on the mat.

Then due to the weight of the person, the mat and floor are pressed together harder, increasing the friction too much. Thus, then it is very difficult to drag the person.