Sandpaper is used to remove materials, such as dirt, paint, etc from the surface of metals or ceramics.  Sandpapers consist of sheets of paper, one side of which is coated with an adhesive.  The paper can be held and rubbed against the surface, which is to be cleaned. 

Originally, Sand, glass or seashells were used as abrasives. Today, however, the abrasives used are aluminium oxide and silicon carbide. 

Sand papers are also known as emery papers.  The sandpapers are standardized according the size of particles they contain. The sand paper number specifies the finenes of the abrasive used.  A higher number indicates greater fineness. A number 6 will have coarser particle than a number 20 sand paper.

Backing refers to the paper or fabric to which the abrasive crystals are bonded.  The backing can be cloth, polyester, rayons. 

The abrasive is bound to the backing by means of a adhesive substance called the bonding.  The bonding should be strong enough to hold the abrasive while the rubbing action takes place. 

Clogging is the accumulation of dirt and removed material between the abrasive crystals.  This can affect the effectiveness of sandpaper action.  Wet Sandpapers can be used with a liquid, such as water to remove the clogged impurities.

A Shock absorber is a device which absorbs shock and vibration.  The energy of the shock is converted to heat and dissipated.  Shock absorbers can be mechanical or hydraulic. 

In automobiles, shock absorbers serve to absorbs the bumps and irregularities on the road surface.  They also make sure that the tyres are in contact with the roads at all times.  This is essential for reliable braking and steering action. 

Shock absorbers are designed in many ways.  The simplest shock absorber is a spring based one. The load of the chassis is transmitted through a spring to the axle.  Both coiled springs and leaf springs are used in shock absorbers. 

Leaf springs consist of strips of metals placed on top of one another and rivetted together.  The energy of the shock is dissipated as heat as the strips rub against each other.

Hydraulic shock absorbers consist of a piston and a cylinder containing hydraulic oil.  The piston pushes against the oil which is forced trough a small opening in the piston.  The oil takes time to enter the opening due to its viscosity.  The resistance offered by the oil serves to retard the movement of the piston and absorbs the vibration.

A bumper in an automobile is the plastic component which runs along the length of the vehicle in the front.  It is made of a flexible and tough polymer, fibreglass or a composite material.

The bumper serves to absorbs the energy of low velocity collisions.  The bumper deforms and absorbs the energy of a collision.  Bumpers are designed to regain their original dimension after the energy of the collision has been dissipated.

Besides absorbing energy of collisions, they also serve to protect pedestrians by minimizing the impact.  Some bumpers are designed to guide the people to topple over the vehicle rather than under it.

The Air conditioning system is a vital aspect of today's automobile.  It is difficult to imagine a car without it.  The Air Conditioning unit, as the name suggests, "conditions" air.  Air is conditioned in terms of temperature and humidity.  The temperature of the air is reduced and its humidity is removed.

The Air conditioning cycle or the refrigeration cycle works by compressing a gas, cooling it, allowing it to expand and absorb heat from the air to be cooled and the compressing it to a liquid again.  The refrigerant is the gas which is used as the working fluid.  In earlier times, ammonia was used as the refrigera 

The air conditioner consists of the following components.

Compressor:  The compressor is the central part of the air conditioning system. The compressor compresses the refrigerant (the gas used for refrigeration).  It is driven by the engine of the automobile through a belt. 

Condenser: The compressed air from the condenser is cooled by the condenser by means of a cooling fan. 

Evaporator: The Evaporator is place where the liquid refrigerant transforms into a gas.  As this happens, heat is absorbed from the surrounding regions.  There is a expansion valve which regulates the evaporation of the liquid and consequently the temperature.  Outside, air is passed through the tubes.  The air loses heat and becomes cool and is blown into the car as cool air. 

Accumulator.  The accumulator is used as reservoir for the liquid has not still evaporated.  This can cause damage to the compressor.  The Accumulator serves to hold the liquid refrigerant.