Mechanical devices - OCRLevers

Levers use mechanical advantage to make lifting or applying pressure easier. All levers are made of a bar and a pivot, called a fulcrum. Levers have three main parts:

• effort - the amount of force applied by the user, also referred to as the input
• fulcrum - where the lever pivots
• load - the weight that needs to be moved, also referred to as the output

Mechanical advantage is the amount of help you get using a machine in comparison to doing something with just human effort, and it is created by levers.

It is measured by dividing the load by the effort applied to moving it, both measured in Newtons (N) - this could also be described as the output (load) divided by the input (effort):

Example

A person lifting a load of 200 N but only using 100 N of effort:

Therefore, the mechanical advantage = 200 ÷ 100 = 2.

This can also be written as 2:1. The person is able to lift twice the load using 100 N of effort.

The mechanical advantage can also be calculated theoretically by measuring the distance between the load and pivot and the effort and pivot.

In the picture below the distance between the load and fulcrum is 2 m. The distance between the effort and fulcrum is 6 m.

Therefore, the mechanical advantage = 6 ÷ 2 = 3 or 3:1

The person will find this load three times easier to lift.

In both examples we were able to calculate the mechanical advantage. It is possible to calculate any part of the formula as long as we have two pieces of information from the formula:

• mechanical advantage = load ÷ effort
• load = mechanical advantage × effort
• effort = load ÷ mechanical advantage

Classes of lever

There are three different types of levers. They are chosen for their ability to produce the most mechanical advantage for a particular task. These classes of lever arrange the effort, fulcrum and load in a different order:

First order levers

First order levers (Class 1) place the fulcrum between the effort and the load. An example would be a seesaw, which places the fulcrum in the centre and allows equally weighted children to lift each other up.

If the load is closer to the fulcrum it becomes easier to lift. When the fulcrum is in the centre, like a seesaw, the effort and the load have to be equal to balance them. If a person is slightly heavier at one end or leans back, moving the weight, one end of the seesaw moves down.

A crowbar is an example of a first order lever that puts the load closer to the fulcrum - this gives it more power to move a load. When the fulcrum is moved nearer the load it takes less effort to move it.

Second order levers

Second order levers (Class 2) place the fulcrum at one end of the lever and the effort at the other, with the load in the centre. The closer together the fulcrum and load are, the easier it is to lift the load. Examples include wheelbarrows, nutcrackers and some bottle openers.

Third order levers

Third order levers (Class 3) place the effort between the fulcrum and the load. If the effort and the fulcrum are further apart, it becomes easier to lift. A third order lever does not have the mechanical advantage of first order levers or second order levers so are less common. They are generally used for moving small or delicate items. Examples include tweezers or fishing rods.