Ups and downs
Suppose someone asked you to lift a hippopotamus out of its
hippopotamus-sized bathtub! It sounds impossible—but with a simple
machine called a pulley, you could do it!
A pulley is a special kind of wheel and axle. The rim of the wheel is
grooved so that a rope or steel cable can fit around it. When one end of
the rope is pulled down, the rope slides over the wheel, which turns on
the axle. Then the load at the other end moves up.
With one pulley, the load moves up as far as you pull the rope down. You
work just as hard to pull the rope as you would to pick up
the load—but you can pull in a direction that is easier for you.
With two pulleys, you can make lifting even easier. The second pulley is
attached to the thing you lift. Each part of the rope between the
pulleys holds half the weight—so you pull only half as hard to move
the load. But the load is held up by twice as much rope as with one
pulley. So you’ll have to pull the rope twice as far as the distance you
want the load to move.
Three pulleys or four pulleys will make lifting even easier. But the
more pulleys you use, the more rope you’ll have to pull.
You might be able to lift that hippopotamus with a hundred pulleys—but
you will have to pull a lot of rope!
