Force

Push or pull on an object

Internal forces

- Act within an object or system whose motion is under investigation
- Forces come in pairs (action and reaction)

Internal forces: tensile force

When pulling forced act on the ends of an internal structure

Internal force: compressive forces

When pushing forces act oj the ends of an internal structure

External forces

Act on an object as a result of its interaction with the environment surrounding it (when two objects touch)

Weight

W = m*g

w (newtons)= m(in kilograms)*g(9.81m/s^2)

Translation:

linear motions in which all parts of a rigid body move parallel to & in the same direction as every other part of the body

Rotation:

- motion in which a (assumed) rigid body moves in a circular path around some pivot point,
- thus the entire body moves in the same angular direction & across the same number of degrees

Kinematics can be_____

active or passive

Degrees of freedom:

the number of independent directions of movements allowed at a joint

max degrees of angular freedom

3 degrees for 2 cardinal planes

Arthrokinematics: the trio

- roll
- slide
- spin

Roll-slide

- As primary movement of bone is to rotate across another bone’s surface
- concurrent but equal + opposite side

spin

- Spinning of one articular surface on another
- NO SLIDE

THE RULE: Convex-on-concave =

opposite

THE RULE: Concave-on-convex

same

Joint articulation fits “the best” where?

- at end range = close packed
- provide more stability

Kinetics

The study of mechanics that describes the effect of forces on the

body

Rectilinear:

- all points on a body/object move in a straight line.
- Direction of motions& orientation of object do not change.
- All points on object move the

same distance.

Curvilinear:

- all points on a body or object move
- orientation does
not

change & all points move the same distance - path the object follows is

curved.

Angular motion

- rotation/rotary motion
- More numerous than linear motion (human movement)

General motion

- Combination of angular motions producing linear motions of multiple body parts

Displacement

- vector quantity
- has direction & magnitude

Speed

- rate of motion
- AVG= d/change in time

Velocity

- rate of motion in a specific direction
- AVG= velocity(or d)/change of time

Baseball fastpitch example

- baseball released @16.8m

- velocity @47.162m/s

- v= d/change of time
- change of t= d/velocity
- 16.8m/47.163(m/s)=
**0.356 seconds**

**hang time** when initial vertical velocity (20 m/s) and
initial horizontal velocity (15 m/s) is known

L = mv

- L = linear momentum
- m = mass
- v = instantaneous velocity
- L constant => sum of forces = 0

elastic collisions

momentum is conserved (transferred) and no energy is lost

inelastic collisions

momentum is conserved but energy is lost

Impulse

the product of force and the time the force acts

Torque equations

- T= F*r
- F = force
- r = moment arm (perpendicular distance from axis of rotation)

Stability

- dependent upon height of center of gravity, size of base of support, and the weight of an object

Locating Center of gravity

sum of force

centric forces

act through the center of gravity of an object and cause linear translation

Eccentric forces

- (not muscle contractions) do not act through the center of gravity
- they cause both linear and angular motion

Force couples

- create angular motion
- couples are forces that

are equal in size, non-colinear, and act in opposite directions

1st class levers

- HUGE forces
- distance is really small.

2nd class levels

- middle
- output forces are modest, input distance is modest

3rd class:

- WEAK output forces
- HUGE distances

Potential Energy (2 types)

- Gravitational
- Strain (or spring) energy

Power

- strongest contraction force is produced at the slowest speed
- max POWER occurs at 1/3 -1/2 of a muscles max contraction velocity

Energy

the capacity to do work

Tension

the stress that acts perpendicular (or normal) to the analysis plane

Shear

A transverse stress that acts parallel to the plane of analysis

Bending

- More complicated
- creates different stresses at the
analysis

plane

• creates both tensile & compressive forces

Wolff's Law

Bone remodels according to the stresses place upon it

Torsion

Occur when torques act about the long axis of an object

Poisson’s Ratio

- Each material has a property (Poisson’s ratio)
- determines the amounts of

axial strain to transverse strain experienced during loading

Elasticity

The ability to stretch under tensile load but then return to it’s original shape

Elastic modulus

The ratio of stress-to-strain is called elastic modulus of a material

Resilient:

the ability to absorb stress/shock/assualt and return to a previous state