EQUILIBRIUM a state all forces/torques in all directions cancel out ΣF = 0 ΣT = 0 a body/system that is not accelerating

EQUILIBRIUM Static Dynamic 11 22 33

BALANCE  a process  control/maintain Equilibrium  use of inner ear and proprioceptors

STABILITY a state resistance of a body/system to disruption of Equilibrium

STABILITY Linear Rotary resistance to accel. directly related to Ft required to upset Eqlb. depends on - mass & Friction pirouette resistance to tipping resistance to a change in Angular Momentum depends on - size of b.o.s. - action line of gravity - height of c of g - direction of tip Force somersaulting diver

3 States of Rotary Equilibrium 1.Stable - c of g well inside b.o.s. 2.Unstable - c of g close to edge of b.o.s. 3.Neutral - spherical object on a level surface

B.O.S. - Base of Support  outermost limits of contacting surfaces  region bounded by a body or part in contact with a surface  the applied Force of the body/part receives a reaction Force from the surface of contact

BALANCE FACTORS  mass/weight of body or part  height of c of g of body or part  size of b.o.s. of body or part  magnitude of friction that determines “sticking” to form axis of rotation  net Tt applied to body or part

Standing on a Slope  as slope angle increases: 1. Perpendicular F normal pressing body to hill decreases 2. Parallel component of F weight in the direction of slope increases

Common “Athletic Stance” feet spread apart - lateral stability feet staggered - forward/backward stability lower body’s c of g - rotary stability

Pushing/Pulling Loads  Pulling with some upward lift reduces load friction and increases feet friction  High friction load = pull with short rope  Low friction load = pull with long rope

Safe Lifting Can I do it? b.o.s. as close as possible to load stabilize vertebral column in upright position with natural curve lower body to load using hip/knee flexion raise load using hip/knee extension

STARTING and STOPPING c of g low c of g forward only a small Tt will upset equilibrium c of g low c of g backward

Changing Directions  body maintains some motion in original d  greater change requires greater F  when running curves: 1. during air phase of curve running, body travels a straight path 2. during contact lateral stress on feet/legs