Pulley Mechanical Advantage

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Fig. 1 Each number corresponds to pulley systems with different mechanical advantages(found in the demo section) 

Equipment:

  • Assortment of Pulleys (On the pulley page there is a list of all the pulleys and a photo)
  • Various lengths of string (works best if you use the wax fishing line)
  • Weights (Best weights to use include the 200g and 500g. Anything below is too light to account for non-massless and frictionless pulleys and string)
  • Pulley systems board
  • Newton Scale (OHAUS 0-200N scale)

Demos:

Each demo below corresponds to figure 1(above).

  1. Mechanical advantage of 2
  2. Mechanical advantage of 4
  3. Mechanical advantage of 6
  4. Mechanical advantage of 8

Explanation:

Pulleys act as simple levers. The side where the rope is being pulled is called the effort and the weight being lifted is called the load.

Mechanical Advantage is the ratio of load to effort. Pulleys and levers alike rely on mechanical advantage. The larger the advantage is the easier it will be to lift the weight. In other words, you would need to put less effort into lifting the weight.

For a single pulley that is fixed to the ceiling, we would have an advantage of one. There is no mechanical advantage with this single pulley. However if the pulley is movable and we attach a weight to it will have a mechanical advantage of two. A rope passes around it, with one end attached to a fixed point and a pulling force is applied upward to the other end with the two lengths parallel. In this situation the distance the lifter must pull the rope becomes twice the distance the weight travels, allowing the force applied to be halved.

pulley diagram

Fig. 2 Pulley with mechanical advantage of one.

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Fig. 3 Pulley system with a mechanical advantage of two.

For a more complicated system with more than one pulley, greater meachanical advantages can be achieved. As we can see in figure four below, we have four pulleys. The weight is now distributed between four ropes. This gives us a mechanical advantage of four. Even though we are lifting an object that has a total force of one hundred newtons, we would only have to use 25N to lift it. It is easier to lift, however you will have to pull four times the length of rope to lift the weight the same length as a pulley system with a mechanical advantage of one.

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Fig. 4 Pulley system with a mechanical advantage of four.

Written By: Nick McCabe