John Elliott, age 11, of Pepperell, Massachusetts, for his. question:

What makes a pullev work?

Pulleys are mechanical movers and weight lifters, but they do not supply the effort to do the work. A simple type of pulley can help you lift a load more smoothly. Another type can help you lift a 60 pound burden with the same amount of strength you would need to lift a 20 pound load.

The Machine Age makes use of five simple gadgets and the pulley is one of them. It has been used as a weight lifter for at least 2,000 years. It can be a simple pulley or a complicated block and tackle. Its various forms are used in cranes and hoists and countless pieces of wheel turning machinery. It has no lifting strength of its own. But it can reduce the effort needed to do a job. The effort needed may be provided by human muscle, or a sturdy horse, by electricity or by some other form of energy.

A machine should have two or more moving parts. The simplest pulley has only one moving part so it is a mere gadget. It is a thick, solid wheel with a groove around its rim. The groove is a slot to hold a rope that goes up and around the wheel. This pulley has a hook to hang it from a sturdy beam and it is called a single fixed pulley. A load is tied to one end of the rope. The other end is pulled by the person with the muscle power.

Suppose the weight of the load is 60 pounds. The man must use 60 pounds worth of effort to lift it. He pulls on the rope, hand over hand, with his 60 pounds of effort and up comes the load. With every foot of rope that goes through the pulley, the load lifts,: one foot above the ground. A pulley of this type may be used to lift a bucket of water from a well. It makes it easier to lift a load from a tight spot. But it cannot reduce the effort needed to do the lifting job. The pulling effort equals the weight because the load is supported by only one strand of rope.

Suppose we make the single pulley movable. Unhook it from the beam and hang one end of the rope in its place. Fix the load to the hook on the pulley and loop the rope around the groove. Lift the load by pulling the free end of the rope. You will be astonished to find that the weight seems lighter. To lift the 60 pound load you exert only  enough effort to lift about 30 pounds. (It would be exactly 30 pounds if some energy were not lost by friction.) This is because the load is supported by two strands of rope, instead of a single strand. The two supporting strands share the load and cut the work power in half. A single movable pulley has a mechanical advantage of two.

A rope can be wound around two or more pulleys. Each loop adds an extra supporting strand to the load and reduces the effort needed to lift it. If there are four strands, each takes one fourth of the load. You can hoist a 60 pound load with the same muscle power needed to lift 15 pounds. In every case, you pay for this mechanical advantage with distance. If the advantage is two, you must pull the rope two feet to lift it one foot. If the advantage is four, you must pull four feet of rope to lift the load one foot.

There are countless ways to take advantage of the pulley. Some machines use the continuous pulley. The two ends of the rope are woven together to form an unbroken ring of rope. Some machines use a continuous belt instead of a rope. In this case, the rim of the wheel bulges out like a barrel and the belt rides around over the hump. A machine may use two or many more connected pulleys. But some kind of power or energy is needed to turn one of the wheels. This wheel turns the other wheels and keeps the belts and other parts moving.