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Physics

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Measure muscle strength with dynamometer, spring balance, or tensiometer ... Magnifies effort to take less effrot to move resistance (wheelbarrow, doorknob) ... – PowerPoint PPT presentation

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Title: Physics


1
Physics
  • Chapter 3

2
Nature of force
  • Produces, stops, prevents motion increase,
    decrease speed or change direction
  • Types of force
  • Internal forces exerted by bodies within a
    defined system
  • Differences in body shape muscle force
  • Measure muscle strength with dynamometer, spring
    balance, or tensiometer
  • External forces exerted by bodies on other
    bodies outside the system
  • Cause body displacement, weight, gravity
  • Also wind/water resistance, friction, other forces

3
Motion
  • Motion is relative you change position in
    respect to a reference point
  • Can be at rest and at motion at the same time!
  • Cause of motion a form of force
  • Magnitude of force compared to resistance
    determines resulting motion
  • Kinds of motion
  • Translatory/linear
  • Angular/rotary
  • Reciprocating repetitive, oscillation
  • Most are a combination of 2 or more motions

4
Body motions
  • Most body segments undergo angular motion
  • The body as a whole experiences rectilinear
    motion
  • Sledding?
  • Twirling on skates?
  • Diving/jumping?
  • Swinging?

5
Newtons laws of motion
  • 1st Law of inertia
  • Body continues in its state of rest or of uniform
    motion unless an unbalanced force acts on it
  • Amount of force needed to alter velocity is
    directly related to its inertia
  • Why its hard to initiate movement

6
Newtons laws of motion
  • 2nd Law of acceleration
  • Acceleration is directly proportional to the
    force causing it, is in the same direction as the
    force, and is inversely proportional to the mass
  • Momentum mass x velocity
  • Force can be applied to increase or decrease
    momentum
  • Short stopping time requires large stopping force
    (and vice versa) to reduce momentum to zero (i.e.
    giving with the ball or landing)
  • Mass weight acceleration due to gravity
  • Linear velocity distance time

7
Newtons laws of motion
  • 3rd Law of reaction
  • For every action there is an equal and opposite
    reaction (balanced force system)
  • To walk forward, must push backwards first (get a
    push from the ground)
  • Ground reaction force is equal in magnitude and
    opposite in direction to the force vector applied
    to the ground
  • Conservation of momentum momentum is constant
    in a system where forces act on each other (can
    be transferred between bodies)

8
Center of gravity
  • Bodys balance point
  • Concentration of body weight
  • Location of COG remains fixed if body shape
    remains unchanged
  • Location will change with movement or added
    weight
  • COG in humans
  • Varies with body build, age, and gender
  • Postural sway
  • Measured in sagittal and frontal planes
  • Affected by age, fatigue, injury, bracing,
    obesity, and environmental stability
  • Related to falls in the elderly

9
Stability and equilibrium
  • Line of gravity
  • Falls through COG to center of earth
  • All objects at rest are in equilibrium
  • Sum of all linear forces and torques is zero
  • Stable equilibrium
  • Unstable equilibrium
  • Neutral equilibrium

10
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11
Principles of stability
  • Low COG ? stability
  • Wide base of support ? stability
  • Line of gravity should intersect base of support
    in the direction of force
  • ? mass ? stability
  • Segmented body is most stable when COG of each
    segment is aligned over base of support
  • ? friction between supporting surfaces ?
    stability
  • Fixation necessary to prevent muscle substitution
  • Visual focus on stable objects during locomotion
    ? stability
  • Physical and emotional balance improves balance
  • Regaining equilibrium is based on same principles
    as maintaining it
  • Inverse relationship between stability and
    mobility
  • Ability to start, stop, and change directions
    manipulation of stability

12
Levers
  • Simple machines that use torque usually a rigid
    bar rotating about a fixed point to overcome
    resistance
  • Used to overcome a large resistance (more than
    the force applied--crowbar) or increase the speed
    or ROM (golf club)
  • When no motion occurs, the lever is balanced
    (see-saw)
  • Lever arms portion of the lever between the
    fulcrum and the force
  • Effort arm perpendicular distance between the
    fulcrum and effort point
  • Resistance arm perpendicular distance between
    the fulcrum and the resistance
  • The addition of outside force can change lever
    classification

13
Lever classifications
  • 1st class effort ?, axis, resistance ?
  • Mechanical advantage is balance used to magnify
    the effects of effort or to increase speed and
    ROM (scissors)
  • 2nd class axis, resistance ?, effort ?
  • Magnifies effort to take less effrot to move
    resistance (wheelbarrow, doorknob)
  • No 2nd class in body
  • 3rd class axis, effort ?, resistance ?
  • Small effort will produce large motion
  • Advantage is speed and ROM at expense of force
    (body levers)

14
Lever selection and torque
  • External are chosen, internal are adapted
  • Longer levers have greater speed and range, but
    require greater strength to control (concept of
    choking up)
  • Torque about any point product of force
    magnitude and perpendicular distance form line of
    force to axis of rotation
  • Moment arm perpendicular distance between the
    direction of the force vector and the axis
  • Torque can be changed by modifying the force or
    moment arm
  • Heavier weight increases torque shorter arm
    decreases torque

15
Torque and Lever Arms
16
Angle of pull
  • Line of pull long axis of the muscle
  • Angle of pull between long axis of bone and
    line of pull
  • Vertical component is always perpendicular to
    lever (rotary-it moves the lever)
  • Horizontal component is always parallel to the
    lever (non-rotary)
  • When angle of pull is 90, the force is 100
    rotary
  • When angle of pull is 45, the rotary and
    stabilizing forces are equal
  • Construct right triangle with hypotenuse as
    muscles line of pull and adjacent side along
    mechanical axis of insertion bone
  • With pulleys, max resistance occurs when rope is
    90 to the extremity
  • With free weights, max resistance occurs when
    weight is perpendicular to ground

17
Physiological muscle advantage
  • Muscle can shorten to 66 of its resting length
  • Multijoint muscles
  • Active insufficiency cannot hold isometric at
    end range of motion
  • Passive insufficiency ineffective contraction
    if elongated over both joints
  • Work with stable/unaffected end elongated
  • HS seated during knee curls

18
Other concepts
  • Work force x distance
  • Power work time
  • Energy capacity to do work
  • Law of conservation (potential and kinetic
    energy)
  • Velocity rate of change in position
  • Acceleration rate at which velocity changes

19
Other concepts
  • Elasticity and rebound
  • the force acting on the object is called stress
  • The distortion that occurs is called strain
    (proportional to the stress) can be tension,
    compression, bending, or torsion
  • If stress is too large, the elastic limit is
    exceeded and permanent distortion occurs
  • Creep low-level stress applied over time causes
    deformation
  • Friction
  • Force that opposes efforts to slide or roll one
    body over another (impossible to move without
    friction)
  • Increase or decrease friction
  • Smooth surfaces have less friction than rough
  • Friction is proportional to the force pressing
    two surfaces together (acts parallel but in
    opposite direction of movement)
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