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4.2 Work is done in stretching or compressing a material

   Why does stretching a spring involve doing work?

In the process of bending, stretching or compressing, energy is transferred in the process, so work is done.

You are having to do work against an opposing force e.g. a stretched spring or rubber band, a squeezed rubber ball want to return to their original shape

 In order to deform a material to be bent, stretched or compressed two forces must be operating, often in opposite directions.

If only one force was involved the material would stay the same shape and just change position.

Bending occurs when you wind up a mechanical clock. Strictly speaking on the 'outer' surface to the right the force is one of tension (stretching) and the 'inner' surface to the left, experiences compression (think of bending quite a thick plate, ok?).

Stretching happens when you use a spring balance to weigh something or put a rubber band round an object.

Older (still do?) forms of car suspension use steel springs which compress on meeting a bump in the road to absorb the energy of the impact.

Rubber shock absorbers in the under carriage of a car have the same effect.

The large sturdy spring S of the wheel suspension of a Land Rover, which is expected to absorb the impact energy of some pretty hefty bumps! On compression, the stiff spring will store a great deal of elastic energy, if only for a brief moment in time!

Key for the 5 photographic diagrams: S = suspension spring;  H = the pipe conveying the hydraulic brake fluid (see hydraulics)

D = the brake drum and disc on which the brake pads in casing P are forced into contact with the smooth disc by hydraulic pressure when you press the brake pedal.

 

A double spring suspension on a red van, conveniently jacked up!

'Spring' photographs by courtesy of Mark Raw of M T R Autotech Ltd garage, Castleton, North Yorkshire, England

Sub-index of physics notes: FORCES 4. Elastic potential energy

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Keywords, phrases and learning objectives for elastic potential energy

Be able to explain why work is done in stretching or compressing a material and elastic potential energy may stored or absorbed e.g. in car suspension springs or shock absorbers.

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Sub-index of physics notes: FORCES 4. Elastic potential energy