CHAPTER 2 Mechanical Principles: Kinetics

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

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

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

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

Newton’s Laws 1st 2nd 3rd

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.

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.

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

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

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”

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

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

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)

levers What is a lever? 3 classes Characteristics Anatomical examples

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

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)

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)

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)

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.

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

torque Conceptual definition Mathematical definition Right hand rule Finding moment arm

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)

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.

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

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

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

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.

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

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

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

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

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

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