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Green Corner




An A-Z of the environment & issues

by Peter Hatswell



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E  F  G  H  I  J

K  L  M  N  O  P

Q  R   S   T   U   V

W  X  Y  Z



K is for kinetic energy

Not the sort you can buy from a utility supplier or a battery shop but the energy that exists because of the speed and weight of an object. It shows itself when you try and speed up or slow down say a car, train or aircraft, the most energy being used when the weight is greatest and/or the speed rapidly changes.


Acceleration absorbs energy and de-acceleration releases it, usually in the form of heat in brake blocks. In the case of an aircraft you also need to provide energy to lift it from the ground and the steeper the flight path and the higher the cruising height the more energy is required. On landing, all this energy is lost to the friction of air resistance and is revealed by the burning of rubber on the runway.


Kinetic energy can be used to good effect in a flywheel which acts like a battery, capable of releasing or absorbing energy at significant rates but requiring only a relatively small input to maintain speed. For the Luton-Dunstable rail line a system has been proposed for a rail car powered by a small gas engine and harnessing most of the braking energy with a dynamo to keep the flywheel spinning. The result of this system combined with the low friction of rails means that the rail car uses one third of the energy of an equivalent bus with similar acceleration and slowing capabilities.


A more everyday energy saving advantage can be seen in the ’defensive driving’ technique in which a driver accelerates gently, holds a steady reasonable speed and looks far ahead so that the brakes can be applied minimally. Choosing a car with low weight and a smooth aerodynamic profile should provide the best energy efficiency and the longest working life. It is a fact of life that the greatest wear and tear on a machine takes place when the most energy is being used.




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