1. CHAPTER 2 Mechanical Principles: Kinetics

2. Introduction Kinetics What is force?
All about forces (as opposed to kinematics)
What is force?
Conceptual definition
Properties of force
Magnitude, direction, & point of application

3. Four types of forces that affect body motion:
Gravity
Muscles
Externally applied resistances
Friction

4. Gravity What is it? What affects it?
What is the acceleration of gravity on earth?

5. Forces
Forces act on a mass
Mass = amount of matter in an object
Mass versus weight

6. Newton’s Laws
1st
2nd
3rd

7. Newton’s first law: inertia
Forces
Newton’s first law: inertia
A body at rest will stay at rest, and a body in motion will stay in motion, until acted on by an outside force.
Inertia is reluctance of a body to change its current state.

8. Newton’s second law: acceleration
Forces
Newton’s second law: acceleration
Acceleration is proportionate to the magnitude of the net forces acting on it and inversely proportionate to the mass of the body.

9. Newton’s third law: action-reaction
Forces
Newton’s third law: action-reaction
For every action force there is an equal and opposite reaction force.
Example: basketball player jumping

10. How we work with forces
Forces can be represented graphically with vectors
The direction is indicated by the arrow
The magnitude is represented by the length of the line

11. Vector combination
Vectors can be combined/added/multiplied graphically
Must be connected head to tail
Resultant must be drawn from start to finish and pointed correctly
Resultant represents “net force”

12. Free body diagram
Model of a system of interest (such as a body or body part) showing all of the forces acting on the body

13. Vector Resolution Start with a resultant and create component vectors
Use 2 component vectors that are perpendicular to each other
In anatomical examples one component will be parallel to the bony lever and one will be perpendicular

14. Anatomical vector resolution
Perpendicular component (normal force) will be the rotary component that will contribute to torque
Parallel component will either by stabilizing (acting toward joint center) or dislocating (acting away from joint center)

15. levers
What is a lever?
3 classes
Characteristics
Anatomical examples

16. Resistance arm =  distance from axis to line of action of resistance
Levers
Resistance arm =  distance from axis to line of action of resistance
Force arm =  distance from axis to “moving force”
In the human body
Axis = joint
Body segments act as levers

17. Levers First-class lever
Axis of rotation located between force and resistance arm
Similar in appearance to a seesaw
Length of force and resistance arms vary
For bullet: Axis of rotation located between force and resistance arm (Consider inserting Figure 2-8)

18. Levers Second-class lever
Axis of rotation at end; force arm is larger than resistance arm.
Wheelbarrow exemplifies a second-class lever.
Long force arm makes it possible to move large resistances with little force.
For bullet: Axis of rotation at end; Force arm larger than resistance arm (Consider inserting Figure 2-8D)

19. Levers Third-class lever
Axis of rotation at end; force arm smaller than resistance arm
Most common in human body
Designed to produce speed of distal segment
Able to move small weights a long distance
Occurs frequently in an open kinematic chain (OKC)
For bullet: Axis of rotation at end; Force arm smaller than resistance arm (Consider inserting Figure 2-8G)

20. Force Applications to the Body
Levers and muscle activity
Majority of lever systems in body are third class.
Muscles must exert large forces to overcome external resistance due to lever arm lengths.
However, small changes in muscle length create large angular displacements.
Design suggests body’s levers are designed for speed rather than for strength.

21. Levers Mechanical advantage
Ratio between the length of the force arm and the length of the resistance arm
MA may be >1, <1, or equal to 1

22. torque Conceptual definition Mathematical definition Right hand rule
Finding moment arm

23. Torque (τ)
Product of a force times the perpendicular distance from its line of action to the axis of motion
τ = F · d
d =  distance from location of force on body segment to the joint (axis)

24. Clinical Application of Concepts
Pressure
Defined as force per unit of area.
Optimal applications of pressure facilitate growth and hypertrophy.
Excessive force may cause tissue injury.

25. Clinical Application of Concepts
Pressure may be reduced by:
Decreasing the magnitude of the force
Increasing the area of application
Decreasing the time of application

26. Equilibrium Forces sum to zero Torques sum to zero
What happens when these things do not sum to zero?

27. Force Applications to the Body
Weight and center of mass (COM)
COM
Point about which an object is balanced
Origin of gravity’s force vector
Symmetrical objects—center of object
Asymmetrical objects—challenging to identify
In adults, located anterior to S2

28. Force Applications to the Body
Base of support (BOS)
Line of gravity is the vertical line downward from the center of mass.
The body is stable when the line of gravity passes through the center of BOS.
Larger the BOS, more stable an object is.

29. Force Applications to the Body
Stable, unstable, and neutral equilibrium
Degree of stability depends on:
Height of center of gravity above base of support
Size of base of support
Location of “gravity line” within base of support
Weight of the body

30. Force Applications to the Body
Stable, unstable, and neutral equilibrium
Stable equilibrium—body returns to former position after light perturbation
Unstable equilibrium—body seeks a new position after light perturbation
Neutral equilibrium—center of gravity displaced but remains at same level
Rolling ball

31. Balance
In biomechanics, balance is the control of equilibrium but it is not synonymous with equilibrium

32. Fluid Forces Archimedes’ Principle
Buoyant force is equal to the weight of the displaced fluid

33. Bernoulli’s Principle
Explains lift
Inverse relationship between flow velocity ad pressure
Magnus effect is special case

34. Projectile motion Trajectory is parabola
Factors that determine trajectory
Vertical and horizontal components of velocity
Effects of drag friction
Sports applications
Various goals