#### Transcript What is force?

```Brunnstrom’s Clinical Kinesiology Sixth Edition
CHAPTER 2
Mechanical Principles: Kinetics
Brunnstrom’s Clinical Kinesiology Sixth Edition
Introduction
 Kinetics
 All about forces (as opposed to kinematics)
 What is force?
 Conceptual definition
 Properties of force
 Magnitude, direction, & point of application
Brunnstrom’s Clinical Kinesiology Sixth Edition
Forces
 Four types of forces that affect body
motion:
 Gravity
 Muscles
 Externally applied resistances
 Friction
Brunnstrom’s Clinical Kinesiology Sixth Edition
Gravity
 What is it?
 What affects it?
 What is the acceleration of gravity on earth?
Brunnstrom’s Clinical Kinesiology Sixth Edition
Forces
 Forces act on a mass
 Mass = amount of matter in an object
 Mass versus weight
Brunnstrom’s Clinical Kinesiology Sixth Edition
Newton’s Laws
 1st
 2nd
 3rd
Brunnstrom’s Clinical Kinesiology Sixth Edition
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.
 F  0
Brunnstrom’s Clinical Kinesiology Sixth Edition
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.
F
a 
m
Brunnstrom’s Clinical Kinesiology Sixth Edition
Forces
 Newton’s third law: action-reaction
 For every action force there is an equal and
opposite reaction force.
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
Vector combination
graphically
 Must be connected head to tail
 Resultant must be drawn from start to finish and
pointed correctly
 Resultant represents “net force”
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
Vector Resolution
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
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)
Brunnstrom’s Clinical Kinesiology Sixth Edition
levers
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What is a lever?
3 classes
Characteristics
Anatomical examples
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
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.
Brunnstrom’s Clinical Kinesiology Sixth Edition
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)
Brunnstrom’s Clinical Kinesiology Sixth Edition
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.
Brunnstrom’s Clinical Kinesiology Sixth Edition
Levers
 Ratio between the length of the force arm and
the length of the resistance arm
Force Arm Length
 MA 
Resistance Arm Length
 MA may be >1, <1, or equal to 1
Brunnstrom’s Clinical Kinesiology Sixth Edition
torque
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Conceptual definition
Mathematical definition
Right hand rule
Finding moment arm
Brunnstrom’s Clinical Kinesiology Sixth Edition
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)
Brunnstrom’s Clinical Kinesiology Sixth Edition
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.
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
Equilibrium
 Forces sum to zero
 Torques sum to zero
 What happens when these things do not sum
to zero?
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
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.
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
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
Brunnstrom’s Clinical Kinesiology Sixth Edition
Balance
 In biomechanics, balance is the control of
equilibrium but it is not synonymous with
equilibrium
Brunnstrom’s Clinical Kinesiology Sixth Edition
Fluid Forces
 Archimedes’ Principle
 Buoyant force is equal to the weight of the
displaced fluid
Brunnstrom’s Clinical Kinesiology Sixth Edition
Bernoulli’s Principle
 Explains lift
 Inverse relationship between flow velocity ad
pressure
 Magnus effect is special case
Brunnstrom’s Clinical Kinesiology Sixth Edition
Projectile motion
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Trajectory is parabola
Factors that determine trajectory
Vertical and horizontal components of velocity
Effects of drag friction
Sports applications
 Various goals