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Glossary
centre of gravity. The point around which a body's weight is equally balanced in all directions. The total weight of the object is concentrated at this point.
coefficient of friction. The ratio of the force that is necessary to move an object and the weight of the object. It is a measure of the amount of friction that exists between two materials as one slides over the other. The coefficient of friction is zero if there is no friction, and it is infinite if no motion is possible. For more information see How Brakes Work: Friction (How Stuff Works, USA).
greenhouse gas. A gas that is transparent to incoming solar radiation and absorbs some of the longer wavelength infrared radiation (heat) that the Earth radiates back. The result is that some of the heat given off by the planet accumulates, making the surface and the lower atmosphere warmer. For more information see The greenhouse effect (CSIRO Atmospheric Research, Australia).
mass. The amount of matter in an object.
traction. The amount of forward thrust that a wheel can provide before it slips. It is the product of the weight bearing down on the wheel (generally 25 per cent of the vehicle weight on a level road) and the coefficient of friction, which depends on the nature of the tyre and the surface of the road. Traction helps determine the steepest road a vehicle can climb.
Posted December 2003.