A pulley is a wheel that is mounted on an axle, which helps in supporting movement of a belt along its circumference. Pulleys serve for transmitting power, as well as elevating loads. Some pulley systems are complex, while others are simple devices for local production. A simple pulley that has a single fixed pulley alters the direction of lifting the force. For instance, a single pulley for lifting some load up the roof will consist of a fixed pulley with a rope for pulling down instead of pushing up.
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The effort that is required to uplift the force is equal to the effort that is required to lift the load without the pulley. However, it is easier in uplifting a load with a single fixed pulley because the pulley enables one to pull the load towards the gravitational acceleration, rather than against it like in the case when one is uplifting a load without the pulley. Figure 1 below shows a single pulley, which is uplifting a load of 100N with a force of 100N. The single pulley helps in changing the direction of uplifting the load from upward to downward, but does not affect the force required to uplift the load, which is constant with or without the pulley.
A Single pulley is made-up of a grooved wheel, plus a block that holds it. A single pulley has a rope or a belt that runs through the groove around the wheel with one end attached to a weight or a fixed block as shown in figure 1 above. Pulleys are made of materials such as belt, rope and steal. Fixed pulleys do not have mechanical advantage, while single moveable pulleys have it.
- Mechanical Advantage (M.A.) = Load/Effort.
For single fixed pulleys, the M.A is equal to one because the effort that is required to lift the load is equal to the weight of the load. However, for single movable pulley, the M.A. of the system is equal to two. This is because the effort that is required to uplift the load is equal to half the weight of the load.
A combined pulley has several advantages over single pulleys. A fixed pulley has a disadvantage of applying more effort than the load uplifted because of the presence of friction force. Similarly, a movable pulley has a disadvantage of pulling the load upward, which makes it difficult. When, one combines a fixed and a moveable pulley, one gets a more efficient system than one experienced by single fixed and movable pulleys.
The presence of the fixed pulley helps in changing the direction of pulling the load from upward to downwards, which makes it easier. In addition, the existence of the movable pulley helps in making the effort required to lift the load less than the load uplifted. A combined pulley has a M.A. of two, which makes the effort required to elevate the load half the weight of the load. However, the main problem with this system is that the effort travels a very long distance. Figure 2 shown below illustrates a combined pulley that consists of a single fixed and a moveable pulley.