BASIC PULLEY PHYSICS

This figure shows a single pulley with a weight on one end of the rope. The other end is held by a person who must apply a force to keep the weight hanging in the air (in equilibrium).  There is a force (tension) on the rope that is equal to the weight of the object. This force or tension is the same all along the rope. In order for the weight and pulley (the system) to remain in equilibrium, the person holding the end of the rope must pull down with a force that is equal in magnitude to the tension in the rope. For this pulley system, the force is equal to the weight, as shown in the picture. The mechanical advantage of this system is 1!

In the second figure, the pulley is moveable. As the rope is pulled up, it can also move up. The weight is attached to this moveable pulley. Now the weight is supported by both the rope end attached to the upper bar and the end held by the person! Each side of the rope is supporting the weight, so each side carries only half the weight (2 upward tensions are equal and opposite to the downward weight, so each tension is equal to 1/2 the weight). So the force needed to hold up the pulley in this example is 1/2 the weight! The mechanical advantage of this system is 2; it is the weight (output force) divided by 1/2 the weight (input force).

Each figure below shows different possible pulley combinations with both fixed and moveable pulleys. The mechanical advantage of each system is easy to determine. Count the number of rope/cable segments on each side of the pulleys, including the free end. If the free end is to be pulled down, subtract 1 from this number. This number is the mechanical advantage of the system! To compute the amount of force necessary to hold the weight in equilibrium, divide the weight by the mechanical advantage!  

Here there are 3 sections of rope. Since the applied force is downward, we subtract 1 for a mechanical advantage of 2. It will take aforce equal to 1/2 the weight to hold the weight steady.

This figure has the same two pulleys, but the rope is applied differently and it is pulled upwards. The mechanical advantage is 3, and the force to hold the weight in equilibrium is 1/3 the weight.