EQUILIBRIUM

a state

all forces/torques in all directions cancel outΣF = 0 ΣT = 0

a body/system that is not accelerating

EQUILIBRIUM Static Dynamic

1

2

3

BALANCE

a process

control/maintain Equilibrium

use of inner ear and proprioceptors

STABILITYa 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 partsize of b.o.s. of body or partmagnitude of friction that determines “sticking” to

form axis of rotationnet 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 LoadsPulling 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 LiftingCan 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 lowc of g forward only a small Tt

will upset equilibrium

c of g lowc of g backward

Changing Directionsbody maintains some motion in original dgreater change requires greater Fwhen running curves:

1. during air phase of curve running, body travels a straight path2. during contact lateral stress on feet/legs