Basic Kinetic (study of actions of forces)Concepts:

1. Mass

- quantity of matter composing a body
- no mass = no mechanical identity
- symbol:  m
- standard unit:  kg (kilogram)

2. Inertia

- tendency to resist change in state of motion (at rest or constant velcity)
- proportional to mass (more mass = more resistance to change)
- has no units

3. Force

- a push or pull acting on a body
- cause of motion
- the main mechanical quantity of interest in kinetics
- symbol:  F
- standard unit:  N (Newton)
- 1 N = (1 kg)(1 m/s²)

- Properties:  magnitude, direction & point of application
- Examples:  muscle force, weight (gravity), friction, etc
- Free Body Diagram

• sketch that shows a defined system in isolation with all of the force vectors acting on the system
• results in a net force (resultant of all vector forces acting upon system)

4. Newton's Laws of Motion

1. Law of Inertia
• a body will maintain a state of rest or constant velocity unless acted on by an external force that changes it
• example: barbell sitting on floor, ice skater gliding (should remain at same velocity, however in reality friction and air resistance slows down

2. Law of Acceleration
• the acceleration of a body is proportional to the force causing it and takes place in the direction in which the force acts
• example: ball travels direction hit, goes farther the faster is pitched and harder it is hit
• example: f= ma

3. Law of Reaction
• for every action there is an equal and opposite reaction
• example: lean on wall, wall pushes back with equal and opposite force

5. Center of Gravity (COG) or Center of Mass (COM)

- point around which a body's weight (mass) is equally balanced in all directions

6. Weight

- attractive force that earth exerts on mass
= ma_g
- a_g = 9.81 m/s²
- units are in N

7. Pressure

- amount of force acting over a unit area
P  =  F / A
- standard unit:  Pa (Pascal)   =  1 N/m²
- examples: high heel shoe, shin protector, snow shoe, etc...

8. Volume

- space occupied by body
- three dimensions
- units³

9. Density

- mass per unit of volume (mass/volume)

10. Specific Weight

- weight per unit of volume

11. Torque

- rotary effect of a force
- sumbol: T
T = force times perpendicular distance from the forces line of action to the axis of rotation

12. Impulse

- product of force and the time over which the force acts

Mechanical Loads on the Human Body (p. 70-75)

1. Types of loading

- Compression:  pressing or squeezing force directed axially through a body
- Tension:  pulling or stretching force directed axially through a body
- Shear:  force directed parallel to a surface
- Bending:  asymmetric loading that produces tension on one side of a body's longitudinal axis and compression on the other
- Torsion:  load causing twisting of a body around its longitudinal axis
- Combined loading:  combination of different types of loading

2. Effects of loading

- deformation: change in shape
- acute vs. repetitive: likelihood of injury: load magnitude vs. frequency

3. Mechanical Stress and Strain

- Mechanical stress: distribution of force inside of a solid body (lumbar vs. thoracic vertabrae)
- strain: deformation due to stress
- load-deformation curve (stress-strain curve)

• yield point (elastic limit): permanent deformation
• failure point: loss of mechanical continuity

Vector Algebra

- vector: physical quantity that possesses both magnitude and direction
- scalar: physical quantity that is completely described by its magnitude

- tip-to-tail = resultant

Chapter 3 Key Terms:

acute loading (74)	application of a single force of sufficient magnitude to cause injury to a biological tissue
axial (72)	directed along the longitudinal axis of a body
bending (72)	asymmetric loading that produces tension on one side of a body's longitudinal axis and compression on the other side
center of gravity (64)	point around which a body's weight and mass are equally balanced in all directions
combined loading (72)	simultaneous action of more than one of the pure forms of loading
compression (70)	pressing or squeezing force directed axially through a body
deformation (73)	change in original shape
density (68)	mass per unit of volume
failure (74)	loss of mechanical continuity
force (63)	push or pull; the product of mass and acceleration
free body diagram (63)	sketch that shows a defines system in isolation with all of the force vectors acting on the system
impulse (69)	product of force and the time over which the force acts
inertia (62)	tendency of a body to resist a change in its state of motion
mass (63)	quantity of matter contained in an object
myoelectric activity (75)	electric current or voltage produced by a muscle
net force (64)	resultant force derived from the composition of two or more forces
pressure (66)	force per unit of area
repetitive loading (74)	repeated application of a subacute load that is usually of relatively low magnitude
resultant (77)	single vector that results from vector composition
scalar (76)	physical quantity that is completely described by its magnitude
shear (70)	force directed parallel to a surface
specific weight (68)	weight per unit of volume
stress (71)	distribution of force within a body, quantified as force divided by the area over which the force acts
tension (70)	pulling or stretching force directed axially through a body
torque (68)	rotary effect of a force
torsion (72)	load producing twisting of a body around its longitudinal axis
vector (76)	physical quantity that possesses both magnitude and direction
vector composition (76)	process of determining a single vector from two or more vectors by vector addition
vector resolution (78)	operation that replaces a single vector with two perpendicular vectors such that the vector composition of the two perpendicular vectors yields the original vector
volume (66)	space occupied by a body
weight (64)	attractive force that the earth exerts on a body
yield point (elastic limit) (74)	point on the load-deformation curve past which deformation is permanent