biomechanics final ch. 9-14

what is Newtons 1st law of motion?

everybody continues in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by forces impressed upon it.

what is momentum?

product of an objects mass and its linear momentum (vector quantity)

what is a elastic collision?

when two objects collide, their combined moment is conserved (bounce off)

what is an inelastic collision?

when two objects collide, remain in contact and travel together in one direction at the same velocity

momentum equation

L=mv

L= linear momentum

m= mass

v= instantaneous velocity

elastic collisions at equal masses…

inherit velocity u1=v2 u2=v1

what is the coefficient of restitution?

the absolute value of the ratio of the velocity of separation to the velocity of approach (unitless)

what is e=1?

a perfectly elastic collision

what is e=0

a perfectly inelastic collision

what is newtons 2nd law of motion (law of acceleration)

the change of motion of an object is proportional to the force impressed; and is made in the direction of the straight line in which the forces is impressed

what is the law of acceleration equation (2NL)?

ΣF=ma

what is impulse and momentum?

newtons 2nd law can also determine average acceleration caused by average net force

what is impulse?

product of force times the time that the force acts

what is momentum?

product of mass times velocity

what are the two factors that will increase the momentum of an object?

average net force and

time over which the force is applied

what is newtons third law of motion?

to every action there is always an equal and opposite reaction

what is mechanoical stress?

internal force divided by the cross-sectional area of the surface on which the internal force acts

what is tension?

axial or “normal” stress that occurs at the analysis plane as a result of a force or load that tends to pull apart the molecules that bond the object together at the analysis plane

what is compression?

a compressive stress is the axial stress that results when a load tends to push or squash the molecules of a material more

what is shear stress?

a transverse stress that acts parallel to the analysis plane as a result of forces acting parallel to this plane

what is bending load?

a load that will cause different stresses at the analysis plane

objects with greater sectional area vs. smaller sectional area

larger sectional area can withstand greater bending forces because they have longer moment arms to resist bending “torque”

what is torsion?

a load that occurs when torques act about the long axis at each end

what is a counter torque?

a torque created by shear forces acting at the analysis plane. a larger diameter means greater counter shear force

what is combined loads?

muscles, tendons, and ligaments= uniaxial tensile loads

bones and cartilage= combined loads

what is strain?

quantification of the deformation of a material

what is linear strain?

changes in objects length (pulled apart or pushed together)

what is shear strain?

occurs when a change in orientation of adjacent molecules as a result of these molecules slipping past each other

what is poissons ratio?

when an object is compressed, it becomes wider in the lateral direction and shorter in the axial direction

what is poisson ratio ranges?

.1-.5 or .25-.35 for most materials

what is work?

product of force and the amount of displacement in the direction of that force

force x displacement

what is the equation for work? 

U=Fd

positive work (concentric)

if the displacement is in the same direction as force

negative work (eccentric)

if the displacement direction is opposite to the direction of force

mechanical energy equation

E= KE+PE+SE

what is kinetic energy?

moving objects have the capacity to do work due to motion (horizontal or x-axis)

what is potential energy?

capacity to do work due to the objects position

what is gravitational potential energy?

energy due to an objects position relative to earth

what is strain energy?

energy due to the deformation of an object (stiffness)

what is the work energy relationship equation?

U=ΔE

what is power?

how quickly or slowly work is done

what is power eqation?

P=V/Δt

what is the force velocity relationship?

as force decreases velocity increases

what is power velocity relationship?

as power increases so does velocity (bell)

what is the thorstenssson test?

a noninvasive test to determine the percentage of fast twitch muscle fibers (noninvasive)

what is torque?

the turning effect produced by a force (aka moment of force)

what is centric force?

force applied directly through the center of gravity (linear motion)

what is eccentric force (torque)?

force not directed through the center of gravity (linear and angular)

what is force couple (torque)?

equal and opposite but noncolinear; changes only angular motion

what is torque equation?

