LEVERS
It was Aristotle that said “ Give me a lever and a fulcrum and
I shall move the Earth.” T
~ Archimedes
Introducing… The Lever
• A lever includes a stiff structure (the lever) that
rotates around a fixed point called the fulcrum.
fulcrum
Parts of a lever
• Fulcrum – The fixed point around which the
lever rotates
• Effort Arm – The part of the lever that the
effort force is applied to (measured from the
fulcrum to the point at which the force is
applied)
• Resistance Arm – The part of the lever that
applies the resistance force (measured from
the fulcrum to the center of the resistance
force)
Forces of the lever
• Input or effort Force – Force exerted ON the lever
• Output or resistance Force – Force exerted BY the
lever, object being moved
Classes of Levers
• There are three different classes of
levers. The class of a lever is
determined by the location of the
effort and resistance forces relative to
the fulcrum.
1st Class Lever
• The fulcrum is located between the effort force and
the resistance force. First class levers can multiply
force and distance. A first-class lever always changes
the direction of force (I.e. a downward effort force on
the lever results in an upward movement of the
resistance force)
– Examples: Scissors, see-saw
F
A Class 1 Lever has its fulcrum between the effort and the load.
First Class Lever
• With a first-class lever, when the fulcrum is closer to the resistance,
the output force is increased. However, there is a corresponding
decrease in both output speed and distance. Conversely, when the
fulcrum is closer tot he effort, the output force is decreased and there
is a corresponding increase in both output speed and distance.
2nd Class Lever • resistance is located between the effort force
and the fulcrum. These levers multiply the
force but the direction stays the same.
– Example: Wheelbarrow
L
A Class 2 Lever has its load between the effort and the
fulcrum.
Second Class Lever
• A second-class lever does not change the direction
of force. When the fulcrum is located closer to the
resistance than to the effort force, an increase in
force (mechanical advantage) results. A secondclass lever is illustrated below:
3rd Class Lever • The effort force is located between the
fulcrum and the resistance. The effort arm
is always shorter than the resistance arm so it
cannot multiply the force and the MA is
always less than 1.
– Examples: Rake, hockey stick
E
A Class 3 Lever has the effort between the load and the
fulcrum.
Third Class Lever
• A third-class lever does not change the direction of force;
third-class levers always produce a gain in speed and
distance and a corresponding decrease in force. A thirdclass lever is illustrated below. Because it requires more
force, third class levers have a mechanical disadvantage.
Lever Recap
• Determining the different classes of levers can
be very difficult
• A trick to help you remember how to classify
each lever is FLE 123
• FULCRUM in middle
• LOAD in middle
• EFFORT in middle =
=
class 1
=
class 2
class 3
Leverage Systems and the Body
• Lever: i.e. bones, a
rigid rod that moves
on some fixed point
• Fulcrum: i.e. joint, a
fixed point
• Resistance: the force
opposing movement
• Effort: the force
exerted to achieve
action, provided by
the muscles
Levers and the human body
• Your body contains
muscles attached to
bones in ways that act
as levers.
• Here the biceps
muscle attached in
front of the elbow
provides the force to
lift the weights.
Can you think of other muscle
levers in your body?
Muscle Mechanics – 1st Class Lever
first-class lever [E-F-R = effort-fulcrum-resistance]
• the fulcrum is placed between the effort and the resistance (load)
• e.g., seesaw, scissors
• skull (head)/atlas and cervical vertebrae
– the muscles of the back (splenius, trapezius, etc.) provide the effort
– the mass of the skull provides the resistance (load)
2nd Class Lever
• second-class lever [F-R-E = fulcrum-resistance-effort]
• the fulcrum is at the end of the lever, the effort is at the opposite
end, and the resistance (load) is in between, e.g., a wheelbarrow
• 2nd class levers provide greater power to move a large resistance
(load)
• raising the body on the toes
– the mass of body is the resistance
– the ball of the foot is the fulcrum
– the contraction of the calf muscles to lift the heel upward is the effort
3rd Class Lever
– third-class lever [F-E-R = fulcrum-effort-resistance]
• the fulcrum is at one end, the resistance is at opposite
end, and the effort is in between, e.g., tweezers
• the most common type of lever in the body
• 3rd class levers provide for rapid motion
• e.g., flexing the forearm or adducting the thigh
• such movements can still be powerful
Mechanical Advantage
•
Levers can provide Mechanical Advantage
by reducing the effort needed to lift a load.
The closer the fulcrum is to the load, the
less effort is needed to lift the load. The
load does not move a great distance.
L
E
F
L
Levers can be used to move loads
farther if the fulcrum is placed close to
the effort. In this situation more effort
is needed to lift the load, but the load
will move farther than if the fulcrum
was closer to the load.
E
F
What is the Relationship?
Input Force x Length
of Input Arm
=
Output Force x Length
of Output Arm
Force x Distance = Force x Distance
# of Weights
x Distance
=
# of Weights
x Distance
Calculating Mechanical Advantage
• By using the length of the effort arm and the
resistance arm you can find the ideal
mechanical advantage. We can find the ideal
mechanical advantage of any simple machine
by dividing the effort distance by the
resistance distance.
• IMA = length of effort arm
(Remember
length of resist arm
D=E/R
.
Mechanical Advantage: Lever
• IMA = length of effort arm
length of resist arm
•
D=E/R
•
.
• For our example,
MA = 10/5 = 2
• Distance from effort to fulcrum:
10 feet
• Distance from load to fulcrum: 5 feet
v
Mechanical Advantage
Forces can also be used to
calculate Mechanical
Advantage
Mech. Adv. = Resist Force
Effort Force
Remember: F=R/E
For our example:
M.A. = 20/10 = 2
10#
20#