University of Scranton - Department of Physical Therapy
Edmund M. Kosmahl, PT, EdD
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Dr. Kosmahl
copyright 2000 Edmund M. Kosmahl
MECHANICS AND LEVER SYSTEMS
Objectives:
-
Define the terms listed under the "Terminology"
section.
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Define Newton's Three Laws, and explain
how they affect human motion.
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List the forces acting on the human body.
-
Explain composition of forces.
-
Explain the concept of mechanical advantage.
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Define the three classes of lever systems,
and give an example of each in the human body.
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TERMINOLOGY
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Mass - a quantity of atoms.
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Velocity - speed of movement.
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Acceleration - a positive rate of change in velocity.
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Deceleration - a negative rate of change in velocity.
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Force - a push or pull.
-
Torque - The tendency of a force acting at a
perpendicular distance from an axis to rotate a lever system.
-
Equilibrium - a steady state of motion.
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Static Equilibrium - no motion.
-
Dynamic Equilibrium - a steady state of acceleration
or deceleration.
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NEWTON'S THREE
LAWS
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I - Equilibrium
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II - a : F/m or F = ma
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III - Action and Reaction
Go
to Newton's Laws Links (leaving Dr. Kosmahl web
space)
-
CLINICAL EXAMPLES
-
I - forces required to start and stop
limbs during gait
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II - two joint muscles tend to move lesser mass
joint
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III - floor reaction forces
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FORCES ACTING ON THE BODY
-
Gravity
-
Friction
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Muscles
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External
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FREE BODY DIAGRAM
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Visualization of body or segment of interest
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Representation of forces by vectors
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COMPOSITION
OF FORCES
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Resultant force one vector representing all of
the forces acting on the body of interest
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Parallel forces can simply be added or subtracted
to find resultant
-
Forces at angles resultant determined graphically
or by trigonometry
-
LEVERS
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Rigid bar
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Axis or fulcrum
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Forces tending to rotate the rigid bar
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Force and force arm
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Weight or resistance and weight or resistance
arm
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Large force in one direction, two smaller forces
in opposite direction
See
Lever Picture 7K
-
MECHANICAL
ADVANTAGE
-
MA = Force arm length / Weight arm length
-
Levers can be used to gain either a FORCE
advantage, or a DISTANCE / SPEED advantage
-
Most human levers create a distance-speed advantage
therefore muscle tension forces must be much greater than weight being
moved
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LEVER CLASSES
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Class I
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Force and Weight on opposite sides of axis
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Can be used to gain force advantage or distance/speed
advantage
-
triceps extending elbow
See
Class I Triceps Picture 5K
-
Class II
-
Force and Weight on same side of axis
-
Weight closer to axis than force
-
Used to gain force advantage
-
gastroc-soleus pushing up on toes
See
Class II Gastroc-soleus Picture 12K
-
Class III
-
Force and Weight on same side of axis
-
Force closer to axis than weight
-
Used to gain a distance/speed advantage
-
Most human levers are class III
-
biceps brachii flexing elbow
See
Class III Biceps Brachii Picture 7K
-
TORQUE
-
Moment of force
-
Effectiveness of force tending to rotate a lever
system
-
Force working at PERPENDICULAR distance
to axis line
-
Torque arm (moment arm) = lever arm ONLY
when lever arm is working at 90o to axis line
-
Static equilibrium, sum of torques equals zero
-
F x Fa = W x Wa
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