Rigid bar
fulcrum
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Forces
 The lever consist of two forces:
1)
2)

An effort force: which will tend to rotate the bar around the fulcrum
An resisting force: which will resist the movement
These forces act around a supporting force which provides for a point
of pivot known as the fulcrum
effort
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resistance
Forces
 Force Arm: The distance from the effort to the fulcrum is known as the
force arm
 Resistance Arm: The distance between the resistance and the fulcrum
is known as the resistance arm
effort
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FA
RA
resistance
Law Of Lever
 In order for the lever to be in equilibrium:
 Load(resistance) * Load Arm= Effort * Effort Arm
 E.g. 1 gm of feather has to be balanced by 1 kg of rock
feathers
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FA
rock
RA
Mechanical Advantage
 Mechanical Advantage or leverage is defined as a
ratio of force arm to resistance arm
 MA=FA/RA
 Mechanical Advantage can either be equal to 1, less
than 1 or greater than 1 depending upon the type of
lever
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Types Of Levers
 Class I Levers: In these types of levers the fulcrum lies between the
resistance and the effort
effort
FA
RA
Resistance
 The Mechanical advantage for these levers can be equal to 1,>1 or < 1
depending upon the position of the fulcrum.
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Types Of Levers
 Class II levers: In these types of levers the resistance
lies between the fulcrum and effort.
 Thus the effort(force) arm is always greater than the
resistance arm
 Mechanical Advantage: Thus the mechanical
advantage of this lever is always greater than 1
FA
RA
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Resistance
effort
Types Of Levers
 Class III levers: In these types of levers the effort lies
between the fulcrum and resistance.
 Thus the resistance arm is always greater than the
force arm
 Mechanical Advantage: Thus the mechanical
advantage of this lever is always less than 1
effort
FA
RA
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Resistance
Levers In Human Body
 In human body the bones act as levers, the weight of
that body part acts as the resistance and the associated
muscular contraction(muscle attached to that bone)
acts as the effort. Motion is produced only when the
effort exceeds the resistance .
 For your legs or any other body part to move the
appropriate muscles and bones must work together as
levers
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Examples
 Common Examples: Class I lever
Scissors: The fulcrum
lies at the center, we
apply force at the
handles of the scissor,
the resistance is at the
other end
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Anatomical Examples
 Our head is connected to the spine at the atlanto-
occipital joint(fulcrum).
 A coronal plane passing through the body divides the
head into a greater anterior part and a smaller
posterior part, such the weight of the anterior part is
greater than the posterior part
 Thus the anterior part of the head tends to fall
(bend)forwards(Load)
 This bending action is prevented by weight of the
posterior part of the head and occipital muscles
(effort)
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Anatomical Examples
 Class I lever:
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Coronal plane
Examples
 Common Examples: Class II lever:
The fulcrum lies at one
end ,the load is present
at the center, and the
effort force is applied at
the other end
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Anatomical Examples
 Class II Lever: When we raise our body on the toes
,the toes act as the fulcrum, the weight of the body acts
as the resistance force and the calf muscles act as the
effort force
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Anatomical Examples
 Class II lever:
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Anatomical Examples
 Common Examples: Class III lever:
Forceps:The fulcrum lies
at one end, the effort
force lies at the center
,the resistance force lies
at the other end
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Anatomical Examples
 Class III Lever: When we hold a object in our hand the
object acts the load, the elbow joint acts as the fulcrum
and the muscles in the arm(Biceps) act as the effort
force.
 Generally most of the levers in the human body are
class III levers
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Anatomical Examples
 Class III lever:
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Anatomical Examples