Humans have been using pulleys for centuries to help lift objects that they otherwise could not transport. The pulley is one of the six famous simple machines, designed to make difficult work easier. Along with the lever, incline plane, wedge, screw, and wheel and axle, the pulley has played a key role in facilitating some of mankind's greatest feats.
What Are Pulleys?
Pulleys consist of wheels and ropes or chains. The wheels often are grooved so that a rope or chain may fit more snugly against the wheels with less risk of slipping off. By pulling on one rope, you can transfer force throughout the machine to lift a heavy or unwieldy object at the other end. A pulley must have at least one wheel and one rope, but may have many more of either or both.
One-Wheel Systems - Redirection
Pulleys that are made up of only one wheel and one rope help with lifting by redirecting force. You must still exert the same force to move the object, but in a different direction. Since it is usually particularly difficult to lift heavy objects, a pulley may allow you to lift the object while exerting a downward force. You can use your body weight to help pull down on a rope, which makes that work seem easier than dead-lifting the object.
Multi-Wheel Systems - Mechanical Advantage
In more complicated multi-pulley arrangements, which feature multiple wheels and ropes, lifting heavy objects seems even easier. The interlocking pulleys create what is known as a mechanical advantage (ME), in which the machine allows the user to exert less force to accomplish a task. The more wheels and ropes that a pulley contains, the easier it is to lift an object. In industrial or construction settings, complex block-and-tackle pulleys help lift tremendous weights.
Work and Force
Lifting the same object the same distance will always require the same amount of work, regardless of whether or not a pulley is used. Work is a measure of force times distance. Complex pulleys with multiple ropes increase the distance over which the force is exerted, so less force is necessary to accomplish the lift. That is why it is easier to lift objects with pulleys. Simple pulleys do not reduce the force necessary, but allow you to exert that force in a more convenient direction.