Everything Science Grade 11

CHAPTER 12. FORCE,MOMENTUM AND IMPULSE 12.6

DEFINITION: Lever

A lever is a rigid object that is used with an appropriate fulcrum or pivot point to

multiply the mechanical force that can be applied to another object.

effort load

Figure 12.17: A lever isused to put in a small effort to get out a large load.

FACT

Archimedes reputedly

said: Give me a lever

long enough and a

fulcrum on which to

place it, and I shall

move the world.

The concept of getting out more than the effort is termed mechanical advantage, and is one example
of the principle of moments. The lever allows one to apply a smaller force over a greater distance. For
instance to lift a certainunit of weight with a lever with an effort of half aunit we need a distancefrom
the fulcrum in the effort’s side to be twice thedistance of the weight’sside. It also means thatto lift
the weight 1 meter weneed to push the leverfor 2 meters. The amount of work done is always the
same and independent of the dimensions of the lever (in an ideal lever). The lever only allows to trade
force for distance.

Ideally, this means that the mechanical advantage of a system is the ratioof the force that performs the
work (output or load) tothe applied force (inputor effort), assuming there is no friction in the system.
In reality, the mechanical advantage will be lessthan the ideal value by an amount determined by the
amount of friction.

mechanical advantage=

load

effort

For example, you want toraise an object of mass 100 kg. If the pivot is placed as shown in Figure 12.18,
what is the mechanicaladvantage of the lever?

1 m 0.5 m

F? 100 kg

Figure 12.18: A lever isused to put in a small effort to get out a large load.

In order to calculate mechanical advantage, weneed to determine the load and effort.

Tip

Effort is the input force
and load is the output
The load is easy to calculate, it is simply the weight of the 100 kg object. force.

Fload= m· g = 100kg· 9 ,8m· s−^2 = 980N