wearing away quickly. You add oil to a car’s engine so that the pistons will slide back and forth with less friction. Even water can be used as a to reduce friction between objects if they are not too hot. Ball bearingsBall bearings reduce friction on spinning objects (Figure 13.8). Ball bearings change sliding motion into rolling motion, which has much less friction. For example, a metal shaft rotating in a hole rubs and generates a great amount of friction. Ball bearings that go between the shaft and the inside surface of the hole allow it to spin more easily. The shaft rolls on the bearings instead of rubbing against the walls of the hole. Well-oiled bearings rotate easily and greatly reduce friction.MagneticlevitationAnother method of decreasing friction is to separate the two surfaces with a cushion of air. A hovercraft floats on a cushion of air created by a large fan. Magnetic forces can also be used to separate surfaces. A magnetically levitated (or maglev) train uses magnets that run on electricity to float on the track once the train is moving (Figure 13.9). There is no contact between train and track, so there is far less friction than with a standard train on tracks. The ride is smoother, so maglev trains can move at very fast speeds. Maglev trains are not yet in wide use because they are much more expensive to build than regular trains. They may become much more popular in the future.Figure 13.8: The friction between a shaft (the long pole in the picture) and an outer part of a machine produces a lot of heat. Friction can be reduced by placing ball bearings between the shaft and the outer part.Figure 13.9: With a maglev train, there is no contact between the moving train and the rail — and thus little friction.
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