Levers

A lever is a rigid bar that pivots (rotates) about a fixed

point. Levers are used to change the magnitude,

direction, or speed of forces that act on an object.

Levers are simple machines used to obtain a Mechanical

Advantage.

What Is a Lever?A lever can be a bar or rod that rotates on a pivot, called a fulcrum.

When an unbalanced force is applied to the lever, the lever rotates about the fulcrum

Parts of a Lever

Distance From Fulcrum to The

Effort

Load or Resistance

FULCRUM

Force or Effort

Distance From Fulcrum to The

Resistance

Class 1 LeverThe Fulcrum lies between the Resistance and the Effort

10 Inches

Load or Resistance

FULCRUM

Force or Effort

10 Inches

Note: The movement of the force and the resistance are in opposite directions.

Note that in this example the masses are different, but the distances from the fulcrum are equal. The lever rotates about

the fulcrum. The unbalanced force that causes the lever to rotate is called a Torque

Class 2 Lever

Note: The direction of the force, and the movement of the resistance are in the same direction.

10 Inches

Load or Resistance

FULCRUMForce or Effort

10 Inches

The resistance is between the Effort and the Fulcrum

Note: In this example the Effort acts further from the Fulcrum then the Resistance. The force (in lbs.) needed to move the resistance is less than the weight of the resistance (in lbs.)

Class 3 Lever

10 Inches

Load or Resistance

FULCRUM

Force or Effort

10 Inches

The Effort is between the Resistance and the Fulcrum

Note: The direction of the force, and the movement of the resistance are in

the same direction.

Third class levers are used to increase the rate of motion (speed) of an object with respect to the speed of the force

acting on the object. In this example the Resistance moves twice as far (fast) as the effort (force) acting on it.

Mechanical Advantage

Theoretical Mechanical AdvantageThe ratio between the Output force and the

Input force of a lever system.

10 feet 1 foot

100 lbs. 1000 lbs.

Center of MassCenter of Effort

Output Force = 1000 lb.Input Force = 100 lb.

Theoretical Mechanical Advantage = 10:1

Force or Effort Load or Resistance

100lb x 10ft. = 1000lb. X 1ft.

Calculating Mechanical AdvantageEffort x Distance = Resistance x Distance

10 feet 1 foot

100 lbs. 1000 lbs.

Center of MassCenter of Effort

Input Force = 100 ft.lbs.

Force or Effort Load or Resistance

Output Force = 1000 ft. lbs.

100lb x 10ft. = 1000lb. X 1ft.

If the Effort x distance from the fulcrum is not equal to the the Resistance x distance from the fulcrum, then;

100 lbs. x 20 ft. = 500 x 15 ft.

20 Feet

15 Feet

100 lbs.

500 lbs.

The Lever System is Unbalanced

Force or Effort

100 lbs.

Load or Resistance

2000 lb.ft. = 7500 lb. ft.

A net torque of 5500 ft. lbs is created

Example: When the loads are equal but the distances are not, the lever is not balanced.

100 x 10 = 100 x 30

10 Feet

30 Feet

100 lbs.

100 lbs.

There are many types of levers found around the home

Nail ExtractorFulcrum Effort

Resistance or Load

Class 1 Lever

Hammer

Effort

Resistance or Load

Fulcrum

Class 3 Lever

Salad Server

EffortResistance or Load

Fulcrum

2 Class 1 Levers Sharing a Common Fulcrum

This backhoe is an example of a multiple lever. It has three

different fulcrums.

The asterisks show the locations of each fulcrum

* **

The TrebuchetA class 1 lever used to

maximize the speed of a relatively lightweight projectile (Resistance)

by using a massive counterweight (Effort)

to create a large unbalanced force