Rotary Motion

1
Rotary Motion

• Chapter 13

2
Rotary force

• Eccentric force a force not in line with COG or
  center of rotation
• Torque turning effect of eccentric force
• 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
• In a body segment, change length of moment arm by
  moving segment closer or farther from axis
• In a muscle, moment arm length depends on
  insertion point (distance between axis and
  insertion changes through the ROM)

3
Torque and Lever Arms
4
Torques

• Summation of torques can result in no motion,
  linear motion, or rotary motion
• Equal parallel forces applied in same direction
  on opposite sides produce either no motion or
  linear motion
• Equal parallel forces acting in opposite
  directions is a force couple (steering wheel)

• Principle of torques must consider magnitude
  and direction when summing no rotation when
  equals zero
• Direction of rotation is either clockwise
  (negative) or counterclockwise (positive)

5
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)
• External levers force applied to one end
  rotates around a fulcrum
• Anatomical levers nearly every bone is a lever
  the joint is the fulcrum muscle provides the
  force
• 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
• Figure 13.16

6
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)

7
Speed and ROM in levers

• Speed and ROM in angular movements are
  interdependent
• If two levers move through same angle at same
  speed, the longer one will move farther and
  faster
• The greater the lever length, the greater the
  linear displacement at the end (i.e. club head)
• Anatomical levers favor speed and ROM over force
• Sport implement, arm, and body segments act
  together to make a large lever
• Multiple levers build up speed at the tip
  (greatest force imparted)

8
Lever selection

• Motor performance skill depends on appropriate
  lever selection
• External are chosen, internal are adapted
• Longer levers have greater speed and range, but
  require greater strength to control (concept of
  choking up)
• Mechanical advantage ability to magnify force
• Ratio of the resistance overcome to the effort
  applied
• MAR/E OR MAEA/RA

9
Newtons laws and rotation

• Moment of inertia directly proportional to the
  mass
• As distance between axis and mass increase, so
  does inertia
• If mass is concentrated close to axis, the object
  is easier to turn
• The heavier an object, the greater the force
  required to start or stop it
• In body, change the segment position to change
  the inertia
• Acceleration torque inertia x acceleration
• Angular momentum is measure of force needed to
  stop or start motion
• Action and reaction momentum must be conserved
• Axes for such actions may be in any plane, but
  reactions must be in same or parallel planes
• Good example is swinging arms opposite to legs
  when walking
• Transfer of momentum move from one segment to
  another (throwing, kicking) from large to
  progressively smaller segments

10
Centripetal and centrifugal forces

• Force applied in a manner that causes weight to
  change direction and move in circular path around
  axis
• Centripetal center-seeking force object
  constantly pulled at right angle to direction
  moving
• Centrifugal outward-pulling force equal in
  magnitude to centripetal force
• The amount of acceleration a whirling object
  possesses increases with speed and decreases with
  distance from axis to object (spinning with arms
  in and out)
• Great speeds and small radii create need to bank
  tracks
• Centrifugal force pulls runner outward to linear
  path
• Runners must lean in (create equal centripetal
  force) to maintain equilibrium