FRICTION

IMPORTANT TERMS

Force                            Force is a push or pull acting on an object.

Non-contact Force        Forces that do not need physical contact with the object on which they are acting are called non-contact forces. Examples- Gravitational Force, Magnetic Force and Electrostatic Force etc.

Contact Force              Forces that act on objects by direct or indirect physical contact with the objects are called contact forces. Examples- Applied Forces and Frictional Force are the examples of contact forces.

Static Friction               Static friction is the friction that exists between stationary object and the surface.

Rolling Friction              Rolling friction is the resistance offered by the surface to the motion of an object which rolls over it.               

Sliding Friction              Sliding friction refers to resistance created by two objects sliding against each other.

Fluid Friction                The force of friction offered by the fluids (air and water) is called fluid friction. It is also called drag.

 

FORCE AND ITS EFFECTS

     Force is a push or pull to an object. It has the following effects on the objects.

1. Force can make stationary object move or make a moving object more fast- A toy car can be made to move by giving it a little push. Similarly if we have a moving toy car we can make it move faster by giving it a push in same direction in which it is moving.
2. Force can slow down or completely stop a moving object- A moving toy car be stopped by just applying a force in opposite direction of its motion.
3. Force can change the direction of moving object- In a football game, the players can change the direction of moving ball by kicking it in different direction.
4. Force can change the shape or size of an object- The shape of rubber band changes when force is applied. The objects made up of glass break easily when we apply force on them.

 

CONTACT FORCES- Forces that acts on objects by direct or indirect physical contact with the objects are called contact forces. Examples- Applied Forces and Frictional Force are the examples of contact forces.

 

Applied Forces

The force which is applied on an object by a person or another object is called applied force.

When we do work with our hands like lifting weight, or pulling an object etc. we are applying force by using our muscles power.

 

Frictional Force

Friction is the resistance to motion, experienced, when two surfaces in contact move with respect to each other.

Whenever the surface of one body moves over another, each exerts a force on the other which opposes the motion of the other. This is called frictional force.

Friction opposes the relative motion of the body as it always acts in the direction opposite to the direction of the motion of the object. For Example- if we gently give a push to a book lying on the table it stops after moving some distance. It means there is some force acting in opposite direction which stops the further movement of the book. This force is the force of friction which acts in the opposite direction of the motion.

 

CAUSES OF FRICTION

Any surface, however smooth, has lot of irregularities when seen under microscope, so when two such surfaces come in contact with each other and slide over each other, the irregularities gets interlocked with each other, which resists the motion of the surfaces. This leads to friction. On the rough surfaces, the number of irregularities are more hence friction offered by rough surface is also more than smooth surface.

 

Factors affecting Friction

 

1. Nature of the Surface- The force of friction depends on the nature of the surfaces in contact i.e. smooth surface or rough surface. As we know, friction is caused by irregularities on the surfaces in contact so if the surfaces are smooth then the number of irregularities will be less hence the friction will also be less, but if the surfaces in contact are rough then they have more irregularities on the surface. Such surfaces when slides over each other, gets interlocked due to their irregularities. More the resistance to motion, more will be the friction. So larger force is required to make them move over each other.
2. Mass of the Body- The force of friction depends on the mass of the body which is lying on the surface. More the mass of an object, more will be the friction. For example - One book is lying on the table, a gentle push to the book can it move. But if we put two more of such books having same thickness, more force is required to overcome the friction offered by the table to the books. Hence it is clear that more the mass of the object, more will be the friction.

 

TYPES OF FRICTION

 

1. Static Friction- Static friction is the friction that exists between stationary object and the surface.
2. Sliding Friction- Sliding friction refers to resistance created by two objects sliding against each other.
3. Rolling Friction- Rolling friction is the resistance offered by the surface to the motion of an object which rolls over it.               

 

The force required to overcome friction at the moment when an object starts moving from a position of rest is a measure of static friction. On the other hand, the force required to keep the object moving with the same speed is a measure of sliding friction.

When a box starts sliding, the contact points on its surface, do not get enough time to lock in to the irregularities. So the sliding friction is smaller than static friction.

