Mechanics and Lever Systems

mass

a quantity of atoms

velocity

speed of movement

acceleration

a positive rate of change in velocity

deceleration

a negative rate of change in velocity

force

a push or pull

torque

the tendency of a force acting at a perpendicular distance from an axis to rotate a lever system (rotational force)

equilibrium

a steady state of motion

static equilibrium

forces acting on object but no motion occurring → sum of internal torque and external torque equals zero

dynamic equilibrium

when the velocity of an object is constant but not zero

ie linear motion, angular motion (could be a combination of both)

translatory motion

linear motion where you move in a straight line

rotary motion

angular motin where you move about an axis

combined motion

in a gait cycle you move in both rotary and linear motions

Newton’s First Law

equilibrium, law of inertia

Newton’s 2nd Law

F=ma

Newton’s third law

action and reaction

law of inertia

an object at rest will stay at rest, unless acted upon by an unbalanced force or torque

remain at rest

the sum of all forces or torques must equal zero

Law of acceleration

the acceleration of an object will be in the direction of the net unbalanced force or torque

net unbalanced force

will produce linear motion

net unbalanced torque

will produce an angular motion

a combination of unbalanced force and torque

will produce general motion

Law of Reaction

for every action, there is an equal and opposite reaction → when an object applies a force to a second object, the second object must simultaneously apply a force equal in magnitude and opposite in the direction to the first object

free body diagram

visualization of body or segment of interest - used to visualize the applied forces, movements, and resulting reactions

vectors

representation of forces (an arrow)

parallel forces

can be added or subtracted to find resultant

resultant force

one vector representing all of the forces acting on the body of interest

non-parallel force

described with a resultant force vector, however can not be simply added or subtracted (determined graphically or by trigonometry)

internal moment arm

perpendicular distance from the internal force to the axis of rotation

external moment arm

perpendicular distance from the external force to the axis of rotation

rigid bar

bone

axis

joint line

forces

tend to rotate the rigid arm

external force

always the objects center of gravity

center of gravity

changes on the object when we add weight to it

torque equation

force times perpendicular distance (calculated in Newton meters)

sum of the internal torque and external to

mechanical advantage

the efficiency of the lever system in correlation of movement arms

the greater this is, the less effort required

Class I lever system

axis of rotation is between opposing forces

can be used to gain force advantage or speed

Class II lever system

internal force and external force are on same side of axis, external force is closer to axis creating a longer internal moment arm allowing to lift heavy loads → used to gain internal force advantage, but sacrifices speed

Class III lever system

internal force and external force are on same side of axis, but the internal force is closer to axis than external force → used to gain speed advantage

most commonly found throughout the human body