Levers

h(ps://www.youtube.com/watch?
v=YlYEi0PgG1g  
 
 Levers  
A  lever  is  a  rigid  rod  that  rotates  around  one  
point  to  move  a  load  by  applying  a  force  to  a  
third  point.  
Archimedes  worked  out  how  things  balance  

Balance  occurs  when  moments  

M   M   M   are  equal  and  opposite.  
M2  
M1  

D1   D2  

M   M  
M   M1D1  =  M2D2  

M  
M   M  
 Law  of  the  Lever  
F1D1  =  F2D2  

F2  
F1  

D1   D2  

The  masses  are  applying  a  

downward  force  on  the  lever  
with  their  weight  –  which  is  a  
force.  So  the  simple  idea  of  
balancing  weights  on  a  pivot  
reveals  how  forces  act  on  a  
lever.  
 Three
Classes of
Levers
 Classes of Levers
“First Class Lever”
•  A first-class lever is a lever in
which the fulcrum is located
between the input effort and the
output load.
•  In operation, a force is applied
(by pulling or pushing) to a
section of the bar, which causes
the lever to swing about the
fulcrum, overcoming the Examples:
resistance force on the opposite
side. • Seesaw
• Scissors (double
lever)
Effort  Force   First Class Lever
F  E  

F  L   Load  Force  

 
 
Fulcrum  is  between  FE  (effort  force)  and  FL  (load  force)  
When  the  effort  moves  farther  than  load,  the  Mechanical  advantage  >1  
When  the  effort  moves  less  than  the  load,  the  Mechanical  Advantage  <  1    
 
 
 Lets  apply  the  Law  of  the  Lever  
F1D1  =  F2D2  

Effort  
Force   Load  
10N  
D1   D2  
D1  =  50cm   D2  =  10cm  
Mechanical  Advantage  =  FL/FE  
 
What  is  the  Mechanical  Advantage  
of  this  lever?  
What  force  of  a  load  could  be  liXed?  
 What  happens  if  the  effort  is  closer  to  
the  pivot  than  the  load?  
F1D1  =  F2D2  
Effort  
Force   Load  
10N  
D1   D2  
D1  =  10cm   D2  =  50cm  
Mechanical  Advantage  =  FL/FE  
 
What  is  the  Mechanical  Advantage  
of  this  lever?  
What  force  of  a  load  could  be  liXed?  
 The  mechanical  advantage  of  a  lever  is  the  raYo  
of  the  length  of  the  lever  on  the  applied  effort  
side  of  the  fulcrum  to  the  length  of  the  lever  on  
the  load  force  side  of  the  fulcrum.  
Effort  
Force   Load    

MA  =  DE/DL  
DE   DL  
 Examples  of  first  class  levers  
Common examples
of first-class
levers include
–  crowbars,
–  scissors,
–  pliers,
–  tin snips
–  and seesaws.

 Second Class Lever
Load  

Effort  Force  

Load  is  between  fulcrum  and  Effort    

Effort  moves  farther  than  Load.    
MulYplies  Effort  Force,  but  does  not  change  its  direcYon    
 The  mechanical  advantage  of  a  2 nd  class  lever  is  always  greater  

than  1  
 
Explanation

•  Second  class  lever  

–  Load  is  located  between  the  effort  force  and  the  fulcrum.    
–  Always  mulYplies  a  force  

–  Example:  Wheelbarrow  
L E

Always  mulYplies  a  force.  

Examples  of  second-­‐class  levers  
•  Examples of
second-class
levers include:
•  nut crackers,
•  wheel barrows,
•  doors,
•  and bottle
openers.
 Evaluate  the  Lever  

Effort  
Force  
Load  
10N  
DL  
DE  
DE  =  80cm   DL  =  20cm  
Mechanical  Advantage  =  FL/FE  
 
Mechanical  Advantage  =  DE/DL  
 
What  force  of  a  load  could  be  liXed?   What  is  the  Mechanical  Advantage  of  this  
lever?  
 Third Class Lever
E L

Effort  is  between  fulcrum  and  Load.  

Does  not  mulYply  force    
Load  moves  farther  than  Effort.    
MulYplies  the  distance  the  effort  force  travels  and  the  speed  at  which  
it  moves.  
 The  mechanical  advantage  of  a  3 rd  class  lever  is  always  less  than  1.  
 Classes  of  Levers  
“Third Class Lever”
Examples:
•  For this class of levers, the input
effort is higher than the output
load, which is different from
• Hockey Stick
second-class levers and some
first-class levers. • Tweezers
•  However, the distance moved by
the load is greater than the • Fishing Rod
distance moved by the effort.
•  In third class levers, effort is
applied between the output load
on one end and the fulcrum on
the opposite end.
 Explanation

•  Third  class  lever  

–  Effort  force  located  between  the  load  and  the  fulcrum.    
–  Effort  arm  is  always  shorter  than  load  arm  
–  MA  is  always  less  than  one  

–  Example:  Broom  

E L

There  is  an  increase  distance  

moved  and  speed  at  the  other  end.  
Other  examples  are  baseball  bat  or  
hockey  sYck.  
Examples of Third Class Levers
•  Examples of
third-class
levers include:
–  tweezers,
–  arm hammers,
–  and shovels.

Third class lever in

human body.
 Evaluate  the  Lever  
Effort  
Force  
10N  
Load  

DL  
DE  
DE  =  20cm   DL  =  80cm  
Mechanical  Advantage  =  FL/FE  
 
Mechanical  Advantage  =  DE/DL  
 
What  force  of  a  load  could  be  liXed?   What  is  the  Mechanical  Advantage  of  this  
lever?  
 Mechanical  Advantage  
•  Mechanical  Advantage  is  the  raYo  between  the  
load  and  effort.      

•  Mechanical  Advantage    deals  only  with  forces.  

•  Mechanical  Advantage    >  1  means  that  the  

output  force  will  be  greater  than  the  input  force.    
–  (But  the  input  distance  will  need  to  be  greater  than  
the  output  distance.)  
Mechanical  Advantage  
• First and Second class levers have a positive mechanical
advantage.
• Third class levers have a mechanical disadvantage,
meaning you use more force that the force of the load you
lift.
 Movement  Ra=o  
•  Movement  RaYo  deals  with  the  distance  
gained  or  lost  due  to  a  mechanical  advantage.  

•  Movement  RaYo    >1  means  that  the  input  

distance  (or  effort)  to  move  a  load  will  be  
greater  than  the  output  distance  of  the  load.      
 Mechanical Advantage: Example
Mechanical Advantage =
effort arm
resistance arm

Crazy Joe is moving bricks to build his cabin.

With the use of his simple machine, a lever, he moves
them easily.
The “effort arm” of his wheel barrow is 4ft., while the
resistance arm of his wheelbarrow is 1 ft.