单词	pulley
释义	pulley A simple machine consisting of a wheel with a flat, crowned, or grooved rim to take a belt, rope, or chain with which a load can be raised. See illustration. In fig (a), assuming the system is frictionless, the force P in any part of the rope is constant, therefore 2P = L , where L is the load. In general, nP = L , where n is the number of supporting ropes. In fig (b), the number of supporting ropes is 4. The mechanical advantage of a pulley system is the ratio of the load, L, to the effort applied to the free end of the rope, P, i.e. mechanical advantage = L/P = L(L/n)-1 = n . Thus in fig (b) the mechanical advantage is 4. A combination of ropes and pulleys as in fig (b) is called a block and tackle . Pulleys
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