Lever Classes and Mechanical Advantage

Your body is levers

Every time you move, bones act as levers turning about your joints. Understanding lever systems explains how muscles produce powerful or fast movements. This spec wants three things: the classes of lever, where they appear in the body, and their mechanical advantage.

Four parts of a lever

Every lever has four ingredients, and in the body each one is a specific structure:

Name the parts

In the body the rigid bar of a lever is a _____. The fixed pivot it turns about is a _____. The force that moves it comes from a contracting _____, and the weight being moved is called the _____.

Three classes

Levers come in three classes, set by which part is in the middle. A handy mnemonic: in classes 1, 2, 3 the middle part is Fulcrum, Load, Effort. - First class: fulcrum in the middle. - Second class: load in the middle. - Third class: effort in the middle.

Name each class

Three levers, with the parts marked: F is the fulcrum, E the effort and L the load. Label each bar with its class.

Name the class

A bicep curl bends the elbow, with the biceps (effort) between the elbow (fulcrum) and the weight in the hand (load). Which lever class is this?

• Third class
• First class
• Second class
• It is not a lever at all

Levers in the body

Each class appears in real movement: - First class: nodding the head. The neck joint (fulcrum) sits between the head's weight (load) and the neck muscles (effort). - Second class: standing on tiptoes. The ball of the foot is the fulcrum, body weight (load) is in the middle, the calf (effort) pulls up at the heel. - Third class: a bicep curl. The elbow is the fulcrum, the biceps (effort) is in the middle, the weight (load) is at the hand.

Match each movement to its lever class

The common one

Which lever class is the most common in the human body?

• Third class
• First class
• Second class
• They are all equally common

Advantage or disadvantage?

This spec judges mechanical advantage by lever class, not by any ratio formula. The two classes that matter trade the same thing against each other:

True or false

Select the TWO statements that are true about mechanical advantage in this spec.

• Second class levers can move a large load with small effort
• Third class levers trade lifting power for range and speed
• Mechanical advantage is found with an effort-arm over load-arm ratio
• Third class levers are best for lifting the heaviest loads

Reason it out

Work through the mechanical advantage of two movements, the way an examiner expects.

• You rise onto tiptoes, lifting your whole body weight with your calf. This second class lever gives a...
• You curl a dumbbell at the elbow, a third class lever. Compared with lifting power, it is built for...
• To explain mechanical advantage in this exam, you should answer in terms of...

Lever-systems frame

A lever has a rigid bone, a joint as the _____, a muscle providing the _____, and a load. The class depends on which part is in the middle. A _____ class lever gives a mechanical advantage, moving a large load with small effort, while a third class lever has a mechanical _____.

The trade-off

Why is a third class lever useful despite its mechanical disadvantage?

• It allows a large range and speed of movement
• It can lift the heaviest possible loads
• It removes the need for any muscle effort
• It works without a fulcrum