Biomechanics Flashcards
Types of Motion
Linear, translatory, curvilinear
Planes of Motion
Transverse, frontal, sagittal plane
External Forces
Gravity, wind, water, objects
Internal Forces
Muscles, ligaments, bones
Scalar
Magnitude Examples: Mass Temperature Work Energy Speed
Vector
Magnitude & Direction Examples: Weight Force Velocity Acceleration
Force Vector
Three Components:
Point of application
Action line with direction indicating push or pull
Magnitude (length of vector)
Naming Forces
“Object-on-object”
First object is source
Second object is object being acted on
Contact required for force to act (exception = gravity)
Mass vs. Force
Kilogram: Used to measure mass (a quantity of matter) Scalar - not a vector quantity Don't confuse with "weight" Magnitude, no direction
Newton
Measure of force
A vector quantity
A measure of weight
US equivalent: pounds (lbs)
Force of Gravity
Attraction of Earth’s mass for the mass of other objects
Causes acceleration: 9.8m/s^2
PTs are always dealing with gravity (Gait, resistance)
Weight
Mass (kg) x 9.8m/s^2
Mass is a scalar unit (no direction)
Weight is a vector (force unit): has magnitude and direction
Center of Gravity
Gravity acts at all points on an object
“Point of application” is COG or center of mass
Hypothetical point at which all mass would appear to be concentrated (center of mass)
For symmetircal objects, COG is located in geometric center
Line of Gravity
Action line and direction of the force of gravity
Always vertically downward toward the center of the Earth
Visualized as a string with a weight on the end (plumb line)
Segmental COG
Each segment in the body has its own COG
Adjacent segments grouped together if moving as a unit
Gravity acting on combined segments represented by a single COG
Combined COG is on a line between segmental COGs
COG: Human Body
Anterior to S2 in anatomic position
Really depends on a person’s proportions
Magnitude equal to person’s weight
COG Displacement
Depends on rearrangement of segments and loads
COG can relocate
Stability & the COG
If LOG falls outside of the base of support (BOS) = instability
The larger the BOS, the greater the stability
The closer the COG is to the BOS, the greater the stability
PTs: Using COG and base of support
Lifting mechanics - Lower COG, wide base of support = Stable
Ankle rehab - Narrow base of support, challenge stability and muscular control = Instability
Walker/Assistive Devices - Increase base of support, greater stability
Relocation of the COG
COG location dependant on arrangement of segments, distribution of mass
Most common redistribution of mass: addition of external mass
Adjustments bring LOG closer to center of base support
Newton’s Laws
Law of Reaction (3rd)
Law of Inertia (1st)
Law of Acceleration (2nd)
Newton’s Law of Reaction
Forces always come in pairs
“For every action there is an equal and opposite reaction.”
Most forces arise from “contact” - Push-Pull force; Gravity - can ignore opposite force
If book contacts table, table also contacts book
Analysis of Forces
Summary: accounting for forces on an object
Forces result from contact
Gravity exerts a force on all objects - also has a reaction force (typically ignored)
All forces come in pairs (whenever 2 objects contact)
Equilibrium
Forces on an object determine if object in translatory, rotary, or curvilinear motion
Forces can be applied to an object without causing movement
Statics: the study of conditions under which objects remain in equilibrium (at rest) as a result of forces acting on them
Newton’s Law of Inertia
Deals with objects in equilibrium
An object will remain at rest or in uniform motion unless acted on by an unbalanced force (outside force)
For an object to be in static equilibrium: sum of forces - 0
Constant velocity - infrequent
Inertia: property that makes an object resist initiation and change in motion
Determining Equilibrium of an Object
Identity and magnitude of all forces
Gravity acts on book at books COG; magnitude = wt. of book
Book on table, therefore, table on book (3rd Law)
Determining Equilibrium of a Table
Equilibrium - the sum of all forces acting on the book must equal zero
Other forces are irrelevant
Linear Force System
Two or more forces act on the same object and in the same line Translation Magnitude and Direction Forces to the right or up are + Forces tot eh left or down are -
Newton’s Law of Acceleration
Acceleration (a): change in speed and/or change in direction
a = F/m or F = ma
Inertia is proportional to the mass of the body or object
Law of Acceleration
Acceleration of a body is proportional to and takes place in the direction in which the force acts and is inversely proportional to the body’s mass
F = ma and a = F/m
Joint Distraction in a Linear Force System
Skeletal traction produces joint distraction
Parts in equilibrium
Known force is gravity-on-weight
Refinement of Newton’s 1st Law (Inertia)
Sum of all horizontal forces must be zero
Sum of all vertical forces must be zero
Force of Friction
Potentially exists whenever two objects contact
Has magnitude only when contacting objects moved or attempt to move
Friction is a vector force
Friction forces also come in pairs: equal in magnitude, opposite in direction
Shear Force: force that moves or attempts to move one object on another (parallel to contacting surfaces in the direction of the attempted movement)
Force of Friction
Action lines of friction forces always lie parallel to contacting surfaces
Magnitude of Friction:
Magnitude of contact between objects
Slipperiness or roughness of surfaces (m = coefficient of friction)
Magnitude of shear force
In traction example, leg needs lifting off bed to minimize friction
Force of Friction
Fx = μ*Fc Fx = Friction force μ = Coefficient of friction Fc = Contact force or Normal force To change Fx μ: Soft-Rough > Hard-Smooth Fc : Heavier object > Fc
Examples of μs
Cobalt chrome on polyethylene (typical materials used in joint replacement), μs = 0.01 to 0.05.
Normal cartilage, μs = 0.005 to 0.02.
Dry cartilage μs = 0.27
Wet cartilage μs = 0.002 0.020
