Most people know that pulleys make lifting heavy loads much easier, but not everyone knows exactly how pulleys work. If you intend to design or use a pulley to lift a heavy load, you should learn about the rules that govern pulley systems in order to make sure your use of a pulley system is safe and efficient.
Conservation of Energy
The core principle at work in a pulley system is the law of conservation of energy, which states that nothing can create or destroy energy. Therefore, the amount of force in a system will always stay the same no matter how you transfer that force. Pulleys work by changing the direction of force, allowing you to spread that force among multiple supports. Because energy is conserved, the total force in a system will be the same and the weight will divide equally among the different supports.
A support in a pulley system is any force which applies upward force. Because gravity naturally pulls weight downward, forces which apply downward force do not counteract gravity and therefore do not provide support. A support can be a solid surface to which a hanging pulley or the line attaches or a person pulling up on the loose end of the line.
The number of supports determines the number of times you divide the weight among the different segments of the line. This number is known as the mechanical advantage of the pulley, and it effectively multiplies the force which you apply to the system. For example, a pulley with two supports has a mechanical advantage of two, and therefore you would only need 1 pound of force to lift a 2-pound weight hanging from that system.
The basic rules of the pulley system don't take friction into account. As you add more pulleys to a system, the friction will increase the resistance on a line. For most pulleys, the friction is not significant. Still, this means there are a limited number of pulleys which you can add to a system before the built-up friction becomes a significant source of resistance. Unfortunately, there is no simple equation for determining resistance because it will depend upon the material, size and shape of both the line and the pulleys.