An ordinary man can do one sixth of a horse power. The average
locomotive of a railroad has more than 500 H.P., while the engines of
an ocean liner may have as high as 70,000 H.P.
169. Waste Work and Efficient Work. In our study of machines we
omitted a factor which in practical cases cannot be ignored, namely,
friction. No surface can be made perfectly smooth, and when a barrel
rolls over an incline, or a rope passes over a pulley, or a cogwheel
turns its neighbor, there is rubbing and slipping and sliding. Motion
is thus hindered, and the effective value of the acting force is
lessened. In order to secure the desired result it is necessary to
apply a force in excess of that calculated. This extra force, which
must be supplied if friction is to be counteracted, is in reality
waste work.
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