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A spring is an object that upon application of a force in a given direction compresses and then, after removal of the force, decompresses or springs back to its original dimension. Spring force is the description of the force that causes the spring to rebound. It is a characteristic of the material at a molecular level and its three-dimensional shape on a macro level. Hooke’s law is the customary formula for calculating this force.
Constant pressure on a surface may be all that is required of a spring in a mechanism. Springs in a car’s shock absorbing system are not calibrated to measure deflection but to absorb the energy transmitted from the wheels to the car. In many small appliances or electrical devices, switches consist partly of a piece of metal stripping acting as a spring. The strip changes shape as pressure is applied, which then connects a new set of electrical contacts.
In other applications, springs provide a proportional quantitative feedback to the user. When measuring a weight, the spring force is compressed a distance proportional to the pull of gravity on the weight. The more closely a linear response is observed, the better a given material and configuration will suffice in a metrology application. Materials stretched beyond their elastic limit will no longer respond as a spring.
Springs are not considered a type of simple machine, as they do not transfer force through a distance. They need to deflect or stretch through a distance to absorb the energy applied to the spring. Most of the energy is released back in the original incoming direction. The spring force is always applied at the incident angle. Some energy is lost as heat.
Hooke’s law states that force is equal to the negative of the spring constant multiplied by the distance. As long as a spring is functioning within its elastic limits and responds proportionately to the applied force, the spring is a Hookes-law spring and its material is considered so. These materials are said to have a linear-elastic property and will have a characteristic spring constant. The negative sign is the result of the resultant force from the opposite direction of the incident force.
In the new field of microelectromechanical systems (MEMS), also called micro-machines, microscopic springs have been made. These nano-scale springs are made from films etched in a way similar to integrated circuit (IC) packages. Researchers have demonstrated Hookean behavior and have used them as tiny rulers or probes to detect surface variations.