Biomechanics Ch. 12

free body diagrams

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magnitude

amount of force being applied

critical angle

angle at which the object begins to slide

anatomical pulley

changes the angle of pull of the muscle providing the force -this increase in angle of pull increases the rotary component (patella for the quadriceps)

direction

direction of a force is along its action line -gravity is a downward directed vector through the center of gravity of the object -direction of a muscular force vector is the direction of line of pull of the muscle

angle of pull >90°

dislocating force is directed away from fulcrum does not occur often muscle is at limit of shortening range and not exerting much force

friction force

product of a relationship between 2 objects and their surfaces

angle of pull =45°

rotary and non-rotary components are equal

mechanical axis

strait line that connects the midpoint of the joints at either end of the bone

Force

that which pushes or pulls through direct mechanical contact or through the force of gravity to alter the motion of an object -vector quantity: has a magnitude, direction and a point of application

law of acceleration

the acceleration of an object is directly proportional to the force causing it and inversely proportional to the mass of the object -F=ma -defines impulse-momentum relationship

muscle angle of pull

the angle between the line of pull and the mechanical axis of the bone -most resting muscles have an angle of pull <90°

energy

the capacity to do work

weight

the force of gravity is measured as the weight of the body applied through the center of gravity of the body and directed toward the earth's axis -W=mg

angle of pull

x-axis: always the mechanical axis of the bone, parallel to the lever, called the "non-rotary component" or "stabilizer" y-axis: always perpendicular to the mechanical axis of the bone (lever) called the "rotary component" or "mover"

potential energy

energy based on position -the product of the weight of an object and the distance over which it can act (also known as gravitational potential) -PE=mgh

point of application

point of which force is applied to an object -where gravity is concerned this point is always through the center of gravity -for muscular force, this point is assumed to be the muscle's attachment to a bony lever (the point of intersection of the line of force and the mechanical axis of the bone)

friction

the force that opposes efforts to slide or roll one body over another -proportional to the force pressing two surfaces together -force of friction acts parallel to the surfaces and opposite to the direction of motion FF=coefficient of friction X the normal force

impulse

the product of force and the time it is applied Ft=m(vf-vi) -a change in momentum -motion produced by force

work

the product of force expended and the distance over which force is applied -W=Fs with s being "displacement" -units are a combination of force and distance (Nm=joules)

momentum

the product of mass and velocity -Ft=mv -any change in momentum is equal to the impulse that produces it -force applied in direction of motion will increase momentum -force applied opposite to direction of motion will decrease momentum

power

the rate at which work is done -P=Fs/t -P=W/t -P=Fv

coefficient of friction

the ration of force needed to overcome the friction, P, to the force holding the surface together, W -large coefficient surfaces cling together -small coefficient surfaces slide easily -coefficient of 0.0 = frictionless surface -tan(angle)=coefficient of friction

law of conservation of energy

the total amount of energy possessed by a body or an isolated system remains constant

law of inertia

a body continues in its state of rest or of uniform motion unless an unbalances force acts on it -a resistance to change in motion -sum of F=0 (when velocity is constant) -defines inertia (static) and momentum (dynamic)

concurrent forces

act at the same point of application at different angles -resultant of two or more concurrent forces depends on both the magnitude of each force and the angle of application

kinetic energy

energy based on motion -work done is equal to the kinetic energy required -work can add to, or take away, energy from a system

law of universal gravitation

explains how masses of 2 objects react with one another -F=G[(m1m2)/r^2]

law of reaction

for every action there is an equal and opposite reaction

negative work

force acts in the direction opposite to that of the objects motion

positive work

force acts in the same direction as that of the objects motion

angle of pull =90°

force is all rotary (no longer pulling towards fulcrum)

parallel forces

forces not in the same action line, but parallel to each other -effect of parallel forces on an object depends on magnitude, direction, and application point of each force

muscle cross section

if force of the muscle is not known, it is computed from the muscle's cross section (width x thickness)

muscular work

if the internal structure of a muscle is rectangular, a simple geometric cross-sectional measure can be used -for penniform and bipenniform muscle, physiological cross section (PCS) must be determined (W=average force x PCS (sq cm) x .5 length of fibers (cm)) -"s" represents 1/2 the length of the average fiber

conservation of momentum

in any system where forces act on each other the momentum is constant -an equal and opposite momentum change must occur to object producing reaction force

angle of pull <90°

non-rotary force is directed toward fulcrum helps maintain integrity of the joint (stabilizes)

normal force

opposite and equal perpendicular reaction between 2 surfaces coming in contact to one another -N or R

joint reaction force

opposite and equal reaction of the body/joints to outside/inside forces -JRF

ground reaction force

opposite and equal reaction of the earth/ground to outside forces -GRF -jumper pushing off the ground and the ground pushing back