T=Fxr

what must happen for an object to be in static equilibrium?

the external forces must sum to 0 and the external torques must sum to 0

what is equilibrium equation?

ΣT=0=F1r1+ F2r2=0

what is center of gravity?

the point in a body or system around which its mass or weight is evenly distributed or balanced

what is center of mass?

point at which the entire mass may be assumed to be concentrated

what is center of gravity equation?

=Σ(wxr)= (ΣW) x reg

what is stability?

the capacity of an object to return to equilibrium or to its original position after being displaced

what factors affect stability?

• height of the center of gravity

• size of base of support

• weight of object

how does height of the center gravity affect stability?

lower height increases stability (-)

how does size of base support affect stability?

increase base of support increase stability (+)

how does weight of an object affect stability?

increase weight of object increases stability (+)

what is stability equation?

Pxh=Wxb

what are elastic properties of the stress strain relationship?

removing the load (stress) returns the objects shape

what are plastic properties of the stress strain relationship?

permanent or lasting change in deformation

what is elastic modulus?

stress-strain curve where the slope represents “elastic modulus”

what is the equation for elastic modulus?

E= Δσ/ Δe

what does a steeper slope mean for elastic modulus?

larger elastic modulus and a stiffer material

what does a gradual slope mean for elastic modulus?

smaller elastic modulus and compliant material

what does small loads mean for plastic behavior?

elastic response

what does large loads mean for plastic behavior?

plastic deformation

what is material strength?

how much stress (or strain) can it withstand before failure?

what is yield strength?

stress beyond this point=plastic deformation
(failure to regain shape)

what is stress at the elastic limit?

yield strength

what is maximum stress a material can withstand?

ultimate strength?

what is ultimate strength?

total load that can be carried

what is rupture or failure strength?

stress corresponding to the end of the stress strain curve

what is failure strain?

strain exhibited when breakage occurs and at the end of the strductiess-strain curve

what property are large failure strains?

ductile

what property are small failure strains?

brittle

what is tougness?

ability to absorb energy> a material is tougher if it takes more energy to break (area under curve)

what are the mechanical properties of musculoskeletal system?

bones (passive)

tendons/ligaments (passive)

muscles (active or passive)

what are the extracellular components of musculo skeletal?

collagen, elastin, ground substance, minerals and water

collagen

very stiff, high tensile strength and unable to resist compression

elastin

compliant and extensible

ground substance

carbs, gel like matrix for collagen and elastin fibers

what are synthetic materials?

isotropic

what are connective tissues?

anistropic

what is isotropic?

materials have the same mechanical properties in every direction

what is anisotropic?

mechanical properties differ based on the direction of the load

tendons parallel to fibers

have a high tensile strength

tendons applied perpendicularly to fibers

have a low tensile strength

bone properties

mostly compressive loads

strongest, stiffest material

cortical= compact

trabecular= spongy

high in mineral content

weakest in shear

bones according to the stress-strain curve

bone is stronger and stiffer is load is applied quick

bone is weaker and less stiff is the load is applied slow

tendons properties

more elastin than tendons

high % collagen

high tensile strength

low compression and shear

tendons according to the stress-strain curve?

less tensile stress is needed to “stretch out” crimps

much higher tensile stress is needed to actually stretch the collagen

tendons vs. ligaments

tendons are more stiff unable to resist non-axial loads (parallel fibers)

ligaments are less stiff and slightly weaker and able to resist non-axial loads (crosshatch fibers)

what is creep?

tendons under a constant tensile stress will not result in a constant strain

(under constant stress creep will increase strain over time)

what is muscle active state?

muscle stiffness is determined by the number of attached cross bridges

what is muscle passive state?

contractile elements: low tensile resistance

non-contractile elements: higher tensile resistance

what is flexibility?

ability to move a joint through its complete range of motion

what is static flexibility?

a measure of the total ROM at the joint and is thought to be limited by the mechanical properties of the muscle tendon unti