Levers

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 the mechanical advantage could be calculated. It is possible to calculate any part of the formula as long as there are two pieces of information from the formula available.

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 (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 must 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.

When a lever is balanced it has equilibrium - the load is balanced on either side.

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.

Scissors are a first order lever. The hand’s grip is the applied force, the fulcrum is the pin at the centre of the scissors and the blade applies force to the load.

The class 1 lever is probably the most familiar to most people. We often use it when we want to lift something we can't lift with just our muscles. The fulcrum is between the load and the applied force.

One of many class-1 levers in humans is the system that rotates the skull up and down. The fulcrum is the point where the skull connects to the spine. The applied force is the muscles on the back of the neck and the load is the weight of the front of the skull. Normal muscle tension in the bundle of muscles at the back of the neck holds the head level.

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.

In a class-2 lever system, the load is between the fulcrum and the applied force, as shown below.

The mechanical advantage in this system is the same as a class-1 lever. One common example is a door. The fulcrum is at one edge, the load is the weight of the door, which we can concentrate at its center of mass, and the applied force is at the edge opposite the hinges.

The muscles that stand you on tip-toe are the calves. They attach to the bottom of the femur and to the heel bone. The load of the weight of a human is transferred through the tibia, which is between the fulcrum (the ball of the foot) and the applied force (the constricting calves).

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.

Tweezers or cooking tongs have the fulcrum at the closed end, the load at the open end, and the force in between.

In a class-3 lever, the applied force is between the fulcrum and the load. In this configuration, the bar must be attached to the fulcrum, or else it would lift off.

In a 3^rd-class lever, the maximum mechanical advantage is 1. Any position of the applied force other than directly opposite the load, results in an amplification of the force needed to move the load.