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Wear and tear is damage that naturally and inevitably occurs as a result of normal wear or aging. It is used in a legal context for such areas as warranty contracts from manufacturers, which usually stipulate that damage from wear and tear will not be covered.
Wear and tear is a form of depreciation which is assumed to occur even when an item is used competently and with care and proper maintenance. For example, repeated impacts may cause stress to a hammer's head. In the normal use of a hammer for its designed task, this stress is impossible to prevent, and any attempt to eliminate it would make the hammer useless. At the same time, it is expected that the normal use of a hammer will not break it beyond repair until it has gone through a certain amount of use.
The phenomenon of wear and tear is a good demonstration of the second law of thermodynamics in action—roughly speaking, objects will stray from their original form and function over time (they are said to have higher entropy), unless energy from an external force is used to maintain them and, in some cases, restore them to their original design. In some cases restoration may be impossible—an object that is impossible to restore would be a consumable. Parts that are designed to wear inside a machine, like bearings and O-rings are intended to be replaced with new ones; consumables like paper, cardboard, fabrics, and product packaging are designed with a service life commensurate with their intended use. For example, grocery stores may issue customers a paper or plastic sack to carry out groceries, but it is intended that the sack will have a short lifespan before wear and tear would cause it to fail.
Durable goods (e.g. automobiles, heavy machinery, mainframe computers, musical instruments, handguns, water heaters) are designed with wear parts that are maintained generally by replacement of component parts. One way to determine if a good is durable or not is whether a service technician or repairman would typically attempt repairs on it. A specialist may need to be consulted, such as an auto mechanic, a computer technician, a luthier, a gunsmith, or a plumber. An automobile's engine may repairable with a simple adjustment or replacement of a single and inexpensive broken part. Likewise, the consumer of a durable good like an electric water heater appreciates being able to replace a failed water heater element (subject to extensive wear and tear) rather than the entire water heater. Whereas an automobile needs both fuel and lubricants (consumables) to operate; it has other parts like tires, paint, and fabric seats which are subject to wear and tear. An automobile manufacturer may disclaim any warranty work that would cause it to repair or replace the tires or the fabric seats or to paint the vehicle.
A company which supports a warranty on a product with the possibility of wear and tear will usually limit the warranty to a period of time in which its wear and tear will probably not be enough to impede the use of the product significantly. Other factors such as the willingness of a customer to replace a product through warranty will affect how long the company offers it. A warranty for a durable good would likely be very different from a mass-produced commodity or consumable like a screwdriver.
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