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.
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.
Subjects: Science and Mathematics — Sports and Exercise Medicine.