 

When a body rolls over the surface of another body, the resistance to its motion is called rolling friction. Rolling reduces friction. It is always easier for a body to roll than to slide over another body or surface. That is why it becomes easy to pull luggage fitted with rollers beneath them.

 

Since rolling friction is less than sliding friction, so the sliding mechanism in most machines is replaced by rolling mechanism with the use of ball bearing to reduce the friction.

 

The above effectively means the following in terms of the magnitude of friction –

Static Friction > Sliding Friction >Rolling Friction

 

FLUID FRICTION

The force of friction offered by the fluids (air and water) is called fluid friction. It is also called Drag.

When car and airplanes move at high speeds, their motion is opposed by friction offered by air molecules surrounding them. This friction, which opposes the motion of a vehicle, produced by air is called drag. The same applies to ships and boats in water. To reduce drag, all automobiles, ships and airplanes are given a special shape, called a streamlined shape. A body with streamlined shape experiences minimum resistance when travelling through a fluid like air or water. Even sea animals like fish and shark, and Birds also have streamlined bodies which make it easier for them to move with great speeds.

 

ADVANTAGES AND DISADVANTAGES OF FRICTION

 

Advantages

1. Friction helps us to walk safely on the road as friction helps us to have better grip with the ground while walking. We can move without skidding unless the floor is wet and offers very less friction.
2. Friction helps us to write on the board and on notebooks; we cannot write with chalk or pen in the absence of friction.
3. If an object starts moving, it will not stop if there were no friction on the floor. Friction is required to stop the moving object.
4. We can fix a nail in the wall with the help of friction. Friction have many more advantages in our daily life.

Disadvantages

1. Friction produces heat in moving parts of machine which results in wastage of energy and fuel.
2. Friction cause wear and tear of moving parts. The shoe sole gets flattened after few months due to the friction offered by the ground.

 

Methods of Reducing Friction

1. Friction in moving parts is reduced by introducing a substance between moving surfaces. This process is called Lubrication. The substance that is so introduced is called lubricant. Common lubricants are oil and grease. Decreasing the friction increase the efficiency of machines.
2. Ball bearing is also used to reduce friction. Ball bearing change the sliding friction into rolling friction, as we know rolling friction is less than sliding friction and hence the friction is reduced.
3. By giving a streamlined shape to the object moving in air/water, we can reduce the friction.
4. While playing carrom board, we can sprinkle some powder to reduce friction.

In some machines it is not advisable to use oil as a lubricant. In these cases, air cushion between moving parts are used to reduce friction.

 

Friction can never be entirely removed from a surface. No surface is perfectly smooth, some irregularities are always there even after polishing the surface.

 

Methods of Increasing Friction

Friction can be increased either by making surface rough or by increasing mass of the object that is moving on it.

 

1. Tyres are treaded (designs are made on the tyres) to provide better grip with the ground by increasing friction.
2. Soles of shoes have grooves to increase the friction and provide better grip to the ground.
3. Kabaddi players often rub their hands with soil for better grip of their opponents.
4. We increase the friction by using brake pads in the brake system of all automobiles. While riding bicycle, brake pads do not touch the wheel but when we apply brakes these brake pads arrest the wheel to increase the friction and hence stop the bicycle.

 

SPRING BALANCE

Spring balance is a device used for measuring the force acting on an object. It consists of a coiled spring which gets stretched when a force is applied to it. Stretching of the spring is measured by a pointer moving on a graduated scale. The reading on the scale gives the magnitude of the force.

Some Interesting Facts

• There is no atmosphere in deep space but friction is present in the deep space.
• A Maglev train moves by the force of magnetism. Magnets under the train and on the track push against each other and make the train hover 1/2 inches above the track. Such train does not have engines, but it is pushed forward by other sets of magnets.
• We can run as fast as a sports car, while skydiving. Once we step out of the plane, gravity pulls us down and our free-fall speed can be from 100 to 160mph.
• All the three laws of Newton are not valid at atomic or macroscopic level.
• About 20 percent of engine power of automobiles is consumed in overcoming frictional forces in the moving parts.